Biotite

Lore & Background

Biotite was named by J.F.L. Members of the biotite group are sheet silicates, with iron, magnesium, aluminium, silicon, oxygen, and hydrogen forming sheets weakly bound by potassium ions. The term 'iron mica' is sometimes used for iron-rich biotite, but also refers to a flaky micaceous form of haematite, so the field term Lepidomelane avoids this ambiguity. Biotite is also called 'black mica' as opposed to 'white mica' (muscovite). Like other mica minerals, biotite has a highly perfect basal cleavage and consists of flexible sheets that easily flake off. It has a monoclinic crystal system with tabular to prismatic crystals and a pseudohexagonal form. In thin section, biotite exhibits moderate relief, pale to deep greenish brown or brown color, moderate to strong pleochroism, and high birefringence.

Reader's Guide

Biotite is significant as a widespread rock-forming mineral found in a wide variety of igneous and metamorphic rocks, including the lava of Mount Vesuvius and the Monzoni intrusive complex. It is an essential constituent of many metamorphic schists and forms over a wide range of pressure and temperature. Biotite comprises up to 7% of the exposed continental crust. Its uses include constraining ages of rocks through potassium–argon dating or argon–argon dating, though argon escapes readily at high temperatures, potentially providing only minimum ages. Biotite is also useful in assessing temperature histories of metamorphic rocks because the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature. The largest documented single crystals of biotite were approximately 7 m² sheets found in Iveland Municipality, Norway.

Did You Know?

• The largest documented biotite crystals were approximately 7 m² sheets found in Iveland Municipality, Norway.