13. Muscovite.—Contains 47 per cent. of silica, 3 per cent. of
sesquioxide of iron, and 10 per cent. of alkalies, chiefly potash; and
the characteristic colors are white, gray, and, more rarely, brown and
yellow. Non-dichroic. Usually found in association with acidic minerals.
The mica used in the arts is muscovite. Specimen 21.
14. Biotite.—Contains only 36 per cent. of silica, 20 per cent. of oxide
of iron, and 17 per cent. of magnesia; colors, deep black to green.
Strongly dichroic. Commonly occurs with other basic minerals. Compare
color with per cent. of iron.