Cassiterite ps. after Feldspar

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This is one of the famous Wheal Coates Cassiterite pseudomorphs after feldspar. This type of pseudomorph is exceptionally rare on a world scale with the discovery at Wheal Coates, made in 1828, and reported by Stephen Davey in the Transactions of the Royal Society of Cornwall in 1832. Twinning was surprisingly widespread, and sometimes very complex. This very large thumbnail specimen is multiply twinned. The largest crystal is 2.2 cm in length and presents a really sharp edged crystal form, demonstrating how perfect the replacement, or pseudomorphism was. It is well terminated at one end but its lower edge is a sheared natural fracture, something which was apparently common in many of the crystals found. Alongside the main crystal appears to be another grown in parallel, but to confuse matters this crystal also appears to pass completely through and out of the other side of the main crystal. Ex Martin Kershaw Collection.