Mineral

What is Mineral?

A solid material having a distinct crystal structure and a reasonably well-defined chemical composition that exists naturally in pure form is referred to as a mineral or mineral species in geology and mineralogy.

Compounds that are exclusively found in living things are typically not included in the geological definition of a mineral.

But some minerals—like calcite, for example—are frequently biogenic, or in a chemical sense, organic molecules (like mellite). Furthermore, inorganic minerals found in rocks like hydroxylapatite are frequently synthesized by living things.

Rock is any bulk solid geologic substance that is relatively homogeneous at a big enough scale; the two concepts are not the same.

One type of mineral may make up a rock, or it may be an aggregate of two or more distinct mineral types that have been spatially separated into different phases.

More accurately, mineraloids are some naturally occurring solid materials like opal and obsidian that lack a distinct crystalline structure. Each crystal structure that a chemical compound has when it occurs naturally is regarded as a distinct species of mineral.

Therefore, quartz and stishovite, for instance, are two distinct minerals that both contain silicon dioxide.

The internationally accepted standard organization for the classification and name of mineral species is the International Mineralogical Association (IMA). 5,955 official mineral species are recognized by the IMA as of July 2023.

Small quantities of impurities can cause a recognized mineral species’ chemical makeup to change somewhat.

There are instances when certain species variations have official or conventional names of their own. For instance, the purple variant of the mineral species quartz is known as amethyst.

Certain types of minerals may include different ratios of two or more chemical elements that are positioned in comparable ways within the mineral’s structure; these include the equation for machinate is (Fe, Ni)₉S₈, or Fe_xNi_9-x S₈, where x is an arbitrary quantity between 0 and 9.

A mineral with a varied composition can occasionally be divided into distinct species, essentially at random, to form a mineral group.

One example of this is the olivine group, which is made up of the silicates Ca_xMg_yFe_2-x-ySiO₄.

A description of a mineral species typically includes its common physical characteristics, such as habit, hardness, luster, diaphaneity, color, streak, tenacity, cleavage, fracture, parting, specific gravity, magnetism, fluorescence, and radioactivity.

Taste or smell, and reaction to acid, in addition to its essential chemical composition and crystal structure.

Key chemical components are used to categorize minerals; the two most used approaches are the Dana and Strunz classifications.

About 90% of the minerals that make up the Earth’s crust are silicates.

The native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates are further significant mineral groupings.

Definitions

International Mineralogical Association

According to the International Mineralogical Association, a material must meet the following criteria in order to be classified as a unique mineral:

• It must be a substance that exists naturally on Earth or on other alien worlds, generated by natural geological processes. This does not include substances that are solely and directly produced by humans (anthropogenic) or found in living things (biogenic), such as seashells, tungsten carbide, urinary calculi, and calcium oxalate crystals found in plant tissues. Nonetheless, materials originating from such sources might be acceptable if geological processes had a role in their creation (for example, evenkite, which comes from plant matter; taranakite, which comes from bat guano; or alpersite, which comes from mine tailings). Even if hypothetical materials are expected to exist in inaccessible natural settings like the Earth’s core or other planets, they are not included.
• In its natural state, it needs to be a solid substance. Native mercury is a notable exception to this criterion; included before the current regulations were set, it is nevertheless categorized as a mineral by the IMA despite only crystallizing below -39 °C. Despite being frequently discovered as inclusions in other minerals, carbon dioxide and water are not regarded as minerals; on the other hand, water ice is.
• It needs to have an organized arrangement of atoms or a clearly defined crystallographic structure. This characteristic suggests a number of macroscopic physical characteristics, including cleavage, hardness, and crystal shape. It does not include several other amorphous (non-crystalline) materials found in geologic settings, such as limonite, obsidian, and ozokerite.
• It needs to have a chemical makeup that is rather well characterized. On the other hand, some crystalline materials that have a constant structure but a changeable composition could be categorized as individual mineral species. Solid solutions like mackinawite, (Fe, Ni)₉S₈, which is primarily a ferrous sulfide with a sizable fraction of iron atoms replaced by nickel atoms, are a common class of instances. Other examples are crystals that merely differ in the regular arrangement of vacancies and replacements, or layered crystals with varied layer stacking. However, some substances can be arbitrarily divided into many minerals if they have a continuous series of compositions. The olivine group (Mg, Fe)₂SiO₄ is a typical example, with its magnesium-rich and iron-rich end-members (fayalite and forsterite) being recognized as distinct minerals.

There is some disagreement concerning the specifics of these regulations. For example, the International Mineral Association (IMA) has rejected a number of recent requests to categorize amorphous compounds as minerals.

Although the IMA has not established a minimum crystal size, it is hesitant to accept minerals that are only found in nature as nanoparticles a few hundred atoms across.

Certain writers stipulate that the substance must be a solid at room temperature (25 °C) or metastable. The material must only be sufficiently stable, though, for the IMA to accurately assess its composition and structure.

For instance, despite the fact that meridianiite, a naturally occurring magnesium sulfate hydrate, is only produced and stable below 2 °C, it has recently been acknowledged as a mineral.

The IMA has authorized 5,955 different mineral species as of July 2023. The two other main categories of mineral name etymologies are those based on chemical composition or physical features.

The most popular names for minerals are those derived from people, followed by the region of discovery.

The majority of names end in “-ite”; the exceptions are often names like galena and diamond which were well-known before mineralogy was organized as a field of study.

Biogenic Minerals

The inclusion of biogenic crystalline compounds in the IMA’s list of excluded materials has caused controversy among mineralogists and geologists.

As an illustration, Lowenstam (1981) claimed that “a diverse array of minerals, some of which cannot be formed inorganically in the biosphere, are capable of being formed by organisms.

According to Skinner (2005), all solids have the potential to become minerals, and those formed by an organism’s metabolic processes are referred to as biominerals.

In order to include “element or compound, amorphous or crystalline, formed through biogeochemical processes,” as a mineral, Skinner broadened the prior definition of the term.

This subject may get some fresh light thanks to recent developments in high-resolution genetics and X-ray absorption spectroscopy, which are revealing information on the biogeochemical relationships between microbes and minerals.

The “Working Group on Environmental Mineralogy and Geochemistry,” for instance, was commissioned by the IMA and is responsible for studying minerals in the hydrosphere, atmosphere, and biosphere.

The range of the group includes mineral-forming microorganisms found almost everywhere on the planet’s surfaces, including rocks, soil, and particle surfaces.

These organisms can be found at least 1600 meters below the sea floor and 70 kilometers into the stratosphere (potentially reaching the mesosphere).

For billions of years, biogeochemical cycles have played a role in the production of minerals. Metals can be precipitated out of solution by microorganisms, which helps ore deposits form. They are also capable of catalyzing the mineral’s disintegration.

Until its listing by the International Mineralogical Association, more than 60 biominerals had been found, given names, and made public.

By Skinner’s (2005) definition, these minerals (a subset specified in Lowenstam, 1981) constitute minerals proper.

Many of these biomineral representatives are spread among the 78 mineral classes mentioned in the Dana classification scheme, even though these biominerals are not included in the official list of mineral names published by the International Mineral Association.

This is taken into consideration in Skinner’s (2005) definition of a mineral, which states that a mineral can be either crystalline or amorphous.

Despite not being the most prevalent type of mineral, biominerals aid in defining the parameters of what exactly qualifies as a mineral proper.

Crystallinity was specifically specified in Nickel’s (1995) formal definition as a necessary condition for classifying a material as a mineral.

Icosahedrite, an alloy of aluminum, iron, and copper, was described in a 2011 study as a quasicrystal that gets its name from its distinct natural icosahedral symmetry.

Quasicrystals are organized but not periodic, in contrast to real crystals.

