VII.—Optically Positive Cordierite

Extract

Cordierite was first found in Travancore at Teruwulla (lat. 9° 22′ N. and long. 76° 37′ E.) in a kind of diorite. In the hand-specimen the mineral appears as violet patches and spots. Monazite, magnetite or ilmenite, garnet and biotite with probably a little hornblende, occur in association with it. In the sections the mineral is found to contain numerous globular inclusions which are surrounded by pleochroic halos. The larger of these may be identified as monazite under the microscope. The mineral is itself pleochroic, light vibrating in the direction of the axis of mean velocity (Y) showing a pronounced violet tint. When light vibrates in other directions the plates do not show any distinct colour, but a faint yellow tint may be observed in some plates. When light vibrates in the direction of the Y axis the pleochroic halos round the inclusions disappear and assume the violet tint of the crystal plate. Cordierite is supposed to occur usually in metamorphosed sediments, and the pleochroic halos are regarded as due to the presence of organic impurities. But a sedimentary origin is out of the question in the present case. It is held by some that pleochroic halos are due to the presence of radio-active substances. I kept an Ilford Empress photographic plate exposed in a dark box to a crushed sample of the mineral for more than twelve hours, but on development the plate did not show any indication of having been affected by the mineral.