Spinel. Calton Hill, Derbyshire, Britain
An olivine nodule within alkali olivine basalt. Chrome-rich spinel (light grey, highest reflectance, top centre and left, bottom centre) has a complex symplectite-like intergrowth with coarse-grained olivine (centre and right) which has a slightly lower reflectance than spinel but shows internal reflections and alteration to 'serpentine' along fractures (low reflectance, bottom right). Spinel is surrounded by fine-grained pyroxene (poorly polished areas). In transmitted light the spinel is brown.
Polished thin section, plane polarized light, x 80, air