This type of granite will also yield a good china clay when it is kaolinised.  The minerals which it is composed of - quartz, feldspar and mica, together with smaller amounts of topaz and tourmaline - crystallised out from hot molten magma as it cooled 275 million years ago.  Topaz is a fluorine bearing mineral and is very hard.  The mica is unusual, as it has significant lithium content (but not as high as in the next specimen).  Some of the best ceramic ceramic china clays are derived from this type of granite - so next time you drink out of a cup remember that it may have started off as a hard granite like this specimen.

This type of granite mainly occurs in the western part of the St Austell granite mass in a number of areas.  When looked at closely (ideally with a lens) the granite can be seen to be composed of three main minerals: quartz (grey greasy appearance), feldspar (milky appearance) and a dark brown platy mica called zinnwaldite, which is a lithium-containing mica. The low iron content of the zinnwaldite mica in this granite means that where it is altered to china clay (kaolinised), the resulting clay also has a low iron content as well, making it white.  Whiteness is an important property of china clay, in view of its main uses in paper and ceramics.  Scattered through the granite are almost black crystals of the boron containing mineral tourmaline.  Much of the iron in this granite is in the tourmaline which, fortunately, is not altered by the kaolinising process, so the iron is locked up out of harms way in a mineral which can easily be removed during processing.  Also present in this specimen (but difficult to identify) is the fluorine bearing mineral topaz, hence this variety of granite is called 'topaz granite'.  Topaz is hardness 8 on Moh's scale - next to corundum, which is 9, diamond is 10.  A vein forms one side of the specimen and has irregular patches of green stained quartz, possibly due to a fine grained green mica called gilbertite or a form of silica containing small quantities of the green mineral chlorite.