MadSci Network: Earth Sciences

Re: rocks or minerals

Date: Tue Aug 18 18:25:46 1998
Posted By: Robert Chesson, Other (pls. specify below), Working Geologist (Certified Profssional Geologist), FEC
Area of science: Earth Sciences
ID: 902939620.Es

The material is probably the mineral muscovite, which is a member of the mica family or group. Muscovite generally has a chemical composition of KAl2(AlSi3O10) (OH)2, is usually colorless or pale shades of green, gray, or brown, and occurs in thin sheets. Muscovite has, what mineralogists call perfect cleavage (the splitting or tendency to split, along planes determined by the mineral crystal structure), is flexible, and can be transparent. At one time muscovite was used as “windows” in stoves due to it’s transparency and resistance to heat.

Muscovite occurs in a variety of different geologic environments. In igneous rocks it is usually confined to granites, is a very common mineral in granite pegmatites (coarse grained igneous rocks found in dikes), and provides a important industrial mineral. Muscovite is also a common and abundant mineral in schist and gneiss of low- and medium-grade metamorphism. It tends to be resistant to weathering and is a common component of sediment in stream beds. I would guess from your description of the area, the muscovite that you found occurs either as part of a coarse pegmatite or is associated with medium grade metamorphic rocks.

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