Mica Sheet Crystal

Photographer: Rob Sheridan
Summary Author: Rob Sheridan
 
As magma intrusions such as dikes, sills, and batholiths, slowly cool beneath the ground, they create an astounding variety of different crystalline minerals. This process depends on the rate of cooling and the concentration of various elements in the magma; the process is described by Bowen’s Reaction Series. Heavier minerals with higher concentrations of iron and magnesium (mafic rocks) tend to crystallize first, leaving lighter (felsic rocks) to crystallize subsequently, as the intrusion continues to cool. These lighter minerals include micas, feldspars, and quartz, the three components of granite. Slower cooling allows larger crystals of these minerals to form. If cooling is very slow (over many thousands of years), large-grained granites, called pegmatites, can form. Perhaps the most striking minerals found in such granite pegmatites are sheets of mica.

Mica consists of sheets of silicon-oxygen tetrahedra held together by ionic forces of contained potassium, magnesium or iron. The fine specimen above was found in a New Hampshire granite pegmatite intrusion. In the ancient world, sheet micas were used for decorative purposes. In the industrial world, their high temperature tolerance and electrical insulating properties made them valuable in a wide variety of applications such as insulators and capacitors. Newer materials have now replaced mica in high temperature electronics, but mica is still important as an additive to paint and makeup (to provide sheen), and to drywall compounds (so it can be more easily sanded). It’s also used as window "glass" in stoves and furnaces.