Mineral

Minerals are solid substances found in nature that have a specific chemical makeup and crystal structure. They are important because they make up the Earth’s rocks and are found in many everyday objects. For example, the mineral quartz is used in watches and electronics.

In geology, minerals are different from rocks. Rocks can be made of one or many minerals mixed together. Some natural solids, like opal, don’t have a crystal structure and are called mineraloids instead of true minerals.

There are many types of minerals, and they are grouped by their chemical parts. Silicate minerals make up about 90% of the Earth’s crust. Other groups include oxides, carbonates, sulfides, and halides, each with their own special properties and uses. As of May 2025, scientists recognize 6,145 different kinds of minerals.

Definitions

The International Mineralogical Association has rules for what counts as a mineral. A mineral must happen naturally, like through processes that shape rocks and Earth. It must be solid, though there are a few exceptions. It also needs a special pattern in its tiny parts, called a crystal structure, and have a clear chemical makeup.

Some minerals come from living things, like certain types found in plants or animals. Scientists sometimes debate whether these should count as minerals. Minerals can form with help from tiny living things, too, and these processes have been part of making minerals for a very long time.

Rocks, ores, and gems

A rock is made up of one or more minerals or similar materials. Some rocks, like limestone or quartzite, are mostly made of one mineral — calcite or aragonite for limestone, and quartz for quartzite. Other rocks are defined by the amounts of certain key minerals; for example, a granite has specific amounts of quartz, alkali feldspar, and plagioclase feldspar. Rocks can also be made completely of non-mineral materials; coal is a rock made mostly of carbon from plants.

Some minerals are found much more often in rocks than others; these are called rock-forming minerals. Important examples include quartz, the feldspars, the micas, the amphiboles, the pyroxenes, the olivines, and calcite. Most of these minerals are silicates. Overall, about 150 minerals are considered very important, either because they are common or because people like to collect them.

Minerals and rocks that are valuable for business, besides gemstones or fuels, are called industrial minerals. For example, muscovite, a type of white mica, can be used for windows, as a filler, or as an insulator.

Ores are minerals that have a lot of a certain element, usually a metal. Examples include cinnabar (HgS) for mercury, sphalerite (ZnS) for zinc, cassiterite (SnO₂) for tin, and colemanite for boron.

Gems are minerals that are beautiful and rare, making them special decorations. About 20 minerals can be used as gems, making up around 35 of the most common gemstones. Gem minerals can come in different types, so one mineral can make several different gemstones; for example, both ruby and sapphire come from corundum, Al₂O₃.

Etymology

The word "mineral" was first used in English during the 15th century. It came from a Medieval Latin word, minerale, which is related to minera meaning mine or ore.

The word "species" comes from the Latin word species, which means a particular kind or type with a distinct look or appearance.

Chemistry

The types and amounts of minerals we find depend on their chemical makeup, which is tied to the elements found in the Earth. Most minerals come from the Earth's crust. Eight elements make up almost all of the crust's weight. These are, from most to least common: oxygen, silicon, aluminium, iron, magnesium, calcium, sodium, and potassium. Oxygen and silicon are the most important, making up 47% and 28% of the crust respectively.

Minerals form based on what conditions allow them to be most stable. For example, in most kinds of rocks, aluminium and metals like sodium and potassium mainly combine with oxygen, silicon, and calcium to make feldspar minerals. But if there is extra sodium, it can form other minerals like riebeckite. If there is extra aluminium, it can form muscovite instead. These changes depend on the temperature and pressure where the minerals form.

Physical properties

Classifying minerals can be simple or difficult. We identify minerals using physical properties, which help us tell them apart. Sometimes these properties are enough to identify a mineral right away. Other times, we need more advanced tools like optical, chemical, or X-ray diffraction analysis. These tools can take time and cost money.

We classify minerals using properties like crystal structure, hardness, lustre, how light passes through them (diaphaneity), color, streak, how they break (cleavage and fracture), and specific gravity. We also test for other properties like fluorescence, phosphorescence, magnetism, radioactivity, and how they react to mechanical changes or acids.

Classification

Earliest classifications

In 315 BCE, Theophrastus presented his classification of minerals in his treatise On Stones. His classification was influenced by the ideas of his teachers Plato and Aristotle. Theophrastus classified minerals as stones, earths or metals.

