Geevor Hematite

The tin and copper won from the tin mines in the West Penwith mining district of Cornwall was derived from the ore mineral cassiterite, tin sulphide. The cassiterite was hosted in veins along with other non-valuable ores, the majority of which were rich in iron, particularly in the form of the oxides hematite and magnetite. Once the ores were extracted, they were broken up at the stamps and ground down prior to smelting. The iron oxides were considered waste, not being concentrated enough to be of economic value in their own right and were discarded after the cassiterite had been picked out. ‘Synthetic’ hematite could also be formed during the smelting of the cassiterite to obtain tin. This is a two stage process. First the tin sulphide must be roasted in the presence of oxygen to drive off the sulphur and convert it to tin oxide or ‘black tin’. It is then reduced to metallic tin in a reducing (oxygen-free) environment. The waste product of the roasting phase, dumped on the slag-heaps at Geevor is also a very pure red iron oxide, analogous to the mineral hematite. The origin of the mine waste on the tips at Geevor is not yet known and it could have been derived from several sources. However it is relatively pure, crystalline hematite, bright cherry-red under the microscope.

 Ruth Siddall