Slightly mineralized granites

Cassiterite, wolframite and zircon. Glen Gairn, Scotland

Click hereEuhedral cassiterite (medium grey, bottom right) with small inclusions of wolframite (higher reflectance, centre of cassiterite) is replaced by phyllosilicates (white mica) (bottom centre) and quartz. Euhedral tabular wolframite (light grey, highest reflectance, centre top) is zoned with poorly polished outer rims which have altered to fine-grained scheelite (centre left). Euhedral zircon (grey with very faint internal reflections, centre left) has two wolframite crystals growing from it. The low reflectance core of the larger wolframite (centre left) is scheelite. White mica (tabular, poorly polished) is intergrown with quartz and unaltered feldspar; the latter two cannot be distinguished in this section. Opaque phases below the surface of the section are responsible for the 'shadows' within the silicates.

Polished thin section, plane polarized light. x 160, air

Wolframite and cassiterite. Glen Gairn, Scotland

Click hereEuhedral cassiterite (grey, bottom centre) is twinned, shown by the differences in bireflectance (light and dark grey). Tabular wolframite (light grey-brown and grey-blue) is zoned, the inner zone is blue and the outer zone is grey-brown. Quartz (top left) and feldspar (bottom centre) show internal reflections, some of which are due to limonitic staining (orange, bottom right).

Cassiterite and wolframite. Glen Gairn, Scotland

Click hereCassiterite (light grey, top left and right) is euhedral and zoned (top right). This zoning is shown by differences in reflectance. Wolframite (centre) is simply twinned and zoned with an inner zone which has altered to fine-grained poorly polished scheelite, enclosed within unaltered wolframite. A thin outermost zone also shows alteration to scheelite and hence is less well polished. Quartz is featureless and feldspar shows minute opaque phases and fluid inclusions along its cleavage (top left).

Polished thin section. plane polarized light, x 160, oil

Cassiterite and molybdenite. Glen Gairn, Scotland

Click hereCoarse-grained cassiterite (light grey, left) is intergrown with radiating laths of molybdenite which show bireflectance and reflection pleochroism (brown-grey to grey, top right). The main silicates are coarse-grained white mica with cleavage (top left) and quartz (bottom right). Black areas are polishing pits.

Polished block, plane polarized light, x40, air

Wolframite and scheelite. Glen Gairn, Scotland

Click hereCoarse-grained wolframite (light grey) has been extensively replaced by scheelite (medium grey, centre). Much of the scheelite has a rhombic habit (centre bottom). Feldspar (grey, hard, bottom left) and phyllosilicates (lowest reflectance, poorly polished, top left) are the silicate gangue phases.

Polished thin section, plane polarized light, x80, air

Magnetite, molybdenite, pyrite and chalcopyrite. Newry, County Down, Britain

Click hereChalcopyrite (yellow, centre) is intergrown with euhedral pyrite (light yellow-white, high reflectance, bottom left) and magnetite (brown-grey, top centre) which carries chalcopyrite inclusions. Curved laths of molybdenite (right) show strong bireflectance and reflection pleochroism (light grey to dark brown-grey) and also carry chalcopyrite inclusions (top right). Quartz (dark grey) shows internal reflections (centre right).

Polished block, plane polarized light. x80, air