CHEMICAL FORMULAS OF MINERALS
The chemical formula of a mineral expresses its chemical composition, determined on a qualitative basis, followed by quantitative chemical analysis. The quantitative relations between the atoms of the elements included in the minerals give empirical formulas, not informing about their internal structure. These patterns can also be represented as oxide patterns, which can make it easier to remember the chemical composition of the mineral and to quickly find out about the oxide ratios, e.g.:
anhydryt — CaSO4 lub CaO • SO3
kalcyt - CaCO3 or CaO • CO2
diopsyd - CaMg(SiO3) the CaO • MgO • 2SiO2
ortoklaz - KAlSi3O8 lub K2O • Al203 • 6SiO2
Information about certain features of the internal structure of minerals contains structural formulas, determined not only on the basis of quantitative chemical analysis, but also on the basis of research on their internal structure. In the structural formulas in square brackets, groups of elements that constitute the basic elements of the lattice of a given compound are given. For example, the formula of beryllium Al2Be3[And6O18] informs, that the mineral is a silicate containing complex silica anions [And6O18]12-, which are in the form of rings composed of six groups [SiO4]4- bound by common oxygen atoms. In the pattern orthoclase K[AlSi3O8] in square brackets is the complex aluminosilicate anion. Anions occurring together in the mineral structure are sometimes separated in structural formulas by a vertical line.
In mixed crystal formulas, the symbols of the atoms or ions that replace atoms or ions are given in parentheses, separated by commas, in order of their decreasing quantitative share. Olivine pattern (Mg, Fe)2[SiO4] informs, that there is more magnesium in it, than the iron replacing it. Polymorphic forms of minerals are usually identified with Greek letters in front of the chemical formula. The presence of water molecules in the spatial lattice of the mineral is marked by specifying their stoichiometric ratio at the end of the pattern, after the dot,
e.g.. gips CaSO4 • 2H2O.
Water contained in amorphous minerals not bound in the mineral lattice is given with the letter n also at the end of the pattern, after the dot, e.g.. opal SiO2 • n H2O.