Monday, June 21, 2010

Pyrophyllite from Goldfield

And we've got another mineral from Goldfield, NV, this time pyrophyllite, a monoclinic phyllosilicate (sometimes also reported with a triclinic form). Although often a metamorphic mineral, at Goldfield it's a hydrothermal alteration mineral. Hard to say what the original rock was, and I don't see any remanent quartz eyes or textures.
Pyrophyllite can be a relatively nondescript clay-like or sericite-like mineral: white, with crystals too small to see. Compared to alunite, the feel of this pyrophyllite-rich rock was talc-like to greasy, and the sheen was pearly. I didn't try the stick-tongue method of checking for kaolinite; I presume it would fail but don't know for sure.
Although crystals of pyrophyllite are more commonly seen forming radiating clusters, these crystals show a somewhat tabular form (if enlarged, this photo will be larger than the previous one). In hydrothermal areas, pyrophyllite is often mistaken for sericite, a very fine-grained white mica. If you can find masses large enough for a scratch test, it's much softer (H = 1-2) than alunite (H = 3.5-4) and slightly softer than sericite (H = 2.5). It's often worth getting an x-ray diffraction or thin-section analysis for positive identification.

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