Georgius Agricola's classification of minerals in his book De Natura Fossilium, published in 1546, divided minerals into three types of substance: simple (stones, earths, metals, and congealed juices), compound (intimately mixed) and composite (separable).

Linnaeus

An early classification of minerals was given by Carl Linnaeus in his seminal 1735 book Systema Naturae. He divided the natural world into three kingdoms – plants, animals, and minerals – and classified each with the same hierarchy. In descending order, these were Phylum, Class, Order, Family, Tribe, Genus, and Species. However, while his system was justified by Charles Darwin's theory of species formation and has been largely adopted and expanded by biologists in the following centuries, it had little success among mineralogists.

Modern classification

Minerals are classified by variety, species, series and group, in order of increasing generality. The basic level of definition is that of mineral species, each of which is distinguished from the others by unique chemical and physical properties. When there exists a range of composition between two minerals species, a mineral series is defined. In contrast, a mineral group is a grouping of mineral species with some common chemical properties that share a crystal structure.

Two common classifications, Dana and Strunz, are used for minerals; both rely on composition, specifically with regard to important chemical groups, and structure. James Dwight Dana, a leading geologist of his time, first published his System of Mineralogy in 1837; as of 1997, it is in its eighth edition. The Dana classification assigns a four-part number to a mineral species. Its class number is based on important compositional groups; the type gives the ratio of cations to anions in the mineral, and the last two numbers group minerals by structural similarity within a given type or class. The less commonly used Strunz classification, named for German mineralogist Karl Hugo Strunz, is based on the Dana system, but combines both chemical and structural criteria.

As the composition of the Earth's crust is dominated by silicon and oxygen, silicates are by far the most important class of minerals in terms of rock formation and diversity. However, non-silicate minerals are of great economic importance, especially as ores. Non-silicate minerals are subdivided into several other classes by their dominant chemistry, which includes native elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, borates, sulfates, phosphates, and organic compounds.

The largest grouping of minerals by far are the silicates; most rocks are composed of greater than 95% silicate minerals, and over 90% of the Earth's crust is composed of these minerals. The two main constituents of silicates are silicon and oxygen, which are the two most abundant elements in the Earth's crust. Other common elements in silicate minerals correspond to other common elements in the Earth's crust, such as aluminium, magnesium, iron, calcium, sodium, and potassium.

Silicates

The base unit of a silicate mineral is the [SiO₄]⁴⁻ tetrahedron. In the vast majority of cases, silicon is in four-fold or tetrahedral coordination with oxygen. In very high-pressure situations, silicon will be in six-fold or octahedral coordination, such as in the perovskite structure or the quartz polymorph stishovite (SiO₂). These silica tetrahedra are then polymerized to some degree to create various structures, such as one-dimensional chains, two-dimensional sheets, and three-dimensional frameworks.

The silicate subclasses are described below in order of decreasing polymerization.

Tectosilicates

Tectosilicates, also known as framework silicates, have the highest degree of polymerization. With all corners of a tetrahedra shared, the silicon:oxygen ratio becomes 1:2. Examples are quartz, the feldspars, feldspathoids, and the zeolites. Framework silicates tend to be particularly chemically stable as a result of strong covalent bonds.

Forming 12% of the Earth's crust, quartz (SiO₂) is the most abundant mineral species. It is characterized by its high chemical and physical resistivity. Quartz has several polymorphs, including tridymite and cristobalite at high temperatures, high-pressure coesite, and ultra-high pressure stishovite. The latter mineral can only be formed on Earth by meteorite impacts, and its structure has been compressed so much that it has changed from a silicate structure to that of rutile (TiO₂). The silica polymorph that is most stable at the Earth's surface is α-quartz. Its counterpart, β-quartz, is present only at high temperatures and pressures (changes to α-quartz below 573 °C at 1 bar). These two polymorphs differ by a "kinking" of bonds; this change in structure gives β-quartz greater symmetry than α-quartz, and they are thus also called high quartz (β) and low quartz (α).

