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Pyrophyllite

The Silicate Minerals

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Pyrophyllite, Al2Si4O10(OH)2, is the white matrix in this specimen. It looks like talc (which has Mg instead of Al) but may be blue-green or brown. (more below)
Hydrous aluminum silicate
Photo courtesy Ryan Somma of Flickr.com under Creative Commons license
Pyrophyllite gets its name ("flame leaf") for its behavior when heated on charcoal: it breaks into thin, writhing flakes. Although its formula is very close to that of talc, pyrophyllite occurs in metamorphic rocks, quartz veins and sometimes granites whereas talc is more likely to be found as an alteration mineral. Pyrophyllite may be harder than talc, reaching Mohs hardness 2 rather than 1. Even the definitive test between them may be difficult: when tested with cobalt nitrate, pyrophyllite turns blue while talc turns violet.

Other Metamorphic Minerals

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