Silver-nickel-cobalt-arsenic-bismuth±uranium. The five metals association

Rammelsbergite, bismuthinite, skutterudite and chalcopyrite. Great Bear Lake, Canada

The cobalt-nickel arsenides are rammelsbergite and skutterudite which are zoned and overgrow each other. The main arsenide is rammelsbergite, which shows faint reflection pleochroism (white and brown-white, bottom centre) and zoning from brown-white cores to white rims (top right). Rammelsbergite is overgrown by euhedral skutterudite (white, centre), which is euhedral and harder (bottom right) and which, in turn, has zoned rammelsbergite overgrowths (centre, bottom right). Thin dark zones between skutterudite and overgrowing rammelsbergite (centre) are due to replacement and alteration of skutterudite. Extensive replacement of a large skutterudite crystal by fluorite (black) is seen on the extreme bottom right where only a relict outer zone of skutterudite remains. Bismuthinite (blue-white) shows reflection pleochroism and bireflectance (top right) and has replaced cobalt-nickel arsenides. A single crystal of chalcopyrite (yellow, centre) is present.

Polished block, plane polarized light, x 160, oil

Rammelsbergite. Great Bear Lake, Canada

A typical dendritic habit of rammelsbergite (white) in this association. The cores of the dendrites are carbonates, especially dolomite (grey, centre, centre left). Calcite, the main gangue, shows bireflectance between adjacent crystals (centre top). The colour and reflectance of rammelsbergite in this photomicrograph are not affected by the presence of native silver and so the mineral appears white.

Polished block, plane polarized light, x80, air

Niccolite, native silver, acanthite and maucherite. Great Bear Lake, Canada

Native silver (white, scratched, centre left) forms the cores to botryoidal niccolite (pink-brown) showing faint reflection pleochroism (light to dark pink-brown, centre right) that is difficult to see. Thin rims of maucherite (grey-blue, centre bottom) surround niccolite. Acanthite (light grey, bottom right) has replaced native silver in the core of a niccolite dendrite. Dark grey areas are calcite showing faint bireflectance (top centre). Black areas are polishing pits.

Polished block, plane polarized light. x 80, air

Niccolite, native silver, maucherite, acanthite and safflorite. Great Bear Lake, Canada

Native silver (white, centre) forms a Marienberg cross within radiating niccolite (pink-brown, centre) that shows faint reflection pleochroism (light to dark pink-brown). Other silver crystals are rhombic (centre right) and may have pseudomorphed dolomite. Niccolite has thin rims of maucherite (blue-grey), which shows very faint zoning (bottom centre). The outermost zone is safflorite but the two arsenides are difficult to distinguish at this magnification. Acanthite (dark grey, right centre) has replaced native silver. Yellow areas (top right) within silver are due to tarnishing of the silver. Black areas are calcite. Scratches within native silver do not pass into the nickel arsenides and demonstrate the low hardness of silver.

Polished block, plane polarized light, x 160. oil

Native silver, niccolite and maucherite. Great Bear Lake, Canada

A radiating aggregate of niccolite (pink-brown, left) has a thin maucherite (blue-grey, left) rim. Native silver (white, centre) appears to be overgrowing the arsenides. The blue-grey colour of maucherite is due to the very high reflectance of native silver.

Polished block, plane polarized light, x 160, oil