Feldspars are the most abundant group in the Earth's crust, at about 50%. In the feldspars, Al³⁺ substitutes for Si⁴⁺, which creates a charge imbalance that must be accounted for by the addition of cations. The base structure becomes either [AlSi₃O₈]⁻ or [Al₂Si₂O₈]²⁻ There are 22 mineral species of feldspars, subdivided into two major subgroups – alkali and plagioclase – and two less common groups – celsian and banalsite. The alkali feldspars are most commonly in a series between potassium-rich orthoclase and sodium-rich albite; in the case of plagioclase, the most common series ranges from albite to calcium-rich anorthite. Crystal twinning is common in feldspars, especially polysynthetic twins in plagioclase and Carlsbad twins in alkali feldspars.

Feldspathoids are structurally similar to feldspar, but differ in that they form in Si-deficient conditions, which allows for further substitution by Al³⁺. As a result, feldspathoids are almost never found in association with quartz. A common example of a feldspathoid is nepheline ((Na, K)AlSiO₄); compared to alkali feldspar, nepheline has an Al₂O₃:SiO₂ ratio of 1:2, as opposed to 1:6 in alkali feldspar. Zeolites often have distinctive crystal habits, occurring in needles, plates, or blocky masses. They form in the presence of water at low temperatures and pressures, and have channels and voids in their structure. Zeolites have several industrial applications, especially in waste water treatment.

Phyllosilicates

Phyllosilicates consist of sheets of polymerized tetrahedra. They are bound at three oxygen sites, which gives a characteristic silicon:oxygen ratio of 2:5. Important examples include the mica, chlorite, and the kaolinite-serpentine groups. In addition to the tetrahedra, phyllosilicates have a sheet of octahedra (elements in six-fold coordination by oxygen) that balance out the basic tetrahedra, which have a negative charge (e.g. [Si₄O₁₀]⁴⁻) These tetrahedra (T) and octahedra (O) sheets are stacked in a variety of combinations to create phyllosilicate layers.

Because of their chemical structure, phyllosilicates typically have flexible, elastic, transparent layers that are electrical insulators and can be split into very thin flakes. Micas can be used in electronics as insulators, in construction, as optical filler, or even cosmetics.

Inosilicates

Inosilicates consist of tetrahedra repeatedly bonded in chains. These chains can be single, where a tetrahedron is bound to two others to form a continuous chain; alternatively, two chains can be merged to create double-chain silicates. Single-chain silicates have a silicon:oxygen ratio of 1:3 (e.g. [Si₂O₆]⁴⁻), whereas the double-chain variety has a ratio of 4:11, e.g. [Si₈O₂₂]¹²⁻. Inosilicates contain two important rock-forming mineral groups; single-chain silicates are most commonly pyroxenes, while double-chain silicates are often amphiboles.

The pyroxene group consists of 21 mineral species. Pyroxenes have a general structure formula of XY(Si₂O₆), where X is an octahedral site, while Y can vary in coordination number from six to eight. Most varieties of pyroxene consist of permutations of Ca²⁺, Fe²⁺ and Mg²⁺ to balance the negative charge on the backbone. Pyroxenes are common in the Earth's crust (about 10%) and are a key constituent of mafic igneous rocks.

Cyclosilicates

Cyclosilicates, or ring silicates, have a ratio of silicon to oxygen of 1:3. Six-member rings are most common, with a base structure of [Si₆O₁₈]¹²⁻; examples include the tourmaline group and beryl. Other ring structures exist, with 3, 4, 8, 9, 12 having been described. Cyclosilicates tend to be strong, with elongated, striated crystals.

Tourmalines have a very complex chemistry that can be described by a general formula XY₃Z₆(BO₃)₃T₆O₁₈V₃W. The T₆O₁₈ is the basic ring structure, where T is usually Si⁴⁺, but substitutable by Al³⁺ or B³⁺. Tourmalines can be subgrouped by the occupancy of the X site, and from there further subdivided by the chemistry of the W site. The Y and Z sites can accommodate a variety of cations, especially various transition metals; this variability in structural transition metal content gives the tourmaline group greater variability in colour. Other cyclosilicates include beryl, Al₂Be₃Si₆O₁₈, whose varieties include the gemstones emerald (green) and aquamarine (bluish). Cordierite is structurally similar to beryl, and is a common metamorphic mineral.