Rocks, Ores, and Gems

An accumulation of one or more minerals, or mineraloids, is called a rock. Certain rocks, like quartzite and limestone, are mainly made of one mineral: quartz in the latter case, and calcite or aragonite in the case of limestone.

The relative abundances of important minerals can distinguish other types of rocks, whereas the proportions of quartz, alkali feldspar, and plagioclase feldspar characterize granites.

We refer to the other minerals in the rock as accessory minerals and don’t significantly alter the rock’s overall makeup.

Coal is a sedimentary rock that is mostly made of carbon produced from biological matter. However, rocks can also be made entirely of non-mineral stuff.

The term “rock-forming minerals” refers to the mineral species and groupings that are significantly more common than others in rocks.

Quartz, feldspars, micas, amphiboles, pyroxenes, olivines, and calcite are some of the most common examples of these; all except the last are silicate minerals.

In general, about 150 minerals are seen to be especially significant, either because of their abundance or because they make good collector’s pieces.

Industrial minerals are defined as commercially useful minerals and rocks that are not gemstones, metal ores, or mineral fuels.

For instance, white mica called muscovite (also known as isinglass) can be used as an insulator, a filler, or for windows. Minerals known as ores are concentrated sources of a certain element, usually a metal.

Examples include the mercury ore cinnabar (HgS), the zinc ore sphalerite (ZnS), the tin ore cassiterite (SnO₂), and the boron ore colemanite.

Gems are ornamental minerals that stand out from other types of minerals due to their beauty, toughness, and typically scarcity.

Of the approximately 35 most popular gemstones, about 20 mineral species are classified as gem minerals.

Since gem minerals are frequently found in multiple forms, one mineral can be the source of multiple distinct gemstones; for instance, sapphire and ruby are both forms of corundum, or Al2O3.

Etymology

The word “mineral” was first used in the English language (Middle English) in the fifteenth century. The word minerals, derived from minera, mine, and ore, originated in medieval Latin.

The Latin species, which means “a particular sort, kind, or type with distinct look, or appearance,” is where the word “species” originates.

Chemistry

The chemistry of minerals, which is reliant on the Earth’s constituent abundances, directly governs both their diversity and abundance.

Most minerals that are found are found in the Earth’s crust. Because these elements are abundant in the crust, they make up the majority of the essential components of minerals.

These eight elements—which together make up more than 98% of the crust’s weight—are oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, and potassium, in decreasing order of abundance.

By far the two most significant components are silicon and oxygen; silicon makes about 28% of the crust’s weight and oxygen 47%.

Within the constraints imposed by the bulk chemistry of the parent body, the minerals that form are the most stable at the temperature and pressure of formation.

For instance, the alkali metals (potassium and sodium) and aluminum that are present in most igneous rocks are mainly found in association with oxygen, silicon, and calcium as feldspar minerals.

The surplus sodium will produce sodic amphiboles like riebeckite if the rock is abnormally rich in alkali metals since there won’t be enough aluminum to mix with all the sodium to form feldspar.

Should the aluminum be exceptionally high, the extra aluminum will crystallize as muscovite or other minerals that are rich in aluminum.

Feldspathoid minerals will replace some of the feldspar if there is a silicon deficiency. Complex thermodynamic calculations are needed to precisely forecast which minerals will be present in a rock of a given composition that was created at a given temperature and pressure.

However, somewhat basic guidelines, such as the CIPW standard, which provides accurate approximations for volcanic rock generated from dry magma, can be used to make approximations.

Within a solid solution series, end member species may differ in their chemical makeup.

For instance, the four recognized intermediate varieties between sodium- and calcium-rich anorthite (CaAl₂Si₂O₈) and the sodium-rich end member albite (NaAlSi₃O₈) that make up the plagioclase feldspars are oligoclase, andesine, labradorite, and bytownite, listed in order of recognition.

Additional instances of series are the wolframite series, which consists of iron-rich ferberite and manganese-rich hübnerite, and the olivine series, which consists of magnesium-rich forsterite and iron-rich fayalite.

Coordination and chemical substitution Polyhedra elucidate this shared characteristic of minerals. Minerals are not pure substances in nature; instead, they are tainted by other elements that may exist in the particular chemical system.

It is therefore feasible to replace one ingredient with another. Ions with similar sizes and charges will chemically replace one another; for instance, a large difference in size and charge will result in structural and chemical incompatibilities that prevent K⁺ from substituting for Si⁴⁺ and Al³+.

which are similar in charge, size, and abundance in the crust, and are frequently substituted chemically.

The substitution scenario involving plagioclase occurs in three different instances. Every Feldspar is a framework.

Silicates, which have a 2:1 silicon-to-oxygen ratio. The replacement of Al³⁺ for Si⁴⁺ results in a base unit of [AlSi₃O₈]-; in the absence of the substitution, the formula would be charge-balanced as SiO₂, yielding quartz. Coordination polyhedra will provide more light on the significance of this structural feature.

The second substitution is between Na⁺ and Ca²⁺; however, a second substitution of Al³⁺ for Si⁴⁺ is required to account for the difference in charge.

The geometric representation of an anion surrounding a cation is called a coordination polyhedron.

Because oxygen is abundant in the crust, coordination polyhedra are typically thought of in terms of oxygen in mineralogy.

The silica tetrahedron is the fundamental unit of silicate minerals, consisting of one Si⁴⁺ encircled by four O^2-.

An alternative method of characterizing the silicate’s coordination is through a numerical value: in the example of the silica tetrahedron, the silicon is designated as having a coordination number of 4.

There is a range of possible coordination numbers for different cations; for silicon, this is nearly usually 4, with the exception of very high minerals under pressure, where the compound is compressed to the point that silicon and oxygen are in an octahedral (six-fold) coordination.

Due to their increased relative size in comparison to oxygen, larger cations have higher coordination numbers (the final orbital subshell of heavier atoms is also different).

Physical and mineralogical variations result from changes in coordination numbers. For instance, many minerals, notably silicates like garnet and olivine, will transition to a perovskite structure at high pressure, such as in the mantle, where silicon is in octahedral coordination.

Additional illustrations are the coordination number of the Al³⁺ separates the aluminosilicates kyanite, andalusite, and sillimanite (polymorphs since they have the same formula, Al₂SiO₅); these minerals alter from one another in response to variations in temperature and pressure.

Because of the requirement to balance charges, a variety of minerals can be found in silicate materials when Al³⁺ replaces Si⁴⁺.

The common rock-forming minerals replace the trace amounts of the other elements that are normally present, as the eight most frequent elements account for nearly 98% of the Earth’s crust.

The unique minerals of the majority of elements are extremely uncommon; they are only discovered in places where these elements have been concentrated to a degree.

where conventional minerals are unable to contain them due to geological processes like hydrothermal circulation.

A rock sample’s mineralogy is affected by variations in composition, temperature, and pressure. Processes like weathering or metasomatism (hydrothermal alteration) can lead to changes in composition.

When the host rock moves into different physical regimes due to tectonic or magmatic activity, changes in temperature and pressure take place.

Variations in thermodynamic parameters provide favorable conditions for mineral assemblages It is therefore feasible for two rocks to have the same or very comparable bulk rock chemistry without sharing a similar mineralogy.

This is because minerals react with one another to form new minerals.

One mineral that is frequently found in granite, a plutonic igneous rock, is orthoclase feldspar (KAlSi₃O₈).

It reacts with weathering to create silicic acid and the sedimentary mineral kaolinite (Al₂Si₂O₅(OH)₄):

2 KAlSi₃O₈+5H₂O +2H⁺→ Al₂Si₂O₅(OH)₄+4H₂SiO₃+ 2K⁺

Al₂Si₂O₅(OH)₄→ Al₂Si₄O₁₀(OH)₂ + H₂O via SiO₂.

