Skarns

Chalcopyrite, pyrite, marcasite, sphalerite and pyrargyrite. Great Bear Lake, Canada

This is a sulphide skarn. Pyrite (yellow-white, bottom right) is intergrown with marcasite (white to light pink-white, centre bottom), showing strong reflection pleochroism and bireflectance (centre). A lath-shaped marcasite aggregate (top left) is a pseudomorph after pyrrhotite. Chalcopyrite (yellow, top) is intergrown with inclusion-free sphalerite (light grey, centre) and minor pyrargyrite (blue, centre, top left). Dark grey areas are silicate.

Polished block, plane polarized light, x 160, oil

Magnetite and haematite. Skye, Scotland

Euhedral to subhedral magnetite (light brown, centre) has thin rims of haematite (white, higher reflectance, centre top) due to incipient martitization. Coarse-grained garnet (light grey, bottom right) is the main phase and is intergrown with other silicates (dark grey, centre right). Black areas are polishing pits.

Polished block, plane polarized light, x40, air

Magnetite, covelline, malachite and haematite. Skye, Scotland

Anhedral magnetite (light brown-grey, bottom centre) is enclosed within garnet (medium grey, bottom right), the main phase. Covelline (blue, centre) shows cleavage and surrounds a small haematite crystal (white, centre). Malachite (dark grey, top left) shows faint green internal reflections. Dark grey areas are silicates, black areas are polishing pits.

Magnetite and haematite. Skye, Scotland.

Magnetite (pink-brown, right) is coarse-grained and is intergrown with haematite (white, centre), which encloses an euhedral basal section of quartz (dark grey, centre bottom). Silicates are dark grey (left), black areas are polishing pits.Coarse-grained magnetite is often difficult to polish well.

Polished block, plane polarized light, x40, air

Haematite and magnetite. Skye, Scotland

Haematite (white, right) is coarse-grained and shows very faint bireflectance along (101 1) twin planes, which are oriented north-south but difficult to see in plane polarized light. Magnetite (pink-brown, bottom left) is well polished. Dark grey areas are silicates, black areas are polishing pits.

Polished block, plane polarized light, x80, air

Haematite and magnetite. Skye, Scotland

This is the same field of view as the third plate above but with partially crossed polars. Haematite crystals show polysynthetic twinning along (1011). The silicate gangue shows light-coloured internal reflections.

Polished block, partially crossed polars, x 80, air