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mica group

The mica group of sheet silicates is kinda confusing.  There are two basic types, muscovite and biotite.  So here goes:

Muscovite is the commonest mica.  Only feldspar and quartz are more abundant in the earth's crust.  It is formed in granites and other silica-rich rocks, and in metamorphic rocks.  It is also a component of shales.  There is a chromium bearing muscovite called fuchsite that is attractive as a specimen.  

There are many more varieties of biotite than muscovite and many have been given separate names.  Biotites contain much iron and are found in igneous rocks, granites, gabbros, lavas and many metamorphic rocks.  Some of the varieties are haughtonite, siderophyllite, manganaophyllite, wodanite, titanobiotite, and calciobiotite.  

Phlogopite is essentially a biotite with more magnesium than iron.  It typically shows an asterism in transmitted.  Phlogopite too is divided into several varieties:  paragonite, margarite, prehnite, lepidolite, and glauconite.

 I am back in the deep freeze and the cats are back in the house.

Posted by oh5/ojhoff at 9:25 AM CST
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