In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form.[1][2]
The geological definition of mineral normally excludes compounds that occur only in living beings. However some minerals are often biogenic (such as calcite) or are organic compounds in the sense of chemistry (such as mellite). Moreover, living beings often synthesize inorganic minerals (such as hydroxylapatite) that also occur in rocks.
The concept of mineral is distinct from rock, which is any bulk solid geologic material that is relatively homogeneous at a large enough scale. A rock may consist of one type of mineral, or may be an aggregate of two or more different types of minerals, spacially segregated into distinct phases.[3]
Some natural solid substances without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids.[4] If a chemical compound occurs naturally with different crystal structures, each structure is considered a different mineral species. Thus, for example, quartz and stishovite are two different minerals consisting of the same compound, silicon dioxide.
The International Mineralogical Association (IMA) is the generally recognized standard body for the definition and nomenclature of mineral species. As of December 2020, the IMA recognizes 5,650 official mineral species[5] out of 5,862 proposed or traditional ones.[6]
The chemical composition of a named mineral species may vary somewhat by the inclusion of small amounts of impurities. Specific varieties of a species sometimes have conventional or official names of their own.[7] For example, amethyst is a purple variety of the mineral species quartz. Some mineral species can have variable proportions of two or more chemical elements that occupy equivalent positions in the mineral's structure; for example, the formula of mackinawite is given as (Fe,Ni)
9S
8, meaning Fe
xNi
9-xS
8, where x is a variable number between 0 and 9. Sometimes a mineral with variable composition is split into separate species, more or less arbitrarily, forming a mineral group; that is the case of the silicates Ca
xMg
yFe
2-x-ySiO
4, the olivine group.
Besides the essential chemical composition and crystal structure, the description of a mineral species usually includes its common physical properties such as habit, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, parting, specific gravity, magnetism, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid.
Minerals are classified by key chemical constituents; the two dominant systems are the Dana classification and the Strunz classification. Silicate minerals comprise approximately 90% of the Earth's crust.[8][9] Other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates.
Rocks, ores, and gems
A rock is an aggregate of one or more minerals[33] or mineraloids. Some rocks, such as limestone or quartzite, are composed primarily of one mineral – calcite or aragonite in the case of limestone, and quartz in the latter case.[34][35] Other rocks can be defined by relative abundances of key (essential) minerals; a granite is defined by proportions of quartz, alkali feldspar, and plagioclase feldspar.[36] The other minerals in the rock are termed accessory minerals, and do not greatly affect the bulk composition of the rock. Rocks can also be composed entirely of non-mineral material; coal is a sedimentary rock composed primarily of organically derived carbon.[33][37]
In rocks, some mineral species and groups are much more abundant than others; these are termed the rock-forming minerals. The major examples of these are quartz, the feldspars, the micas, the amphiboles, the pyroxenes, the olivines, and calcite; except for the last one, all of these minerals are silicates.[38] Overall, around 150 minerals are considered particularly important, whether in terms of their abundance or aesthetic value in terms of collecting.[39]
Commercially valuable minerals and rocks, other than gemstones, metal ores, or mineral fuels, are referred to as industrial minerals.[40] For example, muscovite, a white mica, can be used for windows (sometimes referred to as isinglass), as a filler, or as an insulator.[41]
Ores are minerals that have a high concentration of a certain element, typically a metal. Examples are cinnabar (HgS), an ore of mercury; sphalerite (ZnS), an ore of zinc; cassiterite (SnO2), an ore of tin; and colemanite, an ore of boron.
Gems are minerals with an ornamental value, and are distinguished from non-gems by their beauty, durability, and usually, rarity. There are about 20 mineral species that qualify as gem minerals, which constitute about 35 of the most common gemstones. Gem minerals are often present in several varieties, and so one mineral can account for several different gemstones; for example, ruby and sapphire are both corundum, Al2O3.[42]
Etymology
The first known use of the word "mineral" in the English language (Middle English) was the 15th century. The word came from Medieval Latin: minerale, from minera, mine, ore.[43]
The word "species" comes from the Latin species, "a particular sort, kind, or type with distinct look, or appearance".[