232 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



in the metamorphic rocks, such as the crystalline schists. It forms pseudomorphs 

 after orthoclase in tin veins," where it is associated with cassiterite, tourmaline, and 

 quartz, and owes its origin plainly to the action of water and other mineralizers, 

 among them undoubtedly fluorine; near the veins the granite is entirely altered 

 to a mixture of quartz and muscovite by the same processes. Weed has described 

 it as produced in granite by the action of hot-spring waters in Montana.* 



DISTINCT CONDITIONS REQUIRED FOR MUSCOVITE AND FOR BIOTITE. 



While muscovite is the alteration product of so many minerals, it seems 

 itself not at all subject to ordinary alteration, but is characteristically fresh, even 

 in highly decomposed rocks.'' Here again it shows its distinction from biotite, 

 which in rocks traversed by waters is easily altered to chlorite, iron oxides and 

 carbonates, quartz, epidote, etc., showing that its conditions of formation were 

 different. In many granites, indeed, muscovite and biotite have been found side 

 by side, and in these rocks the conditions for the formation of the two coincide, 

 but on the one hand stands the range of biotite into the more basic igneous 

 rocks and the lavas where muscovite does not occur, and on the other is the range 

 of muscovite among the minerals formed by circulating waters, where biotite 

 does not ordinarily occur. Plainly, then, the average or ordinary conditions under 

 which biotite forms are more heat and less water than muscovite, in whose 

 formation the evidence of comparatively little heat and abundant water is often 

 conclusive; and the upper extreme of the muscovite range overlies the lower 

 extreme of the biotite range only in the granites, a fact which affords some insight 

 into the conditions of formation of this rock. 



THE 8ERICITIC VARIETY OF MUSCOVITE. 



The fine-grained muscovite (which is often the secondary product of other 

 minerals and occurs as fine fibrous aggregates) is called sericite. Sericite, however, 

 does not differ from muscovite, and has the same relation to the coarser variety 

 (between which and it transitional grades of coarseness are often observable) that 

 the tine secondary quartz has to the coarser grains. For that reason the author 

 uses the words muscovite and sericite interchangeably in referring to the tine- 

 grained variety. 



FLUORINE NECESSARY TO THE FORMATION OF MICA. 



Not only has the presence of fluorine been shown to be necessaiy for the artifi- 

 cial reproduction of mica, but fluorine enters into the composition of the mineral, 

 Iwing most abundant in the best crystallized varieties.** The sericitic variety, then, 



nRosenbiuch-Iddings, Microscopical Physiography of the Rook-making Minerals, p. 286. 

 '' Weed, W. H., Twenty-first Ann. Kept. U. S. Oeol. Survey, pt. 2, p. 247. 

 <-Zlrkel, Lehrbuch d. Petrographie, vol. 1. p. 340. 

 rfBischof. (justav, Chemische Geologic, vol. 2, p. 79. 



