Brown and Black Mica

This substance may be found of almost all colours, except blue; at least there have not yet been observed any specimens of that colour. The darkish brown variety, figured at the upper part of the plate, is not uncommon in small specks or specimens in Granite, but rather more rare of a larger size: the group is not as usual convenient for dividing into plates, but is seemingly a congeries of scaly crystals arranging in a peculiar angular manner, sometimes according with the angles of the primitive rhomb, meeting and mingling so as to become quite zigzag, while the accumulated planes are diverging from different centres in confused order. Its colour gives it rather the appearance of thin horn, but it has a more glassy lustre.

The lower specimen is rather rare, being of a true shining black, i. e. black without any mixture of brownish, blueish, or any other colour: it has a more glassy lustre than the upper specimen. The congeries of plates on one side of the specimen are small and on the other larger, more continuous, partly undulated, and altogether less orderly, and not regularly enough grouped to need particularizing.

Common brown Mica of the shops is often two feet or more square, and when used very thin for microscopes appears nearly colourless.