490 



MINERALOGY 



FIG. 507. Muscovite Crystals from Woodstock, 

 Maine. 



onal, all three should be interchangeable. On the other hand, the 

 figure produced by pressure, or pressure figure, will have the three 

 lines at right angles to the sides of the basal section. Inclusions, 



often oxides of iron, in 

 micas are, in numerous 

 instances, symmetrically 

 arranged along lines in- 

 tersecting at 60, thus 

 forming equilateral tri- 

 angles, the sides of which 

 are parallel to the rays 

 of the above figures, 

 more often to the rays 

 of the pressure figure. 

 In some cases both sets 

 may be present. 



The chemical compo- 

 sition of muscovite 

 varies greatly from the 



formula given above, as some of the potassium may be replaced by 

 sodium, lithium, or caesium, while the hydrogen is replaced by 

 fluorine, and all micas contain small amounts of fluorine. Alu- 

 minium is at times replaced by chromium, as in the green chrome 

 mica, fuchsite, of Pfitsch, Tyrol ; or again by vanadium, as in ros- 

 ccelite, a vanadium mica occurring in some of the gold-bearing 

 veins of California. Ferric iron may also take the place of alumin- 

 ium, yielding varieties of dark color. 



Both composition and physical texture are taken into account 

 in separating the micas into species and varieties, which in some 

 cases may be only artificial distinctions, as it is possible that they, 

 like the plagioclase feldspars, are isomorphous mixtures of various 

 salts which have not been definitely determined ; if so, they should 

 in their composition grade into each other with no sharp line of 

 demarcation. 



In rock sections muscovite is of an irregular outline. When cut 

 parallel to the vertical axis, the sections show fine parallel cleavage 

 cracks, very close together ; the relief in this section is high, and the 

 interference colors are of the second or third order. When the 

 section is parallel to the base or cleavage, there are no cracks or 

 relief, and the interference color is a gray of the first order, and as 

 the cleavage is only slightly inclined to the acute bisectrix, the inter- 