As metamorphic grade increases, the pyrophyllite reacts to form kyanite and quartz:

Al₂Si₄O₁₀(OH)₂→ Al₂SiO₅ +3SiO₂+H₂O

Alternatively, a mineral may undergo a change in its crystal structure without reacting as a result of variations in pressure and temperature.

For instance, under high temperatures and pressures, quartz will transform into a range of SiO2 polymorphs, including coesite, tridymite, and cristobalite.

Physical Properties

Mineral classifications range in difficulty from easy to hard. Numerous physical characteristics can be used to identify a mineral; some of these characteristics are clear enough to allow for a complete identification.

In some situations, the only way to classify minerals is by more intricate X-ray, chemical, or optical diffraction analyses; these techniques can be expensive and time-consuming.

Crystal structure and habit, hardness, luster, diaphaneity, color, streak, cleavage and fracture, and specific gravity are examples of physical characteristics used for classification.

Additional, less universal tests include those for radioactivity, magnetism, phosphorescence, tenacity (reaction to mechanically induced changes in form or structure), piezoelectricity, and reactivity to diluted acids.

Crystal Structure and Habit

The orderly geometric spatial arrangement of atoms within a mineral’s internal structure gives rise to its crystal structure.

The regular internal atomic or ionic arrangement that underpins this crystal structure is frequently portrayed in the geometric shape that the crystal adopts.

X-ray diffraction can be used to establish the underlying crystal structure, which is always periodic, even in cases when the mineral grains are too small to discern or have irregular shapes. Generally, the symmetry content of minerals is used to characterize them.

There are only 32 point groups for crystals, and each group has a unique symmetry.

These groupings are further divided into more inclusive categories, the six crystal families being the most inclusive.

The three crystallographic axes’ relative lengths and the angles that separate them can be used to characterize these families; these relationships correlate to the symmetry operations that define the narrower point groups.

Below is a summary of them: a, b, and c stand for the axes, and a, ß, and ? for the angles opposite the corresponding crystallographic axis (for example, a represents the angle between the b and c axes, which is opposite the a-axis).

In addition, the hexagonal crystal family is divided into two crystal systems: the hexagonal, with its six-fold axis of symmetry, and the trigonal, with its three-fold axis.

A mineral’s chemistry and crystal structure work together to define it. Minerals with distinct chemistries can have the same crystal structure in up to 32-point groups.

As an example, the hexaoctahedral point group (isometric family) includes halite (NaCl), galena (PbS), and periclase (MgO) because of the comparable stoichiometry between its various constituent elements.

Polymorphs, on the other hand, are collections of minerals with the same chemical formula but distinct structural differences.

For instance, the formulas for the iron sulfides pyrite and marcasite are both FeS₂, but the former is isometric and the latter is orthorhombic.

The pyrite and marcasite groups are the collective names for these two groups, which are polymorphic with respect to other sulfides having the general AX2 formula.

Polymorphism is not limited to content with pure symmetry. Three minerals, kyanite, andalusite, and sillimanite, belong to the aluminosilicates group and have the same chemical formula, Al₂SiO₅.

While sillimanite and andalusite are both orthorhombic and part of the dipyramidal point group, kyanite is triclinic.

These variations result from the way that aluminum is arranged within the crystal structure. One aluminum ion is in six-fold coordination with oxygen in all minerals.

All minerals typically contain silicon in four-fold coordination, yet some minerals, like stishovite, are an exception. (SiO₂, a rutile-structured quartz polymorph under extremely high pressure).

The second aluminum in kyanite is in six-fold coordination; to represent its crystal structure, its chemical formula is AlAlSiO₅.

The second aluminum in sillimanite is in four-fold coordination (AlAlSiO₅), whereas the second aluminum in andalusite is in five-fold coordination (AlAlSiO₅).

Other physical characteristics of the material are significantly influenced by variations in chemistry and crystal structure.

Crystal family	Lengths	Angles	Common examples
Isometric	a = b = c	α = β = γ = 90°	Garnet, halite, pyrite
Tetragonal	a = b ≠ c	α = β = γ = 90°	Rutile, zircon, andalusite
Orthorhombic	a ≠ b ≠ c	α = β = γ = 90°	Olivine, aragonite, orthopyroxenes
Hexagonal	a = b ≠ c	α = β = 90°, γ = 120°	Quartz, calcite, tourmaline
Monoclinic	a ≠ b ≠ c	α = γ = 90°, β ≠ 90°	Clinopyroxenes, orthoclase, gypsum
Triclinic	a ≠ b ≠ c	α ≠ β ≠ γ ≠ 90°	Anorthite, albite, kyanite

Diamond and graphite are carbon allotropes with extremely distinct qualities. Diamond is the hardest material in nature, has an adamantine sheen, and is part of the isometric crystal family; on the other hand, graphite is very soft, has a greasy shine, and forms hexagonal crystals.

Variations in bonding account for this discrepancy. A diamond, In contrast to graphite, is made up of sheets of carbons in sp² hybrid orbitals, where each carbon is covalently bonded to only three other carbons.

The carbons in sp³ hybrid orbitals form a framework where each carbon is covalently bonded to four neighbors in a tetrahedral manner.

There are significant macroscopic disparities because the van der Waals forces holding these sheets together are substantially weaker.

The intergrowth of two or more crystals of the same mineral species is known as twinning. The symmetry of the mineral determines the twinning’s geometry.

Contact twins, reticulated twins, geniculated twins, penetration twins, cyclic twins, and polysynthetic twins are among the various forms of twins that occur from this.

Contact twins, also known as simple twins, are prevalent in spinel; they are made up of two crystals linked at a plane.

Reticulated twins are interlocking crystals that resemble netting and are frequently found in rutiles.

The beginning of the twin is what causes the bend in the middle of geniculated twins.

The two single crystals that make up penetration twins have grown into one another; Cross-shaped staurolite twins and Carlsbad twinning in orthoclase are two instances of this type of twinning.

The recurrent twinning around a rotation axis results in cyclic twins. Three lines, Fourlings, Fivelings, Sixlings, and Eightlings are the equivalent patterns for this sort of twinning, which takes place around three, four, five, six, or eight-fold axes.

Agagonite frequently contains sixlings. When repeating twinning is present, polysynthetic twins resemble cyclic twins. However, polysynthetic twinning happens along parallel planes, typically on a microscopic size, as opposed to around a rotational axis.

The general form of a crystal is referred to as its crystal habit. This feature is described by multiple terms.

Common habits include prismatic (elongated in one direction), equant (typical of garnet), bladed, dendritic (tree-pattern, common in native copper), tabular (different from bladed habit in that it is platy whereas the latter has a defined elongation), and acicular (describes needlelike crystals as in natrolite).

In connection with crystal shape, certain minerals can be identified by their crystal faces, particularly when examining them under a petrographic microscope.

Crystals that are halfway between euhedral and anhedral forms lack a distinct exterior shape; these forms are referred to as subhedral.

Hardness

The degree to which a mineral can withstand scratches depends on its hardness. The crystalline structure and chemical makeup of a mineral determine this physical attribute.

The hardness of a mineral varies depending on its crystal structure; certain directions are softer than others due to crystallographic weakness.

Kyanite is one material that exemplifies this feature; it has a Mohs hardness of 51/2 parallel to yet 7 parallel to The ordinal Mohs hardness scale is the most widely used measurement system.

A mineral with a higher index scrapes those that are lower, as determined by ten signs.

Mohs hardness	Mineral	Chemical formula
1	Talc	Mg3Si4O10(OH)2
2	Gypsum	CaSO42H2O
3	Calcite	CaCO3
4	Fluorite	CaF2
5	Apatite	Ca5(PO4)3(OH,Cl,F)
6	Orthoclase	KAlSi3O8
7	Quartz	SiO2
8	Topaz	Al2SiO4(OH,F)2
9	Corundum	Al2O3
10	Diamond	C

The hardest natural material is diamond, a carbon polymorph, and it falls on a scale that starts with talc, a phyllosilicate. Below is the scale: These are some more scales.