Sorosilicates

Sorosilicates, also termed disilicates, have tetrahedron-tetrahedron bonding at one oxygen, which results in a 2:7 ratio of silicon to oxygen. The resultant common structural element is the [Si₂O₇]⁶⁻ group. The most common disilicates by far are members of the epidote group. Epidotes are found in variety of geologic settings, ranging from mid-ocean ridge to granites to metapelites. Epidotes are built around the structure [(SiO₄)(Si₂O₇)]¹⁰⁻ structure; for example, the mineral species epidote has calcium, aluminium, and ferric iron to charge balance: Ca₂Al₂(Fe³⁺, Al)(SiO₄)(Si₂O₇)O(OH).

Other examples of sorosilicates include lawsonite, a metamorphic mineral forming in the blueschist facies (subduction zone setting with low temperature and high pressure), vesuvianite, which takes up a significant amount of calcium in its chemical structure.

Orthosilicates

Orthosilicates consist of isolated tetrahedra that are charge-balanced by other cations. Also termed nesosilicates, this type of silicate has a silicon:oxygen ratio of 1:4 (e.g. SiO₄). Typical orthosilicates tend to form blocky equant crystals, and are fairly hard. Several rock-forming minerals are part of this subclass, such as the aluminosilicates, the olivine group, and the garnet group.

The aluminosilicates –kyanite, andalusite, and sillimanite, all Al₂SiO₅ – are structurally composed of one [SiO₄]⁴⁻ tetrahedron, and one Al³⁺ in octahedral coordination. The remaining Al³⁺ can be in six-fold coordination (kyanite), five-fold (andalusite) or four-fold (sillimanite); which mineral forms in a given environment is depend on pressure and temperature conditions. In the olivine structure, the main olivine series of (Mg, Fe)₂SiO₄ consist of magnesium-rich forsterite and iron-rich fayalite. Both iron and magnesium are in octahedral by oxygen. Other mineral species having this structure exist, such as tephroite, Mn₂SiO₄. The garnet group has a general formula of X₃Y₂(SiO₄)₃, where X is a large eight-fold coordinated cation, and Y is a smaller six-fold coordinated cation. There are six ideal endmembers of garnet, split into two group. The pyralspite garnets have Al³⁺ in the Y position: pyrope (Mg₃Al₂(SiO₄)₃), almandine (Fe₃Al₂(SiO₄)₃), and spessartine (Mn₃Al₂(SiO₄)₃). The ugrandite garnets have Ca²⁺ in the X position: uvarovite (Ca₃Cr₂(SiO₄)₃), grossular (Ca₃Al₂(SiO₄)₃) and andradite (Ca₃Fe₂(SiO₄)₃). While there are two subgroups of garnet, solid solutions exist between all six end-members.

Other orthosilicates include zircon, staurolite, and topaz. Zircon (ZrSiO₄) is useful in geochronology as U⁶⁺ can substitute for Zr⁴⁺; furthermore, because of its very resistant structure, it is difficult to reset it as a chronometer. Staurolite is a common metamorphic intermediate-grade index mineral. It has a particularly complicated crystal structure that was only fully described in 1986. Topaz (Al₂SiO₄(F, OH)₂, often found in granitic pegmatites associated with tourmaline, is a common gemstone mineral.

Non-silicates

Native elements

Native elements are those that are not chemically bonded to other elements. This mineral group includes native metals, semi-metals, and non-metals, and various alloys and solid solutions. The metals are held together by metallic bonding, which confers distinctive physical properties such as their shiny metallic lustre, ductility and malleability, and electrical conductivity. Native elements are subdivided into groups by their structure or chemical attributes.

The gold group, with a cubic close-packed structure, includes metals such as gold, silver, and copper. The platinum group is similar in structure to the gold group. The iron-nickel group is characterized by several iron-nickel alloy species. Two examples are kamacite and taenite, which are found in iron meteorites; these species differ by the amount of Ni in the alloy; kamacite has less than 5–7% nickel and is a variety of native iron, whereas the nickel content of taenite ranges from 7–37%. Arsenic group minerals consist of semi-metals, which have only some metallic traits; for example, they lack the malleability of metals. Native carbon occurs in two allotropes, graphite and diamond; the latter forms at very high pressure in the mantle, which gives it a much stronger structure than graphite.

