Fig. 3.

Photomicrographs of preserved chill zones of the Shiant Isles Main sill. a Olivine phenocrysts, together with small rounded grains of chromite, set in a fine-grained groundmass. Sample S82/41. Plane polarised light. Scale bar is 1 mm long. b Same field of view as a but with crossed polars. Note the grain clustering. Scale bar is 1 mm long. c Sample SC986 showing the details of the groundmass. Note the randomly oriented elongate plagioclase laths, set in a granular matrix dominated by pyroxene. Granular oxide grains commonly form strings with a preferred orientation but these are most likely recrystallized along fractures during hydrothermal alteration. Plane polarised light. Scale bar is 200 µm long. d Sample S82/41, under crossed polars, showing the non-compact structure of many of the olivine phenocrysts. Examples are arrowed. Many of these grains have a recognisably orthorhombic symmetry and therefore are likely to be partially annealed dendrites. Note that some clusters are formed of grains with very little crystallographic misorientation. Scale bar is 1 mm long. e Small cluster of rounded olivine grains in sample S82/41, joined by small areas of grain boundary. Note the similarity of birefringence colours: these three grains also have very similar extinction positions indicating low degrees of crystallographic misorientation. Crossed polars. Scale bar is 200 µm long. f Cluster of three olivine grains in sample S82/41, showing the close association with chrome spinel. The arrowed spinel grains contain irregular internal pockets filled with groundmass, indicative of dendritic growth. Plane polarised light. Scale bar is 200 µm long