Plane polarised light
Thin section of a fused siltstone/mudstone known as
buchite within a sill. Partial melting has enabled the growth of
various high temperature minerals including sapphires, an aluminium
silicate and a variety of corundum. Their pale blue colour is just
discernible, especially towards the top centre.
Cross polarised light
The above photo in xpl illustrates the high
birefringence of corundum (sapphires). Their interference colours
include purple, yellow and red. The minerals showing grey
interference colours are feldspars.
Plane polarised light, highly magnified, showing a
possible mullite (aluminium silicate) needle towards bottom left and
?crystallites forming within the fused mudstone towards centre right.
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showing locality of Rubh a' Chromain, Carsaig Bay, Mull,
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Geological Conservation Review (GCR) quotes the Rubh'
a'Chromain composite xenolithic sill as belonging to the Scridain
Suite of sills, its significant feature being the distinctive,
high-temperature mineralogy of the thermally metamorphosed aluminous xenoliths.