• Shore’s hardness test gauges a mineral’s resilience by looking at how deeply a spring-loaded device gets indented.
• The Rockwell scale
• The Vickers Hardness Test
• Brinell scale

Lustre and Diaphaneity

The quality and intensity of light reflected from a mineral’s surface are indicated by its luster.

This quality is described by a wide range of qualitative phrases that are divided into metallic and non-metallic categories.

High-reflectivity minerals, such as galena and pyrite, are examples of metallic and sub-metallic minerals.

The following are examples of non-metallic lusters: adamantine, found in diamonds; vitreous, a glassy luster frequently found in silicate minerals; pearly, found in talc and apophyllite; resinous, found in garnet group members; and silky, found in fibrous minerals such asbestiform chrysotile.

A mineral’s diaphaneity is a property that indicates how well light may travel through it. Transparent minerals allow light to travel through them with no reduction in intensity.

Muskovite, or potassium mica, is an example of a translucent mineral; certain types are so clear that they have been used for windows.

Less light can flow through translucent minerals than through clear ones. Examples of minerals with this quality are nephrite and jadeite, which are mineral versions of jade.

Opaque minerals are those in which light cannot penetrate through. The thickness of the sample determines a mineral’s diaphaneity.

Even though a hand sample does not exhibit this characteristic, a mineral may turn translucent when it is thin enough (for example, in a thin section for petrography).

Certain minerals, on the other hand, including pyrite and hematite, are opaque even in thin sections.

Colour and Streak

Although a mineral’s color is its most noticeable characteristic, it is frequently non-diagnostic. Electrons interact with electromagnetic radiation to cause it, with the exception of incandescence, which does not occur in minerals.

The contribution of elements to the color of a mineral is classified into two major types, idiochromatic and allochromatic: A mineral’s composition cannot be completed without idiochromatic components, whose role in determining a mineral’s color is diagnostic.

Azurite (blue) and malachite (green) are two examples of these minerals. On the other hand, allochromatic elements exist as contaminants in trace concentrations in minerals.

The corundum mineral’s ruby and sapphire variations are two examples of this type of material.

Pseudochromatic minerals get their colors from light waves interacting with one another. Bornite and labradorite are two examples.

Different unique optical characteristics, including color play, asterism, chatoyancy, iridescence, tarnish, and pleochroism, can be found in minerals in addition to their basic body color.

Color variation is a component of several of these qualities. Pleochroism explains the change in color as light passes through a mineral in a new direction, whereas play of color, as in opal, causes the sample to reflect varied colors when it is turned.

Iridescence is a range of color variations caused by light scattering off coatings on crystal surfaces, cleavage planes, or layers with subtle chemical gradations.

Opal, on the other hand, has a color play that results from light refracting off of well-organized microscopic silica spheres inside of its physical structure.

The wavy color banding known as “cat’s eye” that is seen as the sample is rotated is called chatoyancy.

A type of chatoyancy called asterism causes the mineral grain to appear to have stars on it. The second characteristic is very prevalent in corundum of gem quality.

A mineral’s powdered form’s color, which may or may not match the mineral’s body color, is referred to as its streak.

The most popular tool for assessing this characteristic is a porcelain streak plate in either black or white color. A mineral’s streak is unaffected by trace elements or surfaces that are deteriorating.

Hematite, a common example of this characteristic, is shown as being black in color. A cherry-red to reddish-brown streak runs through the silver or red hand sample.

Metallic minerals tend to have more noticeable streaks than non-metallic minerals, whose body color is derived from allochromatic components.

The hardness of the mineral limits streak testing; minerals harder than 7 powder the streak plate instead.

A cherry-red to reddish-brown streak runs through the silver or red hand sample. Metallic minerals tend to have more noticeable streaks than non-metallic minerals, whose body color is derived from allochromatic components.

The hardness of the mineral limits streak testing; minerals harder than 7 powder the streak plate instead.

Cleavage, Parting, Fracture, and Tenacity

Minerals are defined by their particular atomic arrangement. There are planes of weakness in this crystalline structure, and the breaking of a mineral along these planes is known as cleavage.

The ease and cleanliness with which the mineral breaks into pieces can be used to characterize the cleavage quality; popular descriptions for cleavage quality include “perfect,” “good,” “distinct,” and “poor.”

When it comes to especially translucent minerals or thin-portion, cleavage appears from the side as a set of parallel lines designating the planar surfaces.

Not all minerals have the ability to cleave; quartz, which is made up of highly linked silica tetrahedra, lacks the crystallographic weakness necessary for cleavage.

Micas, on the other hand, exhibit perfect basal cleavage and are made of very weakly bound sheets of silica tetrahedra.

There are several types of cleavage since it is a function of crystallography. Usually, cleavage happens in one, two, three, four, or six directions.

One characteristic that sets the micas apart is their unidirectional basal cleavage. Prismatic cleavage is a two-directional cleavage found in minerals like pyroxene and amphiboles.

Rhombohedral cleavage occurs when three directions of cleavage are present but not at 90°, as in calcite or rhodochrosite.

Minerals like galena or halite have cubic (or isometric) cleavage in three directions, at 90°.

Fluorite and diamond exhibit four-directional octahedral cleavage, while sphalerite displays six-directional dodecahedral cleavage.

Minerals possessing numerous cleavages may not break uniformly in every direction; gypsum, for instance, cleaves well in one direction but poorly in the other two. Calcite, on the other hand, cleaves well in three ways.

Minerals differ in the angles between their cleavage planes. For instance, the angle between the cleavage planes of the pyroxenes and amphiboles differs because the latter are single-chain silicates while the former are double-chain silicates.

Pyroxenes split in two directions at around 90°, while amphiboles cleave in two directions that are clearly separated by about 60° and 120°.

Similar to a protractor, a contact goniometer can be used to measure the cleavage angles.Similar in appearance to cleavage, parting—also referred to as “false cleavage”—is caused by structural flaws in the mineral rather than by a pattern of weakness.

A mineral’s parting varies from crystal to crystal, but if the atomic structure permits it, all of a given mineral’s crystals will split.

Generally speaking, a crystal experiences some stress as its parts. The stresses might originate from twinning, exsolution, or deformation (such as a rise in pressure).

Minerals such as corundum, hematite, magnetite, and pyroxenes frequently exhibit separation. A mineral is said to be fractured when it breaks in a direction that does not match a plane of cleavage.

Uneven fractures come in several forms. A well-known instance of conchoidal fracture is found in quartz, where rounded surfaces are produced and identified by gently curved lines.

This kind of fracture is limited to minerals that are extremely homogenous. Hackly, splintery, and fibrous fractures are other forms.

The latter depicts a fracture over a ragged, uneven surface; natural copper exhibits this characteristic.

Tenacity is associated with fracture as well as cleavage. Tenacity characterizes a material’s resistance to breaking, whereas fracture and cleavage describe the surfaces formed when a mineral is broken.

Minerals can be classified as elastic, flexible, sectile, brittle, ductile, or malleable.

Specific Gravity

A mineral’s density can be quantitatively described by its specific gravity. Density dimensions are expressed as mass divided by volume in either g/cm³ or kg/m³ units.

Specific gravity is a dimensionless number that is the same in all unit systems and is calculated by dividing the density of the mineral by the density of water at 4 °C.

The quotient of the sample’s mass and the difference between the sample’s weight in air and water can be used to measure it.

It is not a diagnostic feature for most minerals. The specific gravity of rock-forming minerals, which are usually silicates or sporadically carbonates, is 2.5–3.5. A mineral’s high specific gravity can be used as a diagnostic.

