RELATIONS OF ALTERATIONS OF FELDSPARS. 265 



plagioclases to kaolinite, equation (1), and to gibbsite, equation (2), are 

 by reactions of carbonation and hydration; the alterations to the zeolites, 

 equations (4) to (14), inclusive, are by reactions of hydration, and (8) 

 to (14), inclusive, also involve carbonation. The alteration to .prehnite, 

 equation (15), is a reaction of hydration and desilication. The alterations 

 to zoisite and epidote, equations (16), (17), (18), and (19), are by reactions 

 of hydration. All but (1), (3), (16), and (17) take place with ucrease of 

 volume ranging from 1.58 to 52.76 per cent, provided all the compounds 

 formed remain in situ. All take place with liberation of heat. The altera- 

 tions in all particulars are characteristic of the zone of katamorphism. 



The development of kaolin is probably more characteristic of the belt 

 of weathering- than of the belt of cementation. The development of the 

 zeolites and epidotes is known to occur on an extensive scale in the belt of 

 cementation, both within the bodies of other minerals and within the open- 

 ings in rocks. Amygdules and veins of these minerals, with quartz, are of 

 great importance in cementing rocks. The material for this work is doubt- 

 less in large part, though not altogether, derived from feldspathic minerals. 



For the intermediate scapolites, equations (20) and (21), the alterations 

 take place with a slight increase in volume for marialite and a slight 

 decrease for meionite, provided all the compounds which enter into them 

 are solids. The reactions are those of silication and decarbonation to some 

 extent, and this involves absorption of heat. The geological occurrences 

 correspond with the physical-chemical facts. The most common of the 

 scapolites which occurs as a secondary product in the altered rocks is 

 wernerite, an isomorphous mixture of meionite and marialite molecules. 

 As stated by Dana, wernerite "occurs in metamorphic rocks, and most 

 abundantly in granular limestone near its junction with the associated 

 granitic or allied rock." a Wernerite is associated with such minerals as 

 pyroxene, amphibole, and garnet, which occur as deep-seated alterations. 

 The formation of wernerite from feldspar is probably, therefore, a deep- 

 seated change which occurs in the zone of anamorphism. 



The alteration of orthoclase and plagioclase together to albite, zoisite, 

 muscovite, and quartz, equation (22), is a reaction of hydration and desili- 

 cation. It involves an increase in volume. One would therefore expect 

 the reaction to take place in the zone of katamorphism. 



"Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. S. Dana; Wiley & Sons, 

 New York, 6th ed., 1892, p. 470. 



