
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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