Cassiterite Partly Replacing Orthoclase, Wheal Coates, St Agnes, Cornwall, England
Twinned crystals of orthoclase feldspar partially replaced by cassiterite. This rare kind of replacement, or partial pseudomorph, is best seen in specimens from Wheal Coates, which are now world famous. The crystals were locally called "stannite" which should not be confused with the tin, copper, iron sulphide of the same name. In Cornwall, cassiterite is concentrated in hydrothermal veins and pegmatites associated with granite intrusions. Cassiterite is the primary ore of tin, the most significant metal mined in Cornwall.
© RIC, photographer A.G. Tindle