ALTERATIONS OF ZEOLITES. 335 



cent; albite as compared with laumontite, equation (12), 34.92 per cent; 

 orthoclase and prehnite as compared with laumontite, equation (13), 17.75 

 per cent; and analcite as compared with laumontite, equation (14), 4.30 per 

 cent. 



In calculating- the volume relations the carbonates, and in equation 

 (11) the aluminum hydrate, are ignored. If these compounds were taken 

 into account the decreases in volume would in some cases be somewhat 

 more, in others somewhat less. To ignore these side compounds seems the 

 best course, since the added and subtracted salts may be in other forms 

 than those given. 



For the most part the alterations, so far as the bases are concerned, 

 are remarkably simple, involving only the interchange between the alkalies, 

 sodium and potassium, or between sodium and the alkaline earth calcium, 

 or the addition of the bases sodium or calcium. They are all reactions of 

 dehydration, partial or complete. Many of them are reactions of decar- 

 boilatiou and one, equation (4), is a reaction of silication. Presumably 

 they all take place with the absorption of heat. While nothing- can be 

 ascertained as to the a,ctual conditions under which the chang-es take place, 

 one would expect them to occur in the zone of anamorphism, for a number 

 of the reactions reverse those of the zone of .katamorphism. In the latter 

 zone the alteration of the feldspars into the zeolites is well known. The 

 reverse changes, those of analcite, heulandite, stilbite, and laumontite into 

 albite and orthoclase, for which equations are written, would be hardly 

 likely to take place in the same zone. At any rate the alterations of the 

 feldspars into the zeolites and the zeolites into the feldspars present a very 

 interesting case of reversible reactions discussed subsequently. (See 

 pp. 366-369.) 



It is further certain that the zeolites as extensively formed in the belt 

 of cementation, in the belt of weathering break up, by carbonation and 

 hydration, into the simpler compounds, such as the carbonates of the 

 alkalies and the alkaline earths, diaspore or gibbsite, kaolin, and quartz. 

 Hypothetical reactions could readily be written for these changes similar 

 to those worked out for zoisite and epidote (pp. 322-323); but since so 

 little is known as to the definite minerals which are formed from each 

 zeolite, this is hardly worth while at the present stage of knowledge. 



