Cassiterite ps. after Feldspar

Unknown Owner

Help improve this recordNo owner is registered for this specimen yet. If it's yours or you know more about it, you can add the owner, provenance, publications and other details.

If you are looking for an iconic Cornish mineral specimen then you can't get much more classic than a Wheal Coates Cassiterite pseudomorph after feldspar. Bizarrely, the previous collectors index card records it as from "St. Austel, Cornwall" which is clearly incorrect. 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 being the one which everyone talks about because of their perfection in form. The replacement of the feldspar is by fine-grained Cassiterite and Quartz, but in this piece the Cassiterite clearly dominates due to its dark colour and density. The original Orthoclase crystals from Wheal Coates are sometimes twinned, in the Carlsbad habit, or as cross-like forms, with this wonderful example showing two parallel Carlsbad twinned crystals themselves twinned, with other smaller crystals sat on one side. Under magnification the complexity is insane. What makes it even more special is its overall quality in original crystal shape, with such sharp edges, which allows that earlier twinning to be spotted, plus the fact that it is undamaged. The largest known crystal from Wheal Coates is 5.6 cm in length. The two main overlapping/twinned crystals on this really well-formed pseudomorph individually reach 3.4 cm in length which is a really impressive size. Ex Martin Kershaw Collection, no. 384.