<P> Minerals are classified by key chemical constituents; the two dominant systems are the Dana classification and the Strunz classification . The silicate class of minerals is subdivided into six subclasses by the degree of polymerization in the chemical structure . All silicate minerals have a base unit of a (SiO) silica tetrahedron--that is, a silicon cation coordinated by four oxygen anions, which gives the shape of a tetrahedron . These tetrahedra can be polymerized to give the subclasses: orthosilicates (no polymerization, thus single tetrahedra), disilicates (two tetrahedra bonded together), cyclosilicates (rings of tetrahedra), inosilicates (chains of tetrahedra), phyllosilicates (sheets of tetrahedra), and tectosilicates (three - dimensional network of tetrahedra). Other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates . </P> <P> One definition of a mineral encompasses the following criteria: </P> <Ol> <Li> Formed by a natural process (anthropogenic compounds are excluded). </Li> <Li> Stable or metastable at room temperature (25 ° C). In the simplest sense, this means the mineral must be solid . Classical examples of exceptions to this rule include native mercury, which crystallizes at − 39 ° C, and water ice, which is solid only below 0 ° C; because these two minerals were described before 1959, they were grandfathered by the International Mineralogical Association (IMA). Modern advances have included extensive study of liquid crystals, which also extensively involve mineralogy . </Li> <Li> Represented by a chemical formula . Minerals are chemical compounds, and as such they can be described by fixed or a variable formula . Many mineral groups and species are composed of a solid solution; pure substances are not usually found because of contamination or chemical substitution . For example, the olivine group is described by the variable formula (Mg, Fe) SiO, which is a solid solution of two end - member species, magnesium - rich forsterite and iron - rich fayalite, which are described by a fixed chemical formula . Mineral species themselves could have a variable compositions, such as the sulfide mackinawite, (Fe, Ni) S, which is mostly a ferrous sulfide, but has a very significant nickel impurity that is reflected in its formula . </Li> <Li> Ordered atomic arrangement . This generally means crystalline, however, crystals are also periodic, so the broader criterion is used instead . An ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form, hardness, and cleavage . There have been several recent proposals to classify biogenic or amorphous substances as minerals . The formal definition of a mineral approved by the IMA in 1995: "A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes ." </Li> <Li> Usually abiogenic (not resulting from the activity of living organisms). Biogenic substances are explicitly excluded by the IMA: "Biogenic substances are chemical compounds produced entirely by biological processes without a geological component (e.g., urinary calculi, oxalate crystals in plant tissues, shells of marine molluscs, etc .) and are not regarded as minerals . However, if geological processes were involved in the genesis of the compound, then the product can be accepted as a mineral ." </Li> </Ol> <Li> Formed by a natural process (anthropogenic compounds are excluded). </Li>

What are the 5 main characteristics of a mineral