252 A TREATISE ON METAMORPHISM. 



the glass was passing through that zone. If these had gone far, the undoing 

 of the oxidation, hydration, and carbonation would probably absorb heat. 

 But if the glass reached the zone of anamorphism in an anhydrous condition, 

 the crystallization, producing a decrease in volume, would liberate heat. 

 Thus no general statement can be made as to the heat reaction in the zone 

 of anamorphism. 



FELDSPAR GROUP. 



The minerals of the feldspar group are the most abundant of the 

 silicates. According to Clarke,"' the feldspars comprise 60 per cent of the 

 minerals of the lithosphere. The feldspars include minerals of two classes 

 of symmetry, monoclinic or pseudomonoclinic, and triclinic. Those of the 

 first class comprise orthoclase, microcline, and anorthoclase ; those of the 

 second class include albite, oligoclase, andesine, labradorite, bytownite, 

 and anorthite. 



In chemical composition the feldspars vary from orthosilicates, through 

 metasilicates, to polysilicates. The readiness of decomposition is indirectly 

 proportional to the acidity, the orthosilicates being the most easily decom- 

 posed, and the polysilicates being the most difficult to decompose. 



The more frequent alterations of the monoclinic feldspars and of the 

 polysilicate plagioclase feldspars are to mica, especially muscovite, and to 

 hyclrated silicate of aluminum, especially kaolin. In this alteration there 

 is simultaneous liberation of silica, which may separate as quartz. Very 

 frequently also gibbsite is formed at the same time. Where the mica biotite 

 is produced it is necessary that iron and magnesium shall be added. The 

 most common alterations of the orthosilicate plagioclase feldspars are to 

 zeolites, epidote, and zoisite, frequently with the simultaneous formation of 

 another plagioclase and chlorite. Where epidote is produced it is necessary 

 that iron be added from some other source; where chlorite is produced it is 

 necessary that magnesium and iron be added from some other source. 

 All the important minerals produced by the alterations of the feldspars, 

 with the exception of quartz and plagioclase, are hydrated, though in 

 varying degrees; hence, in general, water is added during the alteration of 

 the feldspars. From the intermediate plagioclases there may be produced 

 any of the foregoing minerals 



a Clarke, F. W., Analyses of rocks, laboratory of the U. S. Geol. Survey, 1880-1899: Bull. U. S. 

 Geol. Survey No. 168, 1900, p. 16. 



