ROCK & GEM FIELD GUIDE: Mica
BY DR. STUART MILLS & PAM FREEMAN
Mica is noted for its shimmering appearance and flaky fracturing that allows it to be split into thin plates. Mica is not one mineral, but rather a group of sheet silicate minerals that is known for its versatility and utility in various technological advancements.
Lepidolite, Aracuai, Minas Gerais, Brazil (9.0 x 6.1 x 4.1 cm)
Forming Mica
Mica forms through both metamorphic and igneous processes under temperatures as high as 300°C. Metamorphic rocks such as schists and gneisses get their famous sheen due to micas, which show off the strain the rocks were under when they formed deep within the earth. Micas contain silicon, aluminum, oxygen and hydrogen plus alkali metals such as potassium, sodium or lithium.