Muscovite
KAl₂(AlSi₃O₁₀)(OH)₂ · Monoclinic · also called white mica, isinglass
Muscovite is the silvery mica in granite and schist, and the elastic-sheet test is unmistakable. Its glitter in stream sediment is responsible for a great many false gold discoveries — but mica floats away in a pan, while gold sinks and stays.
Properties
- Mohs hardness
- 2–2.5
- Streak
- white, colorless
- Lustre
- pearly, vitreous
- Colour
- Silvery, colourless, or pale brown. Thin sheets are transparent with a bright pearly shine.
- Cleavage
- Perfect basal cleavage — it peels endlessly into thinner and thinner transparent sheets.
- Fracture
- Not applicable; cleavage dominates completely
- Specific gravity
- 2.76–3 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm muscovite
- Sheets are elastic — bend one and it springs flat again. Chlorite and gypsum stay bent.
- Peels with a fingernail into transparent flakes.
- Glitters in sunlight when spread through sand or soil, which is the usual cause of 'gold in the creek' reports.
What muscovite gets confused with
Each of these is settled by one test. Run that test first.
Where it is found
Granite, pegmatite, and mica schist. Large books of muscovite come from pegmatite bodies.
Is it worth anything?
No cash value. Large clean sheets have some novelty interest.