Silicates & Quartz
Mica
What it is
Sheet silicates that split into thin, flexible, transparent sheets. Muscovite (the most common mica) was once used as window glass in Russia — hence "Muscovy glass." Lepidolite is the lithium-bearing mica, and fuchsite is the chromium-bearing variety that gives it a sparkling green color.
How it forms
Forms in igneous and metamorphic rocks, especially granite pegmatites. The sheet structure comes from the way silicon-oxygen tetrahedra link in flat layers, bonded by weaker forces between layers.
KAl₂(AlSi₃O₁₀)(OH)₂ and variants
