Muscovite is the most common mica mineral, appearing as silver-white, translucent to transparent sheets in granites, pegmatites, and schists. Its basal cleavage is so perfect that thin flexible sheets can be peeled off with a fingernail — this physical property is unique and instantly diagnostic. The sheets are elastic: bend one and it springs back. Muscovite has been used as a window material ("isinglass") before glass was widely available. In the field, the characteristic sparkle of fresh granite surfaces is almost always mica catching the light.
Muscovite Mica Identification Sheet
The sparkly sheet mineral in every granite — and the one that makes students understand cleavage for the first time.
CleavagePerfect basal cleavage — splits into thin, flexible, elastic sheets; the defining diagnostic property
Common lookalikes
Talc (greasy feel, softer at 1, not elastic); chlorite (darker green, less perfect cleavage, not elastic); selenite gypsum (softer at 2, not elastic, heavier). The elastic sheet test is definitive — no other common mineral does this.