Mineral

Minerals are solid substances found in nature. They have a specific chemical makeup and crystal structure. Minerals are important because they make up the Earth’s rocks and are 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. These are called mineraloids instead of true minerals.

There are many types of minerals. They are grouped by their chemical parts. Silicate minerals make up most of the Earth’s crust. Other groups include oxides, carbonates, sulfides, and halides. Each group has its own special properties and uses.

Definitions

The International Mineralogical Association has rules about what is a mineral. A mineral must be found in nature, like in rocks and Earth. It must be solid, with just 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 in plants or animals. Scientists sometimes talk about whether these should be called minerals. Minerals can form with help from tiny living things, too, for a very long time.

Rocks, ores, and gems

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

Some minerals are found 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. About 150 minerals are important because they are common or 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 in the 1400s. It came from a Latin word, minerale, which is related to minera meaning mine or ore.

The word "species" comes from the Latin word species, meaning a special kind or type with a unique look.

Chemistry

The types and amounts of minerals we find depend on their chemical makeup, which comes from the elements in the Earth. Most minerals come from the Earth's crust. Eight elements make up most 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 most of the crust.

Minerals form based on the conditions that help them stay stable. For example, in most rocks, aluminium and metals like sodium and potassium combine with oxygen, silicon, and calcium to make feldspar minerals. 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

We can tell minerals apart by looking at their physical properties. Sometimes, just looking at these properties is enough to know what mineral it is. Other times, we need special tools like X-ray diffraction to help us. These tools can take time and cost money.

Minerals are grouped based on things like their shape inside (crystal structure), how hard they are, how shiny they look (lustre), and if light can pass through them (diaphaneity). We also look at their color, the color of powder they leave (streak), how they break (cleavage and fracture), and how heavy they feel for their size (specific gravity). We can also test if they fluorescence, phosphorescence, magnetism, radioactivity, or change when pushed or dropped in acid.

Classification

Earliest classifications

In 315 BCE, Theophrastus described minerals in his book On Stones. His ideas were shaped by his teachers Plato and Aristotle. Theophrastus grouped minerals as stones, earths or metals.

Georgius Agricola described minerals in his book De Natura Fossilium in 1546. He divided minerals into three types: simple (stones, earths, metals, and congealed juices), compound (mixed together) and composite (can be separated).

Linnaeus

Carl Linnaeus described minerals in his important book Systema Naturae in 1735. He divided the natural world into three groups – plants, animals, and minerals – and classified each in the same way. However, his system worked well for biologists but was not useful for mineral experts.

Modern classification

Minerals are grouped by type, group, and family. The basic level is a mineral type, which has unique chemical and physical properties. When two minerals have a range of composition between them, they are called a mineral series. A mineral group is a collection of minerals with some common chemical properties that share a crystal structure.

Two common classification systems, Dana and Strunz, are used for minerals; both focus on composition and structure. James Dwight Dana first published his System of Mineralogy in 1837; as of 1997, it is in its eighth edition. The Dana classification gives a four-part number to each mineral. The less common Strunz classification, named for German expert Karl Hugo Strunz, is based on the Dana system, but uses both chemical and structural features.

Because the Earth's crust has lots of silicon and oxygen, silicates are the most common minerals. However, non-silicate minerals are important for industries, especially as ores. Non-silicate minerals are divided into several groups by their chemistry, including native elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, borates, sulfates, phosphates, and organic compounds.

The largest group of minerals are the silicates; most rocks contain more than 95% silicate minerals, and over 90% of the Earth's crust is made of these minerals. The two main parts of silicates are silicon and oxygen, which are the two most common elements in the Earth's crust. Other common elements in silicate minerals include aluminium, magnesium, iron, calcium, sodium, and potassium.

Silicates

The basic part of a silicate mineral is the [SiO₄]⁴⁻ tetrahedron. In most cases, silicon is surrounded by four oxygen atoms. In very high-pressure situations, silicon can be surrounded by six oxygen atoms, as in the perovskite structure or the quartz form stishovite (SiO₂). These silica tetrahedra can link together in various ways to create different structures.

The silicate groups are described below from the most to the least linked together.

Tectosilicates

Tectosilicates, also called framework silicates, have the most linking together. With all corners of a tetrahedron shared, the silicon:oxygen ratio becomes 1:2. Examples are quartz, the feldspars, feldspathoids, and the zeolites. Framework silicates are very stable because of strong bonds.