Sulfides

The sulfide minerals are chemical compounds of one or more metals or semimetals with a chalcogen or pnictogen, of which sulfur is most common. Tellurium, arsenic, or selenium can substitute for the sulfur. Sulfides tend to be soft, brittle minerals with a high specific gravity. Many powdered sulfides, such as pyrite, have a sulfurous smell when powdered. Sulfides are susceptible to weathering, and many readily dissolve in water; these dissolved minerals can be later redeposited, which creates enriched secondary ore deposits. Sulfides are classified by the ratio of the metal or semimetal to the sulfur, such as M:S equal to 2:1, or 1:1. Many sulfide minerals are economically important as metal ores; examples include sphalerite (ZnS), an ore of zinc, galena (PbS), an ore of lead, cinnabar (HgS), an ore of mercury, and molybdenite (MoS₂, an ore of molybdenum. Pyrite (FeS₂), is the most commonly occurring sulfide, and can be found in most geological environments. It is not, however, an ore of iron, but can be instead oxidized to produce sulfuric acid. Related to the sulfides are the rare sulfosalts, in which a metallic element is bonded to sulfur and a semimetal such as antimony, arsenic, or bismuth. Like the sulfides, sulfosalts are typically soft, heavy, and brittle minerals.

Oxides

Oxide minerals are divided into three categories: simple oxides, hydroxides, and multiple oxides. Simple oxides are characterized by O²⁻ as the main anion and primarily ionic bonding. They can be further subdivided by the ratio of oxygen to the cations. The periclase group consists of minerals with a 1:1 ratio. Oxides with a 2:1 ratio include cuprite (Cu₂O) and water ice. Corundum group minerals have a 2:3 ratio, and includes minerals such as corundum (Al₂O₃), and hematite (Fe₂O₃). Rutile group minerals have a ratio of 1:2; the eponymous species, rutile (TiO₂) is the chief ore of titanium; other examples include cassiterite (SnO₂; ore of tin), and pyrolusite (MnO₂; ore of manganese). In hydroxides, the dominant anion is the hydroxyl ion, OH⁻. Bauxites are the chief aluminium ore, and are a heterogeneous mixture of the hydroxide minerals diaspore, gibbsite, and bohmite; they form in areas with a very high rate of chemical weathering (mainly tropical conditions). Finally, multiple oxides are compounds of two metals with oxygen. A major group within this class are the spinels, with a general formula of X²⁺Y³⁺₂O₄. Examples of species include spinel (MgAl₂O₄), chromite (FeCr₂O₄), and magnetite (Fe₃O₄). The latter is readily distinguishable by its strong magnetism, which occurs as it has iron in two oxidation states (Fe²⁺Fe³⁺₂O₄), which makes it a multiple oxide instead of a single oxide.

Halides

The halide minerals are compounds in which a halogen (fluorine, chlorine, iodine, or bromine) is the main anion. These minerals tend to be soft, weak, brittle, and water-soluble. Common examples of halides include halite (NaCl, table salt), sylvite (KCl), and fluorite (CaF₂). Halite and sylvite commonly form as evaporites, and can be dominant minerals in chemical sedimentary rocks. Cryolite, Na₃AlF₆, is a key mineral in the extraction of aluminium from bauxites; however, as the only significant occurrence at Ivittuut, Greenland, in a granitic pegmatite, was depleted, synthetic cryolite can be made from fluorite.

