Alloclasite

De Dictionnaire geolien
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Alloclasite


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)