A change in specific gravity is directly correlated with a change in chemistry and, by extension, mineral class.

Because they contain elements with larger atomic masses, oxides, and sulfides among more common minerals typically have greater specific gravity.

As a general rule, minerals with adamantine or metallic luster typically have greater specific gravities than non-metallic or dull luster.

Due to their high iron and lead contents, respectively, hematite, Fe₂O₃, and galena, PbS, have specific gravities of 5.26 and 7.2–7.6, respectively.

Native metals typically have very high specific gravities; gold has been seen to have a specific gravity of between 15 and 19.3, while kamacite, an iron-nickel alloy frequently found in iron meteorites, has a specific gravity of 7.9.

Other Properties

Minerals can be diagnosed using additional attributes. These are more specific to minerals and are less universal.

Carbonates can be distinguished from other mineral types by dropping diluted acid—typically 10% HCl—on top of a mineral.

The afflicted area effervesces and releases carbon dioxide gas as a result of the acid’s reaction with the carbonate ([CO₃]^2-) group. The mineral can also be tested in powdered or original crystal form using this expanded assay.

When calcite and dolomite need to be distinguished, particularly within the rocks, an example of this test is performed.

(Dolomite and limestone, correspondingly). In contrast to powdered dolomite, which typically needs acid poured to a scratched surface in a rock, calcite effervesces in acid instantly.

Zeolite minerals will not fizz off in acid; instead, they will freeze after five to ten minutes, and after a day in the solution, they will dissolve or turn into a silica gel.

A few minerals have the highly noticeable characteristic of magnetism. Among common minerals, magnetite is the most magnetized, followed by pyrrhotite and ilmenite, though not to the same extent.

Due to insufficient data and inherent variation, electrical properties are rarely utilized as diagnostic criteria for minerals.

One example of an exhibitor of electrical properties is quartz, which is piezoelectric. Taste and smell tests can also be performed on minerals. Table salt is halite or NaCl; sylvite, which contains potassium, tastes strongly of bitterness.

The smell of sulfurides is distinctive, especially when the sample is broken up, reacting, or pulverized.

A unique characteristic of minerals containing radioactive elements is their radioactivity.

The radioactive elements may exist as trace impurities, as in zircon, or as a defining constituent, as in the case of uranium in uraninite, autunite, and carnotite.

A radioactive halo, also known as a pleochroic halo, is an optical phenomenon that can be observed using a variety of methods, including thin-section petrography.

The decay of a radioactive element causes damage to the mineral crystal structure, making it locally amorphous (metamict state).

Classification

Earliest Classifications

Theophrastus described his system of classifying minerals in his book On Stones around 315 BCE. Plato and Aristotle, his tutors, had an impact on his categorization.

Minerals were categorized by Theophrastus as stones, piles of earth, or metals. In his work De Natura Fossilium, which was published in 1546, Georgius.

Agricola classified minerals into three categories: simple (consisting of stones, earth, metals, and congealed juices), compound (intimately combined), and composite (separable).

Linnaeus

Carl Linnaeus provided an early taxonomy of minerals in his influential 1735 work Systema Naturae.

He established a hierarchy for each of the three kingdoms that make up the natural world: plants, animals, and minerals.

These were Phylum, Class, Order, Family, Tribe, Genus, and Species, listed in descending order.

Though each distinct mineral is still formally referred to as a mineral species, his system had little success among mineralogists, despite being justified by Charles Darwin’s theory of species.

Formation and widely adopted and expanded by biologists in the succeeding centuries (who still use his Greek- and Latin-based binomial naming scheme).

Modern Classification

In ascending sequences of more generality, minerals are categorized by variety, species, series, and group.

The fundamental concept of a mineral species is that each one is unique in its chemical and physical characteristics, setting it apart from the others.

For instance, the chemical formula SiO₂ and the unique crystalline structure that sets quartz apart from other minerals that share the same formula (referred to as polymorphs) are what define quartz.

A mineral series is defined as having a range of composition between two species of minerals. For instance, different concentrations of the biotite series are used to indicate endmembers: eastonite, phlogopite, siderophyllite, and annite.

A mineral group, on the other hand, is an assemblage of different mineral species that share a crystal structure and some common chemical characteristics.

The pyroxenes are single-chain silicates that crystallize in either the orthorhombic or monoclinic crystal systems.

The pyroxene group shares the formula XY(Si, Al)₂O₆, where X and Y are both cations, with X usually larger than Y.

Lastly, a mineral variety is a particular kind of mineral species that varies in terms of a physical attribute like color or crystal habit.

Amethyst, a purple type of quartz, is one example. For minerals, there are two widely used classifications: Dana and Strunz. Both of these depend on composition, particularly on significant chemical groups, and structure.

The ninth edition of James Dwight Dana’s System of Mineralogy, which was initially published in 1837 by the eminent geologist of the day, was still in print in 1997.

A mineral species is given a four-part number under the Dana classification.

Important compositional groupings serve as the basis for its class number; the type provides the final two numbers to classify minerals according to structural similarities within a specific type or class, and the ratio of cations to anions in the mineral.

The Dana system serves as the foundation for the less widely used Strunz classification, which is named for German mineralogist Karl Hugo Strunz.

However, it incorporates additional structural and chemical criteria, the latter of which relates to the distribution of chemical bonds.

Since silicon and oxygen make up the majority of the Earth’s crust, silicates are by far the most significant class of minerals in terms of diversity and rock formation.

Non-silicate minerals, however, are extremely valuable economically, particularly when used as ores.

The main chemistry of non-silicate minerals, which includes native elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, borates, sulfates, phosphates, and organic compounds, divides them into numerous other divisions.

The majority of non-silicate mineral species are uncommon, making up only 8% of the crust of the Earth overall, although Certain minerals, like hematite, magnetite, pyrite, and calcite, are rather common. In non-silicates, two main structural patterns are seen: silicate-like connected tetrahedra and close-packing.

Atoms can be packed closely together in formations that minimize interstitial space. In hexagonal close-packing, layers are stacked one on top of the other (“ababab”), while in cubic close-packing, groups of three layers are stacked (“abcabcabc”).

The structures of SO^4-₄ (sulfate), PO^4-₄ (phosphate), AsO^4-₄ (arsenate), and VO^4-₄ (vanadate) are analogs of connected silica tetrahedra.

Since non-silicate minerals concentrate elements more than silicate minerals do, they are of significant economic importance.

Silicate minerals make up the majority of minerals; over 95% of rocks are made up of silicate minerals, which also make up over 90% of the Earth’s crust.

Silicates are the largest class of minerals by far. The two most prevalent elements in the Earth’s crust, silicon and oxygen, are the two major components of silicates.

Other typical elements found in silicate rocks are aluminum, magnesium, iron, calcium, sodium, and potassium, which are also found in the Earth’s crust.

The silicates that create rocks, such as feldspars, quartz, olivines, pyroxenes, amphiboles, garnets, and micas, are significant.

Silicates

The [SiO₄]^4- tetrahedron is the fundamental unit of a silicate mineral. Silicon and oxygen are almost always in a tetrahedral or four-fold coordination.

Silicon will be in six-fold or octahedral coordination at extremely high-pressure conditions, as in the case of the quartz polymorph stishovite (SiO₂) or the perovskite structure.

In the latter instance, the mineral’s structure is that of rutile (TiO₂) and its related group, which are simple oxides, rather than that of silicate.

After that, a certain amount of polymerization is applied to these silica tetrahedra to produce different shapes, such as one-two-dimensional sheets, three-dimensional frameworks, and dimensional chains.

In a basic silicate mineral, the base 4-charge needs to be balanced by additional elements in order to prevent the polymerization of the tetrahedra.