Carbonates

The carbonate minerals are those in which the main anionic group is carbonate, [CO₃]²⁻. Carbonates tend to be brittle, many have rhombohedral cleavage, and all react with acid. Due to the last characteristic, field geologists often carry dilute hydrochloric acid to distinguish carbonates from non-carbonates. The reaction of acid with carbonates, most commonly found as the polymorph calcite and aragonite (CaCO₃), relates to the dissolution and precipitation of the mineral, which is a key in the formation of limestone caves, features within them such as stalactite and stalagmites, and karst landforms. Carbonates are most often formed as biogenic or chemical sediments in marine environments. The carbonate group is structurally a triangle, where a central C⁴⁺ cation is surrounded by three O²⁻ anions; different groups of minerals form from different arrangements of these triangles. The most common carbonate mineral is calcite, which is the primary constituent of sedimentary limestone and metamorphic marble. Calcite, CaCO₃, can have a significant percentage of magnesium substituting for calcium. Under high-Mg conditions, its polymorph aragonite will form instead; the marine geochemistry in this regard can be described as an aragonite or calcite sea, depending on which mineral preferentially forms. Dolomite is a double carbonate, with the formula CaMg(CO₃)₂. Secondary dolomitization of limestone is common, in which calcite or aragonite are converted to dolomite; this reaction increases pore space (the unit cell volume of dolomite is 88% that of calcite), which can create a reservoir for oil and gas. These two mineral species are members of eponymous mineral groups: the calcite group includes carbonates with the general formula XCO₃, and the dolomite group constitutes minerals with the general formula XY(CO₃)₂.

Sulfates

The sulfate minerals all contain the sulfate anion, [SO₄]²⁻. They tend to be transparent to translucent, soft, and many are fragile. Sulfate minerals commonly form as evaporites, where they precipitate out of evaporating saline waters. Sulfates can also be found in hydrothermal vein systems associated with sulfides, or as oxidation products of sulfides. Sulfates can be subdivided into anhydrous and hydrous minerals. The most common hydrous sulfate by far is gypsum, CaSO₄⋅2H₂O. It forms as an evaporite, and is associated with other evaporites such as calcite and halite; if it incorporates sand grains as it crystallizes, gypsum can form desert roses. Gypsum has very low thermal conductivity and maintains a low temperature when heated as it loses that heat by dehydrating; as such, gypsum is used as an insulator in materials such as plaster and drywall. The anhydrous equivalent of gypsum is anhydrite; it can form directly from seawater in highly arid conditions. The barite group has the general formula XSO₄, where the X is a large 12-coordinated cation. Examples include barite (BaSO₄), celestine (SrSO₄), and anglesite (PbSO₄); anhydrite is not part of the barite group, as the smaller Ca²⁺ is only in eight-fold coordination.

Phosphates

The phosphate minerals are characterized by the tetrahedral [PO₄]³⁻ unit, although the structure can be generalized, and phosphorus is replaced by antimony, arsenic, or vanadium. The most common phosphate is the apatite group; common species within this group are fluorapatite (Ca₅(PO₄)₃F), chlorapatite (Ca₅(PO₄)₃Cl) and hydroxylapatite (Ca₅(PO₄)₃(OH)). Minerals in this group are the main crystalline constituents of teeth and bones in vertebrates. The relatively abundant monazite group has a general structure of ATO₄, where T is phosphorus or arsenic, and A is often a rare-earth element (REE). Monazite is important in two ways: first, as a REE "sink", it can sufficiently concentrate these elements to become an ore; secondly, monazite group elements can incorporate relatively large amounts of uranium and thorium, which can be used in monazite geochronology to date the rock based on the decay of the U and Th to lead.

Organic minerals

The Strunz classification includes a class for organic minerals. These rare compounds contain organic carbon, but can be formed by a geologic process. For example, whewellite, CaC₂O₄⋅H₂O is an oxalate that can be deposited in hydrothermal ore veins. While hydrated calcium oxalate can be found in coal seams and other sedimentary deposits involving organic matter, the hydrothermal occurrence is not considered to be related to biological activity.

Recent advances

Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. Recent changes have included the addition of an organic class, in both the new Dana and the Strunz classification schemes. The organic class includes a very rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names adopted in 2009 a hierarchical scheme for the naming and classification of mineral groups and group names and established seven commissions and four working groups to review and classify minerals into an official listing of their published names. According to these new rules, "mineral species can be grouped in a number of different ways, on the basis of chemistry, crystal structure, occurrence, association, genetic history, or resource, for example, depending on the purpose to be served by the classification."

Astrobiology

Scientists think that special minerals made by living things, called biominerals, might help us find life beyond Earth. They could be clues to whether there was or is life on places like Mars.

In 2014, NASA shared that robots named Curiosity and Opportunity were exploring Mars. They looked for signs of very old life, such as tiny organisms that might have lived long ago, and for places that could have been good for life, like old river plains or lakes. Finding these clues is very important for NASA.