ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 273 



product of igneous and calcareous rocks. The schists and gneisses contain- 

 ing wollastonite are often garnetiferous and epidotic. 



The very frequent development of the above pyroxenes in the sedi- 

 mentary rocks which are calcareous, rather than amphiboles, is due to the 

 fact that the pyroxenes are richer in calcium than are the amphiboles. Where 

 sedimentary rocks contain magnesium abundantly with the calcium, the 

 amphiboles are likely to form rather than the pyroxenes. 



Acmite occurs mainly in the eruptive rocks, and especially in those 

 which are rich in alkalies. According to Rosenbusch, it occurs especially 

 in granites and syenites rich in sodium, in the eleeolite- syenites, phonolites, 

 and leucitophyres. As a metamorphic mineral acmite is derived from 

 arfvedsonite. 



Spodumene sometimes occurs as an accessory constituent in the granites, 

 schists, and gneisses, and in some cases as considerable masses. 



Pectolite, while not an abundant mineral, is present as a secondary 

 constituent in many basic eruptive rocks, both plutonic and volcanic. It is, 

 however, especially prevalent in the volcanic rocks, since these are more 

 porous, and pectolite is especially likely to occur in cavities or seams. 

 Occasionally pectolite is found in the metamorphic rocks as a product of 

 apophyllite. 



Alterations of the diopside-augite series. — The most common alteration of the non- 

 aluminous diopside and sahlite is into talc (orthorhombic or monoclinic; 

 sp. gr. 2.7-2.8). They also often alter into serpentine (monoclinic; sp. gr. 

 2.5-2.65). These changes are accompanied by the formation of calcium 

 carbonate, and frequently by the separation of a part of this carbonate as 

 calcite (rhombohedral; sp. gr. 2.7135). 



The aluminous pyroxenes, augite, and diallage, under the conditions of 

 the zone of katamorphism, change into chlorite (monoclinic; sp. gr. 2.08- 

 2.16), with which are usually associated epidote (monoclinic; sp. gr. 3.25- 

 3.5), this mineral often being embedded in the chlorite and calcite. Under 

 conditions of weathering, any of the minerals of the diopside-augite series 

 may be partly or entirely replaced by quartz (rhombohedral; sp. gr. 

 2.6535), chalcedony (cryptocrystalline; sp. gr. 2.6-2.64), or calcite. Such 

 replacements are particularly common in the case of the porous andesites 

 and trachytes, and also in tuffs. Not infrequently this replacement of 

 the pyroxene occurs without the feldspar being greatly affected. 

 JION xlvii — 01: 18 



