GUIDE TO THE MINERALOGIC COLLECTIONS 



111 



Muscovite is mined in South Carolina and New Hampshire. 

 Deposits of good quality also exist in Pennsylvania, Colorado, 

 Nevada, New Mexico, South Dakota, Washington and California. 

 Muscovite has been found in Westchester, Orange, Jefferson and 

 St Lawrence counties, N. Y. 



Muscovite, known commercially as mica and colloquially as 

 isinglass, is much used for the doors of furnaces and stoves, also 

 as an insulating material in dynamos and other electric appli- 

 ances and for many less important purposes. 



Lepidolite (lithia mica) 



Lepidolite is a basic fluosilicate of potassium lithium and 

 aluminium. It occurs in crystalline plates resembling those of 

 muscovite but of a pinkish or violet-gray color often nearly 

 white. More frequently it is found in massive granular aggre- 

 gates of <oarse or fine scales. It cleaves easily parallel to the 

 base into elastic plates. It is distinguished from muscovite 

 mainly by the color. 



It is found in granite and gneiss particularly in pegmatite 

 veins. 



Biotite (black mica) (H,K) 2 (Mg,Fe II ) 2 (Al,Fe II1 ),(SiO l ) :: 



Biotite is a potassium, magnesium, aluminium, ferrous and 

 ferric iron orthosilicate. 



It bccnrs in monoclinic crystals, tabular or short prismatic 

 in habit, similar to those of muscovite. These show the basal 

 cleavage characteristic of the micas, separating into thin elastic 

 plates. It is often found in disseminated scales or in massive 

 aggregates of cleavable scales. The luster is vitreous, pearly, 

 or, in the dark colored varieties, 'submetallic; the color is com- 

 monly dark green to black. 



Biotite occurs as an important constituent in many igneous 

 rocks and is common in most granites, and in many syenites and 

 diorites; also in such eruptive rocks as rhyolite, trachyte and 

 andesite. In small flakes biotite is present in many common 

 rocks and soils. 



Orange, Essex and St Lawrence counties, N. Y. furnish good 

 specimens. 