Different elemental combinations are needed in other silicate formations in order to counteract the resulting negative charge.

Due to similarities in ionic radius and charge, Al³⁺ frequently replaces Si⁴⁺; in these situations, the [AlO₄]^5- tetrahedra form the same structures as the unsubstituted tetrahedra, but they require different charge balancing requirements.

The structure that forms and the number of shared tetrahedral corners, or coordinating oxygens, (for silicon and aluminum at tetrahedral sites) can both be used to characterize the degree of polymerization.

Orthosilicates (or nesosilicates)
possess no polyhedra connecting; so, no corners are shared by tetrahedra.
Disilicates (or sorosilicates)
Have one oxygen atom shared by two tetrahedra.
Inosilicates are chain silicates
Whereas double-chain silicates have two or three common corners, single-chain silicates only have two.
Phyllosilicates
Possess a sheet structure that necessitates three shared oxygens; in double-chain silicates, some tetrahedra are required to share two corners rather than three, as this would lead to a sheet structure.
Framework silicates (or tectosilicates)
Possess four corners shared by your tetrahedra.
Ring silicates (or cyclosilicates)
Tetrahedra only needs to share two corners in order for the cyclical structure to form.
The following describes the silicate subclasses in decreasing order of polymerization.

Tectosilicates

The highest degree of polymerization is found in tectosilicates, often referred to as framework silicates.

A tetrahedron with all of its corners shared has a silicon-to-oxygen ratio of 1:2. Quartz, feldspars, feldspathoids, and zeolites are a few examples.

Strong covalent connections give framework silicates their tendency to be especially chemically stable. The most common mineral species is quartz (SiO₂), which makes up about 12% of the Earth’s crust.

Its strong physical and chemical resistivity is what makes it unique. High-temperature tridymite and cristobalite, high-pressure coesite, and ultra-high-pressure stishovite are only a few of the polymorphs of quartz.

The latter mineral’s structure has been compacted to such an extent that it has only been created on Earth by meteorite strikes.

Transformed from a silicate to a rutile (TiO₂) structure. At the surface of the Earth, a-quartz is the silica polymorph that is most stable.

The opposite substance, ß-quartz, is only found under extreme pressure and temperature conditions (becomes a-quartz below 573 °C at 1 bar)

These two polymorphs are also known as high quartz (ß) and low quartz (a) because of the “kinking” of bonds that separate them; as a result of this structural alteration, ß-quartz has more symmetry than a-quartz.

At almost 50%, feldspars are the most prevalent group in the crust of the Earth. Al³⁺ replaces Si⁴⁺ in the feldspars, resulting in a charge imbalance that needs to be balanced by the addition of cations.

One of two base structures results: [AlSi₃O₈]- or [Al₂Si₂O₈].^2-The two main subgroups of feldspars, alkali, and plagioclase, plus the two less common groups, celsian and banalsite, comprise the 22 different mineral species that make up this group.

The alkali feldspars consist of often a sequence that runs from sodium-rich albite to potassium-rich orthoclase; for plagioclase, the most typical series runs from albite to calcium-rich anorthite.

Feldspars frequently exhibit crystal twinning, particularly in the case of polysynthetic twins in plagioclase and Carlsbad twins in alkali feldspars.

Because orthoclase and albite are unstable in solid solution, the latter subgroup creates exsolution lamellae when it slowly cools from a melt.

In a hand sample, exsolution can range from tiny to easily noticeable; when Na-rich feldspar dissolves in a K-rich host, the perthitic texture is formed.

Rarely occurs in the opposite texture, known as antiperthitic, in which K-rich feldspar dissolves in a Na-rich host.

Although they originate in Si-deficient environments, feldspathoids vary from feldspar structurally because they permit further substitution by Al³⁺.

Feldspathoids are therefore practically never seen in conjunction with quartz. Nepheline ((Na, K)AlSiO₄) is a typical example of a feldspathoid; in contrast to alkali feldspar, nepheline has an Al₂O₃:SiO₂ ratio of 1:2 rather than 1:6. Zeolites frequently appear in needles, plates, or blocky masses and have characteristic crystal behaviors.

They have holes and channels throughout their structure, and they form in the presence of water at low pressures and temperatures. Zeolites are used in many industrial processes, particularly the treatment of wastewater.

Phyllosilicates

Tetrahedra sheets that have been polymerized make up phyllosilicates. Their unique silicon: oxygen ratio of 2:5 results from their binding at three oxygen sites.

The kaolinite-serpentine, mica, and chlorite groups are significant examples. Phyllosilicates have a sheet of octahedra (elements in six-fold coordination by oxygen) in addition to tetrahedra.

These elements balance out the basic tetrahedra, which have a negative charge (e.g., [Si₄O₁₀]^4-). These sheets of octahedra (O) and tetrahedra (T) are layered in different combinations to produce layers of phyllosilicates.

There are three octahedral sites in a unit structure within an octahedral sheet; however, not all of the sites may be occupied.

The mineral is referred to as dioctahedral in one situation, and trioctahedral in another.

Van der Waals forces, hydrogen bonds, or sparse ionic interactions, weaken the bonds between the layers, resulting in a crystallographic weakness that in turn creates a noticeable basal cleavage among the phyllosilicates.

The 1:1 clay minerals that make up the kaolinite-serpentine group are called T-O stacks, and because hydrogen bonds hold the sheets together, their hardness varies from 2 to 4.

Van der Waals forces, rather than T-O-T stacks, hold the 2:1 clay minerals (pyrophyllite-talc) together, making them softer (hardness ranging from 1 to 2).

The octahedral occupation of these two sets of minerals is what divides them; talc and serpentine are trioctahedral, while kaolinite and pyrophyllite are dioctahedral.

Micas are T-O-T-stacked phyllosilicates as well, but they are distinct from other members of the T-O-T and T-O-stacked subclasses in that they include aluminum in the tetrahedral sheets (Al³⁺ is found at octahedral sites in clay minerals).

Micas are commonly found in the biotite series and muscovite. Although they still have excellent basal cleavage, mica T-O-T layers are harder than other phyllosilicate minerals because metal ions hold them together.

Although there is a brucite-like (Mg(OH)₂) layer in between the T-O-T stacks, the chlorite group is connected to the mica group.

Phyllosilicates may be broken into extremely thin flakes and have translucent, elastic, and flexible layers that are electrical insulators due to their chemical makeup.

Micas can be employed in building, electronics as insulators, optical filling, and even cosmetics. Because it poses less of a health risk than amphibole asbestos, chrysotile, a form of serpentine, is the most prevalent mineral species in industrial asbestos.

Inosilicates

Tetrahedra are linked in chains repeatedly to form isosilicates. These chains can be single, in which case a tetrahedron is joined to two additional tetrahedrons to form a continuous chain or double-chain silicates can be produced by merging two chains.

The silicon-to-oxygen ratio of single-chain silicates is 1:3 (for example, [Si₂O₆]^4-), while double-chain silicates have a ratio of 4:11 (for example, [Si₈O₂₂]^12-).

Two significant mineral families that form rocks are found in isosilicates: single-chain silicates.

which are typically pyroxenes, and double-chain silicates, which are frequently amphiboles. Although they are uncommon, higher-order chains do exist (e.g., three-, four-, and five-member chains).

The group Pyroxene comprises twenty-one mineral species. Pyroxenes have the general structure formula XY(Si₂O₆), where Y can have a coordination number ranging from six to eight and X is an octahedral site.

Ca²⁺, Fe²⁺, and Mg²⁺ permutations make up the majority of pyroxene variations, which are used to balance the negative charge on the backbone.

About 10% of the Earth’s crust is made up mostly of pyropes, which are essential to the composition of mafic igneous rocks.

The chemistry of amphiboles varies greatly; they have been compared to a “mineralogical garbage can” or a “mineralogical shark swimming a sea of elements”.

