Altered granite with tourmaline and cassiterite vein
We would like to thank the following for the use of this sample:
This sample comes from reclaimed land near the village of Trethurgy, close to the giant Carclaze china clay pit, St Austell, Cornwall. The rock is a hydrothermally altered granite cut by a cassiterite bearing tourmaline vein that formed asboron-rich late-stage magmatic-hydrothermal fluids from which the tourmaline crystallised, also carried tin. It is from this paragenesis that the entire tin industry of Cornwall is based - an industry that can trace its beginnings from Roman times or possibly even earlier.
The thin section shows a coarse grained igneous texture with large grains of quartz, muscovite mica, and altered plagioclase feldspar. Blue/brown igneous tourmaline is present in the granitic portion of the rock but the majority of the tourmaline grains are found in the vein. This hydrothermal tourmaline is darker in colour with blue/brown pelchroism and the vein also contains cassiterite (tin oxide), distinguished by its higher relief and dark brown colour, although unlike many oxides, it is not opaque.
Ma = Millions of years ago
Rock forming mineral: