46 THE CHICAGO ACADEMY OF SCIENCES. 



perpendicular to the vibration plane of the lower Nicol, help 

 distinguish it from pyroxense, amphibole and biotite. The 

 best specimens are those in the tourmaline granite specimen in 

 which the gold was found. 



The MICAS are represented by three varieties, muscovite, 

 biotite and phlogopite. All have highly perfect basal cleavage, 

 which makes it possible to separate them into extremely fine 

 sheets, whose length and breadth is limited only by the diame- 

 ter of the original crystal. They are elastic and similar in 

 hardness and specific gravity, and all crystallize in the monoclinic 

 system. All of them show on basal sections plane angles of 

 120 and 60, which suggests the hexagonal system, but under 

 the microscope with convergent light and crossed Nicols, ex- 

 hibit ellipses and hyperbolas characteristic of biaxial minerals. 



The true nature of the optical phenomena of the micas is 

 best seen in muscovite, in which the axial angle is 50 to 70. 

 Biotite very closely resembles uniaxial minerals, the hyper- 

 bolas so nearly forming a single cross. These specimens show 

 the percussion figures so characteristic of mica. When a 

 cleavage plate is struck with a dull-pointed awl, a six-rayed 

 figure results. In muscovite the most prominent crack is par- 

 allel to the clinopinacoid, and the plane of the optic axes is at 

 right angles to it. In biotite the plane of the optic axes is 

 parallel to the most prominent crack. 



riuscovite. H 2 K A1 3 (Si O 4 ) 3 . Monoclinic. 



Crystals usually tabular, with rhombic or regular six-sided 

 outlines. 



The planes are strongly striated horizontally, and the min- 

 eral usually occurs in scales and masses. Basal cleavage, per- 

 fect, while a secondary cleavage is shown by the percussion 

 figures parallel to the clinopinacoid (oio) and this is the cause 

 of the narrow strips in which the mineral often appears. The 

 laminae are flexible and elastic. The hardness is 2.2 to 2.5, the 

 specific gravity 2.7 to 3. The lustre is vitreous,, pearly or silky. 

 The colors, grey, brown, pale green, violet and yellow. The 

 streak is uncolored; the diaphaneity, transparent to translu- 

 cent. The pleochroism is feeble. The birefringence is high, 

 hence interference colors are bright. Before the blowpipe 

 with alkaline carbonates muscovite decomposes, giving reac- 

 tions for iron and manganese. It yields water in a closed tube, 



