12 LITHIUM MICA GRANITE WITH TURQUOISE AND IRON STAINING, Gunheath china clay pit We got this boulder simply because the digger driver thought it looked attractive! In fact, it is quite an interesting rock. The basic granite is a lithium mica granite similar to boulder no 4, but it is weakly kaolinised. In places it is impregnated by turquoise, a mineral which contains copper and phosphate, although in this case it is a rather pale variety. On one side is a vein composed of iron oxide. These iron oxide veins were formed as much as 50 million years after the quartz tourmaline veins. They are usually in a north-south or northwest-southeast direction, which is more often than not at right angles to the mainstage veins as in boulder No 7, so they are usually referred to as 'cross-course' veins. The mineralising fluids which formed these veins contained no boron, so all we get in these veins is iron oxide (usually haemetite) and quartz, occasionally with more exotic minerals such as those containing uranium (torbernite, autunite and pitchblende). Also present in the matrix of this specimen is a complex copper phosphate mineral called libethenite, it is dark green in colour. Iron lodes surround the china clay area and probably represent iron expelled from micas during the various alteration stages which the St Austell granite has been subjected to. Many of these iron lodes were exploited in underground mines in the 19th century and appreciable tonnages of iron ore were produced and shipped upcountry. The most famous mine was Restormel Royal, just north of Lostwithiel. It became 'Royal' after it was visited by Queen Victoria and Prince Albert in 1846.