The [Si₈O₁₂]- is the core of the amphiboles; cations can balance it in three different ways, though the third isn’t always employed and one element can fill both of the other two.

Lastly, amphiboles typically have a hydroxyl group, making them hydrated. group ([OH]^–), albeit an oxide ion, fluoride, or chloride may be used in its place.

There are more than 80 species of amphibole due to its varied chemistry; however, unlike pyroxenes, changes mostly include combinations of Ca²⁺, Fe²⁺, and Mg²⁺.

Asbestiform crystal habits can be seen in a number of amphibole mineral types. These minerals that create asbestos have various uses, particularly in building materials.

They form long, thin, flexible, and strong fibers that are electrical insulators, chemically inert, and heat-resistant.

Chrysotile serpentine asbestos is thought to be less hazardous than amphibole asbestos, which includes anthophyllite, tremolite, actinolite, grunerite, and riebeckite. Nevertheless, asbestos is a proven carcinogen and can induce a number of other ailments, including asbestosis.

Cyclosilicates

The silicon-to-oxygen ratio in cyclosilicates, also known as ring silicates, is 1:3. With a base structure of [Si₆O₁₈]^12-, six-member rings are the most prevalent; examples of these are the beryl and tourmaline groups.

There are more ring structures; some have been described: 3, 4, 8, 9, and 12. Strong crystals with elongated striated shapes are characteristic of cyclosilicates.

Tourmalines possess an intricate chemistry, which can be broadly represented by the formula XY₃Z₆(BO₃)₃T₆O₁₈V₃W. The fundamental ring structure is T₆O₁₈, where T is typically Si⁴⁺ but can also be Al³⁺ or B³⁺.

The occupancy of the X site allows for the subgrouping of tourmalines, which can then be further differentiated based on the chemistry of the W site.

The tourmaline group exhibits significant color fluctuation due to the structural transition metal content variability at the Y and Z sites, which can tolerate a wide range of cations, particularly different transition metals.

Other cyclosilicates are beryl (Al₂Be₃Si₆O₁₈), which is used to make the green and blue gemstones aquamarine and emerald. Cordierite is a typical metamorphic mineral that shares structural similarities with beryl.

Sorosilicates

The silicon-to-oxygen ratio in sorosilicates, also known as disilicates, is 2:7 due to tetrahedron-tetrahedron bonding at one oxygen. The ensuing shared structural component is the group [Si₂O₇]^6-.

Members of the epidote group are by far the most prevalent disilicates. Epidotes can be found in a wide range of geological environments, including granites, metapelites, and mid-ocean ridges. Epidotes are constructed around the [(SiO₄)(Si₂O₇)]^10- structure; as an illustration.

The mineral species epidote contains ferric iron, calcium, and aluminum in order to maintain a balanced charge: Ca₂Al₂(Fe³⁺, Al)(SiO₄)(Si₂O₇)O(OH).

An important aspect of petrogenesis is oxygen fugacity, which is mitigated by the presence of iron in the forms of Fe³⁺ and Fe²⁺.

Other sorosilicate examples are vesuvianite, which absorbs a large amount of calcium into its chemical structure, and lawsonite, a metamorphic mineral that forms in the blueschist facies (a subduction zone environment with low temperature and high pressure).

Orthosilicates

Tetrahedra that are separated and have their charges balanced by other cations make up orthosilicates.

This kind of silicate, often known as nesosilicates (e.g. SiO₄), has a silicon-to-oxygen ratio of 1:4. Common orthosilicates are fairly hard and have a tendency to produce blocky equant crystals.

This subclass includes a number of rock-forming minerals, including the garnet, olivine, and aluminosilicate groups.

The structural components of the aluminosilicates, kyanite, andalusite, and sillimanite (all Al₂SiO₅), are one [SiO₄]^4- tetrahedron and one Al³⁺ in octahedral coordination.

The remaining Al³⁺ may be in five-fold coordination (andalusite), four-fold coordination (sillimanite), or six-fold coordination (kyanite); the mineral that forms in a particular environment depends on the temperature and pressure levels.

The two primary olivine series of (Mg, Fe)₂SiO₄ in the olivine structure are iron-rich fayalite and magnesium-rich forsterite.

Due to oxygen, iron and magnesium are both octahedral. Other There are mineral species with this structure, such as tephroite (Mn₂SiO₄).

The general formula for the garnet group is X₃Y₂(SiO₄)₃, where X and Y are the big and tiny eight-fold and six-fold coordinated cations, respectively.

There are two groups of six suitable garnet end members. Pyrope (Mg₃Al₂(SiO₄)₃), Almandine (Fe₃Al₂(SiO₄)₃), and Spessartine (Mn₃Al₂(SiO₄)₃) are the pyralspite garnets with Al³⁺ in the Y position.

Uvarovite (Ca₃Cr₂(SiO₄)₃), grossular (Ca₃Al₂(SiO₄)₃), and andradite (Ca₃Fe₂(SiO₄)₃) are the ugrandite garnets with Ca²⁺ in the X position.

Although garnet has two subgroups, there are solid solutions between each of the six end-members.

Topaz, staurolite, and zircon are some further orthosilicates. Zircon (ZrSiO₄) is helpful in geochronology because U⁶⁺ can replace Zr⁴⁺.

In addition, its very robust structure makes it challenging to reset as a chronometer. One frequent metamorphic intermediate-grade indicator mineral is staurolite.

Its extremely intricate crystal structure was only fully characterized in 1986. A common gemstone mineral is topaz (Al₂SiO₄(F, OH)₂), which is frequently found in granitic pegmatites with tourmaline.

Non-Silicates

Elements that are not chemically bound to other elements are called native elements. This class of minerals consists of several alloys, solid solutions, and native, semi-metal, and non-metal minerals.

Metallic bonding, which gives the metals their unique physical characteristics including their lustrous metallic sheen, ductility and malleability, and electrical conductivity, holds the metals together.

Groups of native elements are separated based on their chemical properties or structural characteristics.

Metals like gold, silver, and copper are members of the group known as gold, which has a cubic close-packed structure. The gold group and the platinum group share a similar structure. Several kinds of iron-nickel alloys define the iron-nickel group.

Two types of iron meteorite are kamacite and taenite, which vary in the quantity of nickel present in the alloy. Kamacite is a type of native iron that contains less than 5-7 percent nickel.

At the same time, taenite’s nickel concentration varies from 7 to 37%. Semi-metals, which are composed of minerals in the arsenic group, are only partially metallic; for example, they do not possess metals’ malleability.

There are two kinds of native carbon: graphite and diamond. The latter originates under the mantle at extremely high pressure, giving it a far stronger structure than graphite.

Sulfides

Sulfur is the most prevalent chalcogen or pnictogen found in sulfide minerals, which are chemical mixtures of one or more metals or semimetals.

Arsenic, tellurium, or selenium can be used in place of sulfur. Sulfides often have a high specific gravity and are soft, brittle minerals. When powdered, many sulfides, including pyrite, smell sulfurous.

Because sulfides are weatherable and many of them dissolve easily in water, they can form enriched secondary ore deposits when the dissolved minerals are redeposited.

Sulfides are categorized according to the metal-to-sulfur ratio (M: S)—for example, 2:1 or 1:1—between the metal and sulfur.

Numerous Sulfide minerals, or metal ores, are valuable economically. Examples of these include zinc ore sphalerite (ZnS), lead ore galena (PbS), mercury ore cinnabar (HgS), and molybdenum ore molybdenite (MoS₂).

The most prevalent sulfide is pyrite (FeS₂), which is present in most geological settings. It is not an iron ore, though; sulfuric acid can be produced by oxidizing it instead.

