On Granite containing Andalusite from the Cheesewring, Cornwall

Extract

The granite of the Cheesewring Quarry is coarse grained and grey in colour. It is essentially composed of white felspar, quartz and two micas. Tourmaline occurs sparingly. The dominant felspar is orthoclase, frequently twinned on the Carlsbad plan. Crystalline form is not well developed. Under the microscope the individual grains do not, as a rule, give detinite extinction. Different portions extinguish at slightly different angles. The arrangement of the different portions is often very irregular; sometimes, however, it gives rise to a striped structure, under crossed nieols, suggestive of micro-perthite ; but in no case has a perfectly definite intergrowth of plagioclase and orthoelase been observed. Examined by Szabo's method the orthoclase is seen to be fairly rich in soda. A well striated plagioclase occurs in the rock, but is present in very small quantity. When it does occur it is often seen to be idiomorphic with respect to quartz.