MINERALOGICAL AND CHEMICAL CHARACTERS. 23 



lines. The absorption of the rays vibrating parallel to the cleavage in 

 biotite is always very great and is sometimes sufficient in the deeply 

 coloured varieties to make the thin section of the mineral appear 

 almost black. 



The colours in ordinary light vary very greatly, but the ferro- 

 magnesian micas are most deeply coloured and are often deep-brown, 

 even in thin films. The characteristic colour of lepidolite is a delicate 

 lilac, pink or grey (Pihra, Hazaribagh district). Some of the musco- 

 vite raised in the Hazaribagh district is noted for its red tint which in 

 thick sheets may be a deep ruby-colour. Amber-coloured, smoky 

 brown and, in one locality, deep grass-green muscovite is obtained 

 in the Nellore district of Madras. The colours and the character 

 of the lustre are changed on hydrous alteration of the mica, 

 or are modified by the presence of inclusions of other mineral 

 matter. A peculiar pearly or silvery lustre, displayed by muscovite 

 obtained near Bendi in the Hazaribagh district and a few other locali- 

 ties, is due to the removal of exceedingly thin films of decomposed 

 mica from the cleavage surfaces. Numerous minute inclusions in phlo« 

 gopite, like that from near Waltair, Madras Presidency, give the 

 mineral a bronzy colour and semi-metallic lustre. The same mineral 

 often exhibits the remarkable phenomenon of aster ism from the same 

 cause. The phlogopite from Waltair when interposed between the 

 eye and a candle-flame or other small point of light, shows a six-rayed 

 star, in which one pair of rays is specially pronounced and appears 

 as a bright band of light always disposed at right angles to a 

 prominent striation noticeable in the mineral, and at right angles also 

 to the leading line of the percussion-figure (figs. 6 and 7). The less 

 prominent rays of the ligh star cross the bright band at angles as 

 nearly as can be measured of 6o°. The pleochroism of this phlogopite 

 is also quite distinct, showing a pink tinge for rays vibrating (nearly) 

 at right angles to the striation, and a greenish tinge for rays vibrating 

 (nearly) parallel to the striation. The optic-axial angle in this speci- 

 men is too narrow to permit a safe determination of the direction of the 

 plane of the optic axes. The cause of asterism has been attributed to 



( '3 ) 



