OCCURRENCE OF ANORTHOCLASE. 257 



produced, and therefore in the belt of rock flowage. In the formation of 

 muscovite and quartz from feldspar by equation (3), as the specific gravity 

 of the separated quartz is somewhat greater than that of the original 

 feldspar, and that of the muscovite is considerably greater than that of the 

 feldspar, the condensation in volume above calculated is accounted for, 

 although the alteration is one involving hydration and possibly carbonation. 

 In the zone of anamorphism the wateradded is doubtless largely derived from 

 other minerals, as this is a belt of dehydration, and destruction of previous 

 minerals containing hydroxides. This passage from one mineral to another 

 would involve no increase in the total volume, the controlling consideration. 

 The most doubtful point concerning equation (3) is the carbonation of the 

 potassium. It might be supposed that the potassium unites with a part of 

 the freed silica and with other elements to form potassium minerals. But it 

 is not easy to suggest such minerals, as leucite is not recorded as a meta- 

 morphic mineral. The more probable solution of the problem is that 

 potassium and a portion of the silica unite with the alumina of the gibbsite 

 or some other minerals and produce one molecule of mica from one of 

 orthoclase, as suggested in equation (4). This suggestion is rendered 

 especially plausible for the slates, schists, and gneisses derived from sedi- 

 ments, for such rocks usually contain residual orthoclase and also aluminum 

 hydroxide. (See pp. 232, 235, 898-900.) The reaction of equation (4) pro- 

 duces great decrease in volume, is one of dehydration, and thus absorbs heat; 

 it is therefore a perfect example of the rules of the zone of anamorphism. 

 The same remarks are applicable to equations (5) and (6), respectively, for 

 the production of biotite, as to (3) and (4) for the formation of muscovite, 

 with the addition that the development of biotite involves silication and 

 decarbonation, and therefore still better than muscovite illustrates the 

 reactions of the zone of anamorphism. 



The physical-chemical principles for the alteration of orthoclase and 

 microcline to epidote are the same as for the alterations of the more basic 

 feldspars to epidote. As the process occurs much more extensively in con- 

 nection with the latter minerals, it is discussed under the basic plagioclases. 

 (See pp. 263-264.) 



ANORTHOCLASE. 



occurrence — This mineral is subordinate in quantity to orthoclase and 

 microcline. It occurs in both deep-seated and effusive igneous rocks; in 

 mon xlvii — 04 17 