The rare sulfosalts, which are related to sulfides, are composed of sulfur and a semimetal like bismuth, arsenic, or antimony bound to a metallic element.

Sulfosalts are usually soft, heavy, and brittle minerals, just like sulfides.

Oxides

Simple oxides, hydroxides, and numerous oxides are the three types of oxide minerals. O^2- is the predominant anion and ionic bonding is the main characteristic of simple oxides.

The proportion of oxygen to cations might further separate them. The minerals in the periclase group are arranged in a 1:1 ratio.

Cuprite (Cu₂O) and water ice are examples of oxides having a 2:1 ratio. Minerals in the corundum group, which includes hematite (Fe₂O₃) and corundum (Al₂O₃), have a 2:3 ratio.

The ratio of rutile group minerals is 1:2; the species that bears this name, The primary ore of titanium is rutile (TiO₂); additional ore types include pyrolusite (MnO₂; manganese ore) and cassiterite (SnO₂; tin ore).

The hydroxyl ion, or OH^–, is the main anion in hydroxides. A heterogeneous mixture of the hydroxide minerals diaspore, gibbsite, and boehmite.

Bauxites are the principal resource of aluminum and originate in regions with a very high rate of chemical weathering, usually under tropical circumstances. Lastly, molecules containing two metals and oxygen are known as multiple oxides.

The spinels, a significant group in this class, have the general formula X²⁺Y³⁺₂O₄. Magnesite (Fe₃O₄), chromite (FeCr₂O₄), and spinel (MgAl₂O₄) are a few examples of species.

Due to its two oxidation states of iron (Fe²⁺Fe³⁺₂O₄), which distinguishes it as a multiple oxide rather than a single oxide, the latter is easily distinguished by its significant magnetic.

Halides

Compounds with a halogen (fluorine, chlorine, iodine, or bromine) as the primary anion are known as halide minerals.

These minerals are typically brittle, mushy, weak, and soluble in water. Sylvite (KCl), fluorite (CaF₂), and halite (NaCl, table salt) are common examples of halides. In chemical sedimentary rocks, sylvite and halite can be the major minerals.

They often originate as evaporites. The mineral cryolite, Na₃AlF₆, is essential for extracting aluminum from bauxites.

Fluorite can be used to create synthetic cryolite since the only known major occurrence of cryolite was depleted in a granitic pegmatite in Ivittuut, Greenland.

Carbonates

The primary anionic group in carbonate minerals, [CO₃]^2-, is carbonate. All carbonates react with acid, many of them have rhombohedral cleavage, and they are generally brittle.

Field geologists frequently carry diluted hydrochloric acid to differentiate carbonates from non-carbonates because of the last attribute.

The dissolution and precipitation of the mineral, which is essential in the creation of limestone caves, is related to the reaction of acid with carbonates, which are most frequently seen as the polymorph calcite and aragonite (CaCO₃).

They include karst landforms like stalactites and stalagmites. In marine environments, carbonates are most frequently generated as chemical or biogenic deposits.

The carbonate group is structurally arranged in a triangle with three O^2- anions surrounding a core C⁴⁺ cation.

Different mineral groups are formed by arranging these triangles in different ways. Calcite is the most prevalent carbonate mineral and the main ingredient of metamorphic marble and sedimentary limestone.

A sizable portion of the magnesium in calcite (CaCO₃) can replace the calcium.

Its polymorph aragonite will instead form under high-Mg conditions; depending on whether the mineral forms preferentially, the marine geochemistry in this regard can be defined as either an aragonite sea or a calcite sea.

As a double carbonate, dolomite has CaMg(CO₃)₂ as the formula for the secondary dolomitization of limestone, which is the conversion of calcite or aragonite to dolomite.

This reaction increases pore space since dolomite has a unit cell volume that is 88% that of calcite, which can result in the formation of an oil and gas reservoir.

These two mineral species belong to groups of minerals with namesake: the dolomite group is made up of minerals with the general formula XY(CO₃)₂, while the calcite group is made up of carbonates with the general formula XCO₃.

Sulfates

The sulfate anion, [SO₄]^2-, is present in all sulfate minerals. They are often delicate, translucent, transparent, and fragile.

When saline waters evaporate, sulfate minerals typically precipitate out as evaporites. Sulfates are also present as byproducts of sulfide oxidation or in hydrothermal vein systems linked to sulfides.

There are two types of sulfates: hydrous and anhydrous minerals. By far the most prevalent hydrous sulfate is gypsum or CaSO₄·2H₂O. It begins as an evaporite; it is related to other evaporites like halite and calcite.

Gypsum can create desert roses if it crystallizes and includes sand grains. Gypsum is employed as an insulator in materials like plaster and drywall because of its extremely poor thermal conductivity, which allows it to retain a low temperature even when heated.

Gypsum loses heat by drying out. Anhydrite is the anhydrous counterpart of gypsum and is formed straight from seawater in extremely dry conditions.

The general formula for the barite group is XSO₄, in which X is a big 12-coordinated cation. A few examples include anglesite (PbSO₄), celestine (SrSO₄), and barite (BaSO₄). Anhydrite is not included in the barite group since its lower Ca²⁺ only coordinates eight times.

Phosphates

Phosphorus can be substituted with antimony, arsenic, or vanadium, and the tetrahedral [PO₄]^3- unit characterizes the phosphate minerals, albeit the structure can be generalized.

The apatite group is the most prevalent phosphate; common species within this group include hydroxylapatite (Ca₅(PO₄)₃(OH)), fluorapatite (Ca₅(PO₄)₃F), and chlorapatite (Ca₅(PO₄)₃Cl). The primary crystalline components of teeth and bones are minerals in this group.

Animals with backbones. The main structure of the relatively common monazite group is ATO₄, where T stands for phosphorus or arsenic and A is frequently a rare-earth element (REE).

In addition to being a REE “sink” that can concentrate these elements to the point where an ore is formed, monazite group elements can contain comparatively large amounts of uranium and thorium, which can be used in monazite geochronology—a method of dating rocks based on the decay of U and Th to lead—making it an important mineral.

Organic Minerals

There is a class for organic minerals in the Strunz classification. These uncommon compounds can arise from geologic processes, but they do include organic carbon.

An oxalate that can be deposited in hydrothermal ore veins is whewellite, CaC₂O₄·H₂O.

Although hydrated calcium oxalate is present in organic-rich sedimentary deposits such as coal seams, its hydrothermal presence is not thought to be connected to biological activity.

Recent Advances

The definitions and categorization systems for minerals constantly changing to reflect the most recent developments in the field.

One of the most recent additions to the Strunz and Dana classification methods is the organic class.

One extremely uncommon class of minerals with hydrocarbons is the organic class.

In 2009, the IMA Commission on New Minerals and Mineral Names established a tiered system for the designation and mineral groupings, group names were classified, and four working groups and seven commissions.

Were formed to examine and categorize minerals into an official listing of their published names.

These new rules allow mineral species to be categorized in several ways, depending on the classification’s intended use. These criteria include chemistry, crystal structure, occurrence, association, genetic history, and resource.

Astrobiology

It has been proposed that biominerals may serve as significant markers of alien life and, as such, may be crucial to the hunt for extant or past life on Mars.

Moreover, it is thought that organic elements, or “biosignatures,” which are frequently connected to biominerals, are essential to both pre-biotic and biotic processes.

NASA said in January 2014 that the Curiosity and Opportunity rovers on Mars would be looking for signs of ancient water, such as fluvial-lacustrine environments (plains connected to ancient rivers or lakes) that may have supported life in the past.

These investigations would also be looking for evidence of ancient life, such as a biosphere based on autotrophic, chemotrophic, and/or chemolithoautotrophic microorganisms.

NASA’s main goal is to find evidence of organic carbon, taphonomy (the study of fossils), and habitability on the planet Mars.

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