Biotite is a phyllosilicate mineral that is part of the mica group. Its general chemical composition can be expressed as:
\[ (K, Na)(Mg, Fe^{2+}, Al)(Si_3Al)O_{10}(OH)_2 \]
In this formula:
- \(K\) (potassium) and \(Na\) (sodium) are the alkali cations.
- \(Mg\) (magnesium), \(Fe^{2+}\) (iron in the +2 oxidation state), and \(Al\) (aluminum) are the cations that occupy the octahedral sites.
- \(Si\) (silicon) and \(Al\) (aluminum) are the cations that occupy the tetrahedral sites.
- \(OH\) groups represent hydroxyl ions that are part of the structure.
Biotite can vary in its specific composition, particularly in the proportions of iron, magnesium, and aluminum, leading to a range of colors from dark brown to black, and it typically forms in igneous and metamorphic rocks.
1 answer