Quartz (SiO₂) makes up 12% of the Earth's crust and is the most common mineral. It is known for its high chemical and physical resistance. Quartz has several forms, including tridymite and cristobalite at high temperatures, high-pressure coesite, and ultra-high pressure stishovite. The latter forms only on Earth from meteorite impacts. The silica form that is most stable at the Earth's surface is α-quartz. Its form, β-quartz, exists only at high temperatures and pressures.

Feldspars are the most common group in the Earth's crust, making up about 50%. In feldspars, Al³⁺ replaces Si⁴⁺, which creates a charge imbalance that must be balanced by adding cations. The basic structure becomes either [AlSi₃O₈]⁻ or [Al₂Si₂O₈]²⁻ There are 22 types of feldspars, divided into two main groups – alkali and plagioclase – and two less common groups – celsian and banalsite. The alkali feldspars are most often a mix between potassium-rich orthoclase and sodium-rich albite; for plagioclase, the most common mix ranges from albite to calcium-rich anorthite. Crystal twinning is common in feldspars.

Feldspathoids are similar to feldspar, but form when there is not enough silicon, allowing more aluminium to replace it. As a result, feldspathoids are almost never found 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, rather than 1:6 in alkali feldspar. Zeolites often form crystals in needles, plates, or blocky shapes. They form in the presence of water at low temperatures and pressures, and have spaces and holes in their structure. Zeolites have several uses, especially in cleaning waste water.

Phyllosilicates

Phyllosilicates consist of sheets of linked tetrahedra. They are connected at three oxygen sites, which gives a 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 surrounded by six oxygen atoms) that balance the charge of the tetrahedra sheets. These T and O sheets stack in various ways to create phyllosilicate layers.

Because of their chemical make-up, phyllosilicates usually have flexible, clear layers that do not conduct electricity and can be split into very thin pieces. Micas can be used in electronics, construction, as filler, or even in cosmetics.

Inosilicates

Inosilicates consist of tetrahedra linked in chains. These chains can be single, where a tetrahedron is linked to two others to form a continuous chain; or two chains can be linked to create double-chain silicates. Single-chain silicates have a silicon:oxygen ratio of 1:3 (e.g. [Si₂O₆]⁴⁻), while double-chain silicates have a ratio of 4:11, e.g. [Si₈O₂₂]¹²⁻. Inosilicates include two important rock-forming groups; single-chain silicates are most often pyroxenes, while double-chain silicates are usually amphiboles.

The pyroxene group has 21 types of minerals. Pyroxenes have a general structure of XY(Si₂O₆), where X is a site with six bonds, while Y can have six to eight bonds. Most pyroxenes are mixtures of Ca²⁺, Fe²⁺ and Mg²⁺ to balance the charge. Pyroxenes make up about 10% of the Earth's crust and are a key part of igneous rocks.

Cyclosilicates

Cyclosilicates, or ring silicates, have a silicon to oxygen ratio of 1:3. Six-member rings are most common, with a basic structure of [Si₆O₁₈]¹²⁻; examples include the tourmaline group and beryl. Other ring structures exist, with 3, 4, 8, 9, 12 members described. Cyclosilicates are usually strong, with long, striped crystals.

Tourmalines have a very complex chemistry described by a general formula XY₃Z₆(BO₃)₃T₆O₁₈V₃W. The T₆O₁₈ is the basic ring structure, where T is usually Si⁴⁺, but can be replaced by Al³⁺ or B³⁺. Tourmalines can be grouped by the X site, and then by the chemistry of the W site. The Y and Z sites can hold various metals, which gives the tourmaline group many colors. Other cyclosilicates include beryl, Al₂Be₃Si₆O₁₈, with varieties such as the gems emerald (green) and aquamarine (blue). Cordierite is similar to beryl, and is a common metamorphic mineral.

Sorosilicates

Sorosilicates, also called disilicates, have tetrahedron-tetrahedron bonding at one oxygen, resulting in a 2:7 silicon to oxygen ratio. The common structure is the [Si₂O₇]⁶⁻ group. The most common disilicates are members of the epidote group. Epidotes are found in many geological settings, from mid-ocean ridges to granites to metapelites. Epidotes are built around the [(SiO₄)(Si₂O₇)]¹⁰⁻ structure.

Other examples of sorosilicates include lawsonite, a metamorphic mineral forming in the blueschist environment, and vesuvianite, which contains a lot of calcium.

