Minerals / Quartz, Tan y Foel.

There are many badly formed prismatic crystals of quartz, SiO2 lining this cavity in sandstone. The crystals are colourless, but many appear to be dull orange in  colour, due to a dusty coating of possibly an iron mineral. The specimen is from Tan y Foel quarry at Cefn Coch in Powys. The quarry is in the Penstrowed grits formation of Mid Silurian age.

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