Alloclasite
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Sommaire
COLOR
White. By comparison with cobaltite, the tint is light grey, and in contact with alloclasite, cobaltite has a creamy shade. Takes a good polish.
REFLECTANCE
High, slightly lower than cobaltite.
ANISOTROPISM
Rather strong in nondiagnoostic brown to yellow-brown tints under perfectly crossed nicols. However, by slightly uncrossing the nicols, bright and more typical purplish pink to yellow-green tints appear.
TEXTURE
Consistently has polysynthetic twins similar to rammelsbergite. In many cases, some twin elements and lamellar groups are formed of cobaltite whereas the others are alloclasite. This seems to indicate rapid variation in depositional conditions permitting simultaneous formation of the two CoAsS dimorphs.
ASSOCIATED MINERALS
Cobaltite, glaucodot, danaite, aikinite, bornite, tetrahedrite-tennantite.
CRITERIA OF DETERMINATION
The optical properties are quite characteristic, especially the purplish pink to green tints observed by uncrossing the nicols, as are lamellar twins and the possible replacement by cobaltite and by other cobalt minerals. Some twins also exhibit highly characteristic alloclasite and cobaltite alternation. Generally, confusion with another mineral is impossible.
Source
ATLAS OF ORE MINERALS (P. Picot and Z. Johan)
