ALTERATIONS OF MUSCOVITE. 337 



Still others from which muscovite is derived, such as the feldspars, may 

 be original constituents of the igneous rocks, or they may be original or 

 secondary constituents of the sedimentary rocks. It is therefore clear that 

 muscovite has an unusual variety of sources, and consequently it may be 

 expected in almost any variety of rock except the volcanics. It is, how- 

 ever, a more characteristic constituent of the acidic and intermediate rocks 

 than of the basic rocks. 



In summary, muscovite is derived from anorthoclase, diaspore, gibbsite, 

 leucite, microcline, nephelite, orthoclase, plagioclase and orthoclase, scapo- 

 lites, sodalite, and spodumene. The muscovite damourite is derived from 

 andalusite, corundum, cyanite, sillimanite, staurolite, and topaz. 



Alterations. — The minerals to which muscovite alters are not nearly so 

 abundant as those from which it is derived. One of the most frequent 

 alterations is that of hydration, a part of the potassium being replaced by 

 hydrogen; or at the same time it may take up other bases and thus the 

 mineral may pass into vermiculite, a somewhat indefinite compound to 

 which no formula can be assigned. Muscovite also alters into serpentine 

 (monoclinic; sp. gr. 2.50-2.65) and into the steatitic form of talc (massive; 

 sp. gr. 2.7-2.8). Probably simultaneously with the formation of these 

 minerals gibbsite (monoclinic; sp. gr. 2.3-2.4) or diaspore (orthorhombic ; 

 sp. gr. 3.3-3.5) forms, although the contemporaneous formation of these 

 minerals is not mentioned. Muscovite also may alter into the soda-mica 

 paragonite (monoclinic; sp. gr. 2.78-2.90). 



The reactions by which muscovite passes into serpentine and talc are 

 very uncertain. If the magnesium were supposed to be derived from a 

 carbonate and all of the silica of the muscovite went into the resultant 

 compounds, the reactions may be written as follows: 

 For serpentine: 



(1) 2KH 2 Al 3 Si 3 12 +9MgC0 3 +13H ;i O=3H 4 Mg3Si 2 9 +6Al(OH) 3 +K 2 C0 3 +8C0 2 +k. 

 For talc: 



(2) 4KH,Al 3 Si 3 12 +9MgC0 3 +17H 2 0=3H 2 Mg 3 Si 4 12 +12Al(OH) 3 +2K 2 C0 3 +7C0 2 +k. 



The increase in volume of the serpentine as compared with the muscovite is 

 16.56 per cent, and the decrease of the talc 25.23 per cent. But if the 

 magnesium carbonate be contributed by solutions, and the gibbsite remains 

 as a solid with the serpentine and talc, the increase in volume of the ser- 

 mon xlvii — 04 22 