Orthosilicates

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

The aluminosilicates –kyanite, andalusite, and sillimanite, all Al₂SiO₅ – are made of one [SiO₄]⁴⁻ tetrahedron, and one Al³⁺ in six bonds. The remaining Al³⁺ can be in six-fold (kyanite), five-fold (andalusite) or four-fold (sillimanite) coordination; which mineral forms depends on pressure and temperature. In the olivine structure, the main series is (Mg, Fe)₂SiO₄ of magnesium-rich forsterite and iron-rich fayalite. Both iron and magnesium are in six bonds by oxygen. Other minerals with this structure exist, such as tephroite, Mn₂SiO₄. The garnet group has a general formula of X₃Y₂(SiO₄)₃, where X is a large eight-bond cation, and Y is a smaller six-bond cation. There are six ideal types of garnet, split into two groups. 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, mixing occurs between all six types.

Other orthosilicates include zircon, staurolite, and topaz. Zircon (ZrSiO₄) is useful in dating rocks as U⁶⁺ can replace Zr⁴⁺; furthermore, because of its strong structure, it is hard to change. Staurolite is a common index mineral for medium-grade metamorphic rocks. It has a complex crystal structure that was fully described in 1986. Topaz (Al₂SiO₄(F, OH)₂, often found in granitic pegmatites with tourmaline, is a common gem mineral.

Non-silicates

Native elements

Native elements are minerals not chemically bonded to other elements. This group includes native metals, semi-metals, and non-metals, and various alloys and solid solutions. The metals are held together by metallic bonding, which gives them shiny looks, flexibility, and the ability to conduct electricity. Native elements are divided into groups by their structure or chemical features.

The gold group, with a 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 includes several iron-nickel alloy minerals. Two examples are kamacite and taenite, found in iron meteorites; these differ in the amount of nickel in the alloy; kamacite has less than 5–7% nickel and is a type of native iron, while taenite has 7–37% nickel. Arsenic group minerals are semi-metals, which have some metallic traits; for example, they lack the flexibility of metals. Native carbon occurs in two forms, graphite and diamond; the latter forms at very high pressure in the mantle, giving it a much stronger structure than graphite.

Sulfides

The sulfide minerals are compounds of one or more metals or semi-metals with a chalcogen or pnictogen, with sulfur most common. Tellurium, arsenic, or selenium can replace sulfur. Sulfides are usually soft, brittle minerals with high density. Many powdered sulfides, such as pyrite, have a sulfur smell when powdered. Sulfides can weather and dissolve in water; these dissolved minerals can be redeposited, creating enriched ore deposits. Sulfides are classified by the ratio of metal or semi-metal to sulfur, such as 2:1, or 1:1. Many sulfide minerals are important for industries 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 common sulfide, and can be found in most rock environments. It is not, however, an ore of iron, but can be oxidized to produce sulfuric acid. Related to sulfides are rare sulfosalts, in which a metal is bonded to sulfur and a semi-metal such as antimony, arsenic, or bismuth. Like sulfides, sulfosalts are usually soft, heavy, and brittle.

Oxides

Oxide minerals are divided into three types: simple oxides, hydroxides, and multiple oxides. Simple oxides have O²⁻ as the main negative part and mainly ionic bonding. They can be divided by the ratio of oxygen to cations. The periclase group has 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 include corundum (Al₂O₃), and hematite (Fe₂O₃). Rutile group minerals have a 1:2 ratio; rutile (TiO₂) is the main ore of titanium; other examples include cassiterite (SnO₂; ore of tin), and pyrolusite (MnO₂; ore of manganese). In hydroxides, the main negative part is the hydroxyl ion, OH⁻. Bauxites are the main aluminium ore, and are a mix of the hydroxide minerals diaspore, gibbsite, and bohmite; they form in areas with high rates of chemical weathering (mainly tropical areas). Finally, multiple oxides are compounds of two metals with oxygen. A major group in this class are the spinels, with a general formula of X²⁺Y³⁺₂O₄. Examples include spinel (MgAl₂O₄), chromite (FeCr₂O₄), and magnetite (Fe₃O₄). The latter is known for its strong magnetism, as it has iron in two states (Fe²⁺Fe³⁺₂O₄), making it a multiple oxide rather than a single oxide.

