Arsenopyrite

COLOR

Variable. Cobalt-bearing varities are white. Nickel-bearing varities are brown. Pleochroism is very strong, ranging from white or light brown to grey or brown-grey. Takes a good polish.

REFLECTANCE

Medium in the lightest position. Cobalt varieties are clearly more reflective and have a reflectance similar to hematite.

ANISOTROPISM

Very strong to extremely strong in both varieties. Cobalt varieties have dull anisotropism: white with weakly greenish subshade. Nickel varieties have a more distinctly bright anisotropism in light brown tints similar to, but less yellow, than those of graphite. No internal reflections.

TEXTURE

These minerals are often very finely crystalline and precipitate from gels. Both varieties may be associated. They occur especially in the alteration zone of ultramafic rocks as radiating aggregates with spherulitic tendency or as bundles of twisted fibers.

ASSOCIATED MINERALS

Other manganese minerals, iron oxides, heterogenite, also primary nickel-cobalt minerals, especially pentlandite.

CRITERIA OF DETERMINATION

Confusion is impossible because of the geological environment (altered ultramafic rocks) and striking optical characteristics, especially the very strong anisotropy. Nickel varieties which are brown and polarize in bright brown tints cannot be misidentified. Cobalt varieties would be confused with lithiphorite, but they polarize a little more strongly with slightly brighter colors. They differ from chalcophanite by the absence of internal reflections.

Source

ATLAS OF ORE MINERALS (P. Picot and Z. Johan)