[Aztlan] Copan muscovite - more than you ever wanted to know

[Bruce Rogers]

Listeros,

More than you ever wanted to know about muscovite

Muscovite is also known as isinglass, Muscovy 
glass, and potash mica. Chemically, it is a 
potassium-aluminum alumino-silicate with loosely 
attached fluorine or hydroxyl molecules.  It is 
usually clear, but may be tinted or spotted gray, 
brown, yellow, green, or, if the right trace 
elements are present, purplish or reddish. 
Bright green-colored chromium-bearing muscovite 
is called fuchsite and is common in some gold 
ores in the California Mother Lode.

Muscovite is found in thin, sheeted crystals 
reflecting its crystal structure of thin layers 
of silica and aluminum molecules loosely held 
together by the hydroxyl molecules. Individual 
crystals are formed of hexagonal plates with a 
slightly inclined central axis.  The mineral 
usually forms as stacks of sheets, called books, 
which may reach immense size.  Some books in the 
Nellore region of India (mid-eastern coastal area 
NNE of Madras) reach 4 meters or better across. 
Usually, however, the metamorphic rock crystals 
are only a few millimeters across.  It is a 
common mineral in metamorphosed rocks such as 
marble and schist and in granitic rocks, 
especially coarsely crystalline veins called 
pegmatites where crystals are commonly 3-8 cm 
across. 

Thin sheets can be bent and spring back to their 
original shape.  In very thin sheets, it is quite 
transparent and was used as poor man's window 
glass.  In post-Medieval Russia, it was commonly 
used as window coverings and thus got is name 
muscovite from the term Muscovy glass.  Because 
it melts at a high temperature (about 1320°C), it 
was also commonly used as the temperature 
resistant material for the peephole in wood 
burning stoves of the 1800's.  A page-sized, 
clear sheet of muscovite today (2008) will cost 
you about $150.00.  Crushed and formed muscovite 
is also desirable as electrical/electronic 
insulation.

This material is not found locally in the Yukatán 
Peninsula, but is present in granitic rocks in 
the Maya Mountains of southern Guatemala and 
Belize. These old rocks were eroded down to sea 
level, then covered with an arm of the Caribbean 
Sea near the end of the Cretaceous Period, some 
60 million years ago at the close of the Age of 
Dinosaurs.  Thick beds of limestone were 
deposited before further earth movements uplifted 
the area and the current cycle of erosion started 
some 5 to 3 million years ago. Continued erosion 
has also bared the granitic rock core of these 
mountains.

Clear sheets of muscovite may be the mica reputed 
to cover at least the top of the major pyramids 
at Teotihuacán.  Since it is not present near 
Copan, its use at that site implies long-distance 
trade of a precious commodity.

Cheers,
Bruce Rogers, earth scientist on a good day