Halides

The halide minerals are compounds where a halogen (fluorine, chlorine, iodine, or bromine) is the main negative part. These minerals are usually soft, weak, brittle, and dissolve in water. Common examples of halides include halite (NaCl, table salt), sylvite (KCl), and fluorite (CaF₂). Halite and sylvite often form from drying lakes, and can be major minerals in sedimentary rocks. Cryolite, Na₃AlF₆, is important for getting aluminium from bauxites; however, as the main deposit at Ivittuut, Greenland, in a granitic pegmatite, is used up, synthetic cryolite can be made from fluorite.

Carbonates

The carbonate minerals have carbonate, [CO₃]²⁻, as the main negative group. Carbonates are usually brittle, many split easily, and all react with acid. Geologists often carry acid to tell carbonates from other minerals. The reaction of acid with carbonates, most commonly calcite and aragonite (CaCO₃), leads to the formation of caves, features inside them such as stalactites and stalagmites, and karst landscapes. Carbonates usually form as remains of living things or chemical sediments in ocean environments. The carbonate group is a triangle shape, with a central C⁴⁺ cation surrounded by three O²⁻ anions; different mineral groups form from different arrangements of these triangles. The most common carbonate mineral is calcite, the main part of sedimentary limestone and metamorphic marble. Calcite, CaCO₃, can have magnesium replacing calcium. Under high-magnesium conditions, its form aragonite will form; ocean chemistry can be described as an aragonite or calcite sea, depending on which form is more common. Dolomite is a double carbonate, with the formula CaMg(CO₃)₂. Secondary dolomitization of limestone is common, in which calcite or aragonite turns into dolomite; this reaction increases space between particles, which can create reservoirs for oil and gas. These two minerals are part of groups named after them: the calcite group includes carbonates with the general formula XCO₃, and the dolomite group has minerals with the general formula XY(CO₃)₂.

Sulfates

The sulfate minerals contain the sulfate anion, [SO₄]²⁻. They are usually clear to cloudy, soft, and often fragile. Sulfate minerals often form from drying lakes, where they precipitate from salty water. Sulfates can also be found in hydrothermal veins with sulfides, or as products when sulfides change. Sulfates can be divided into those without water and those with water. The most common hydrous sulfate is gypsum, CaSO₄⋅2H₂O. It forms from drying lakes, and is found with other minerals such as calcite and halite; if it forms with sand grains, gypsum can create desert roses. Gypsum has low heat resistance and keeps cool when heated as it loses heat by drying out; so, gypsum is used in materials such as plaster and drywall. The anhydrous form of gypsum is anhydrite; it can form directly from seawater in very dry areas. The barite group has the general formula XSO₄, where X is a large 12-bond 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 bonds.

Phosphates

The phosphate minerals have the tetrahedral [PO₄]³⁻ unit, though the structure can be more general, and phosphorus can be replaced by antimony, arsenic, or vanadium. The most common phosphate is the apatite group; common types are fluorapatite (Ca₅(PO₄)₃F), chlorapatite (Ca₅(PO₄)₃Cl) and hydroxylapatite (Ca₅(PO₄)₃(OH)). Minerals in this group are the main parts of teeth and bones in vertebrates. The 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 source, it can concentrate these elements to become an ore; secondly, monazite can hold uranium and thorium, which can be used in monazite geochronology to date rocks based on the decay of U and Th to lead.

Organic minerals

The Strunz classification includes a class for organic minerals. These rare minerals contain organic carbon, but form through geological processes. For example, whewellite, CaC₂O₄⋅H₂O is an oxalate that can form in hydrothermal veins. While hydrated calcium oxalate can be found in coal seams and other sedimentary deposits with organic matter, the hydrothermal form is not linked to biological activity.

Recent advances

Mineral classification systems and their definitions are changing to match new discoveries in mineral science. Recent changes include adding an organic class in both the new Dana and Strunz systems. The organic class includes a very rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names adopted a new system in 2009 for naming and classifying mineral groups. According to these new rules, "mineral types can be grouped in many ways, based on chemistry, crystal structure, where they are found, how they form, their history, or their use, depending on the purpose of the classification."

Astrobiology

Scientists think special minerals made by living things, called biominerals, might help us find life beyond Earth. They could show if there was life on places like Mars.

In 2014, NASA said robots named Curiosity and Opportunity were exploring Mars. They looked for very old signs of life, like tiny organisms, and for places that might have been good for life, like old river plains or lakes. This is important for NASA.