Metamorphosed volcanogenic sulphides

Pyrite, chalcopyrite, sphalerite, covelline and galena. Parys Mountain, Britain

Euhedral pyrite crystals (pale yellow-white, bottom centre) have relict poorly polished cores (top left), suggesting that they have recrystallized. Chalcopyrite (yellow, centre), which has altered to covelline (blue, centre right and left) about its crystal boundaries, is intergrown with sphalerite (light grey, left). Sphalerite contains abundant chalcopyrite inclusions aligned along crystallographic directions (left) and has suffered chalcopyrite disease. A single grain of galena (white, centre left) is intergrown with sphalerite. Dark grey areas are quartz, black areas are polishing pits.

Polished block, plane polarized light. x80, air

Pyrite, sphalerite, covelline, galena and chalcopyrite. Parys Mountain, Britain

Pyrite aggregates (pale yellow-white, top) have been fractured. These fractures have been infilled by chalcopyrite (yellow, bottom right), galena (blue-white, centre left) and covelline (blue, top left). Sphalerite (light grey, left) carries unoriented chalcopyrite and euhedral pyrite inclusions (left). The central aggregates of covelline have relict chalcopyrite (centre bottom left) within them and show intense bireflectance and reflection pleochroism (light to dark blue). Quartz is dark grey (top right), black areas are polishing pits.

Polished block, plane polarized light, x 80, air

Sphalerite, chalcopyrite, galena and pyrite. Parys Mountain, Britain

Fine-grained and complex intergrowths occur between sphalerite, the main sulphide (light grey), chalcopyrite (yellow, top centre), galena (blue-white, centre bottom) and subhedral pyrite (light yellow-white, bottom centre). Dark grey areas are quartz.

Polished block, plane polarized light, x 80, air

Galena, sphalerite, chalcopyrite and pyrite. Parys Mountain, Britain

Galena (blue-white, centre) shows characteristic triangular cleavage pits (black, centre) due to plucking along the (100) cleavage. It encloses euhedral pyrite (light yellow, high reflectance, centre right) and chalcopyrite (yellow, centre bottom), and is intergrown with sphalerite (light grey, left). Sphalerite carries abundant fine-grained chalcopyrite, partly concentrated along grain boundaries but mainly crystallographically oriented within crystals - this is chalcopyrite disease. The gangue is dark grey, the slightly higher reflectance phases are carbonates (top left, bottom right) which have plucked along their cleavage, the darker phase is quartz showing faint internal reflections (top right). Both pyrite and chalcopyrite show relief against galena. The different orientations of the triangular polishing pits within galena show that it comprises a number of separate crystals.

Sphalerite, chalcopyrite, pyrite and covelline. Parys Mountain, Britain

Euhedral pyrite (light yellow-white, centre right) crystals occur within chalcopyrite (yellow, centre right) and sphalerite (light grey). fractures within chalcopyrite and sphalerite are infilled with covelline (deep blue, centre). Sphalerite shows extensive chalcopyrite disease, with chalcopyrite inclusions oriented along crystal boundaries, twin planes and growth zones (top right). Very fine-grained to submicroscopic chalcopyrite (centre) imparts a yellow-brown surface colour to sphalerite. Black areas are polishing pits.

Sphalerite, chalcopyrite, pyrite, galena and covelline. Parys Mountain, Britain

Chalcopyrite (yellow, top right) is intergrown with sphalerite (light grey, centre). Sphalerite is fractured and partially replaced by covelline (blue, bottom centre). Euhedral pyrite (light yellow, high reflectance, top left) is intergrown with galena (blue-white, top centre). Widespread replacement of sphalerite by chalcopyrite (as chalcopyrite disease) is crystallographically controlled (centre right) or very fine-grained so giving a yellow-brown surface colour to the sphalerite (centre left), black areas are voids.

Polished block. plane polarized light. x80, air