|BGS image ID: P521549|
Mineral specimen of pyrite. Cuan Slate Quarry, Argyllshire.
The image shows a number of near-perfect cubic crystals of pyrite. They have a variable colour and are stained with yellow and orange colours due to the presence of secondary iron oxide minerals on the crystal surfaces. When fresh pyrite has a metallic brass-yellow colour.
Crystals of pyrite such of this are common in the slate deposits of the Easdale area. They were referred to as 'diamonds' by the quarrymen, although economically they are practically worthless. Pyrite is an iron sulphide mineral belonging to the cubic crystal system that occurs as striated, cubic, octahedral or pyritohedral crystals. These specimens come from a small quarry in the 25-ft raised-beach platform 150 yards from the quay at Cuan Ferry at the north end of Luing. The thicker seam contains a bluish-grey slate with fairly large pyrites crystals, although the crystals usually cleave through, and do not pull out when the slate is split. The other seam lies immediately west of the thicker seam. It is a darker slate and rather softer in quality, with very small pyrites crystals. The slate and pyrites belong to the Dalradian Supergroup (Precambrian) Easdale Slate, predominantly black carbonaceous pyritic slates.