Biotite Muscovite Granite

Biotite muscovite granite is coarse grained rock and another variation of granite which is felsic in chemistry, meaning that its main constituent minerals are quartz and feldspar. The quartz has a white colouration which is down to the presence of fluid gases present during the formation of the rock which stop the quartz forming clear. Plagioclase, a potasium mineral being white in general due to either being albite or anorthite gives the rock an overall off white tone in colour. However, this rock also contains numerous biotite and muscovite mica crystals which both contrast in colour from one another due to differing chemistries. Muscovite, like all micas form in sheets and is often colourless when the sheets are single, however, when overlaid can give a bronze or brassy appearance with a hint of green. Biotite mica on the other hand, while almost identical in chemistry, also contains higher amount of iron (Fe), thus, the mineral appears black. All of these minerals together comprise of this rock which is better known as ‘Cornish Granite’. The granite however is part of the Cornubian batholith, specifically the St. Austell pluton, an igneous granitic body which formed during the Permian period around 280 million years ago and was the driving force behind various intrusions of heat and fluid infiltrating the local rock already in the area which resulted in mineralisation and later leading to rich ore deposits. The feldspar from exposed occurrences of this granite and others in the area eventually led to the formation of kalonite which would later be used to produce china clay. An excellent rock specimen for serious collectors as well as college or university level education, these particular rock samples can be used to both describe the variations that one may find in granites as well as show the chemistry of the rock and how such an igneous body can alter it’s surroundings and result in mineral ores being deposited. These samples come from St. Austell in Cornwall, UK and are available in three sizes and come supplied in a card tray with an information label.