ALTERATIONS OF ZEOLITES. 333 



The development of the zeolites in nearly all cases requires hydration 

 and expansion of volume, as shown under the discussions of the particular 

 minerals from which they form. Their formation, therefore, tends to fill 

 up the crevices and cracks in rocks, even if no material be furnished from 

 an extraneous source. It may be that the zeolitization combined with other 

 alterations furnishes sufficient material to entirely fill the vacuoles of many 

 amy gdaloids without material being furnished from an extraneous source. 

 (See pp. 631-634.) However, it is doubtless the case that much of the mate- 

 rial of the zeolites which is deposited in the belt of cementation is derived 

 by solution from the belt of weathering'. As shown under the individual 

 minerals from which the zeolites develop, the conditions for the formation 

 of these minerals are those of the zone of katamorphism, both in the belt 

 of weathering and in that of cementation. 



In the belt of cementation, in which hydration is perhaps the most 

 characteristic reaction and alterations can take place with expansion of 

 volume, the zeolites form on a great scale. Conforming with these state- 

 ments are the observations made by Daubre"e a that zeolites can be formed 

 experimentally in the presence of abundant water at temperatures of about 

 50° C. Pointing in the same direction is the fact stated by Renard 6 that 

 phillipsite has extensively formed at the bottom of the sea at temperatures 

 not far from 0° C. 



While the zeolites develop chiefly in the belt of cementation, it is 

 certain that in very humid regions they form in the belt of weathering. 

 But it is also certain that to a great extent the zeolites are also destroyed in 

 the belt of weathering. This is especially the case in hot arid regions. 



Alterations. — The most comprehensive statement as to the alterations of 

 the zeolites is that given by Clarke/ His statements are as follows: (1) 

 Natrolite alters into prehnite (orthorhombic ; sp. gr. 2.875); (2) mesolite 

 alters into prehnite ; (3) scolecite alters into prehnite ; (4) analcite alters 

 into albite (triclinic; sp. gr. 2.635), and (5) orthoclase (monoclinic; sp. gr. 

 2.57) and prehnite; (6) apophyllite alters into pectolite (monoclinic; 

 sp. gr. 2.73); (7) heulandite alters into albite, and (8) into orthoclase; 



"Daubree, A., Etudes synthetiques de geologie experimentale, Paris, 1879, pt. 1, pp. 199, 205-207. 

 ''Murray, John, and Renard, A. F., Report of the scientific results of the voyage of H. M. S. 

 Challenger, 1873-1876; Deep-sea deposits, London, 1891, pp. 400-411. 



c Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, pp. 32^5. 



