272 A TREATISE ON METAMORPHISM. 



The minerals diopside, sahlite, and augite constitute the so-called 

 diopside-augite series. 



occurrence. — The pyroxene group is one of the most widespread and 

 important, One or another variety of pyroxene may occur in almost any 

 rock; but the pyroxenes are much more abundant in the intermediate and 

 basic than in the acidic rocks. Pyroxene is found in the plutonic and 

 volcanic rocks, as an original constituent of the clastic rocks, and as an 

 original and secondary constituent of the metamorphosed rocks, both of 

 igneous and of aqueous origin. The minerals of the pyroxene group occur 

 extensively in veins. 



Diopside occurs in marbles, especially magnesian marbles. Indeed, 

 this is the common form of pyroxene which develops as a secondary 

 constituent during the metamorphism of the magnesian limestones. It also 

 occurs in veins. As a metamorphic mineral diopside is derived from 

 dolomite. 



Sahlite occurs in ferriferous magnesian marbles. Like diopside, it is 

 also found in veins. Unlike diopside, it is a common product in many 

 hornblendic schists and gneisses, such rocks probably having been in their 

 original condition calcareous, magnesian, and ferriferous. Sahlite is derived 

 from ankerite and parankerite. 



Hedenbergite occurs as a rather common constituent of some nepheline 

 syenites and other basic syenites. 



Augite is a common form of pyroxene in the eruptive rocks, both 

 plutonic and volcanic. It occurs in many mechanical sediments. It also 

 is found in metamorphic rocks of both igneous and sedimentary origin, 

 though in the sedimentary metamorphosed rocks it is less common than 

 diopside and sahlite. But augite develops to a considerable extent in the 

 sedimentary rocks which are intermediate between the chemical and 

 mechanical rocks — that is, those which contain abundant calcium carbonate 

 and also are rich in aluminum. Augite is recorded as a metamorphic 

 mineral derived from hornblende. 



WoUastouite occurs especially in the metamorphosed calcareous and 

 sedimentary rocks, it being a secondary product produced by metamorphism. 

 It is found abundantly in marbles, and in schists and gneisses which were 

 originally calcareous, especially the calcareous feldspathic schists. It also 

 develops in calcareous inclusions in eruptive rocks, and is found as a contact 



