SILICATES, TITAN ATKS. ETC 423 



minerals. Minerals crystalli/ing curlier, as apatite, zircon, or 

 magnetite, may occur as inclusions, while lamellar intcrgrowths 

 parallel to 010 with monoelinic pyroxene are not uncommon. 



( rtliorhombic pyroxenes are common pyrogenetic constituents 

 of igneous rocks, especially the more basic varieties, as peridotites, 

 iioriti-s, diorites, and some syenites and their corresponding por- 

 pli\ Tie- and lavas, where they are associated with monoelinic py- 

 roxenes, olivine, plagioclases and micas, but never associated with 

 nepheline or leucite. 



In their weathering and hydration they form bastite, a fibrous 

 substance, in composition much like serpentine; other alteration 

 products are serpentine, talc, carbonates, iron oxides, and quartz. 



Artificial. - There are four modifications of magnesium meta- 

 silicate, two of which are pyroxenes, the other two amphiboles. 

 Of the two pyroxenes one is monoelinic, the other orthorhombic. 

 The transition temperature between these two modifications is 

 about 1100 C. 



When magnesia, iron oxides, and silica are fused in an open 

 crucible and allowed to crystallize near their fusing point, the 

 monoelinic form appears. If crystallization takes place below 

 1100, the orthorhombic form, enstatite, will appear. 



If serpentine is fused, it breaks down, on cooling, into a mixture 

 of enstatite and olivine. 



MONOCLINIC PYROXENES 



In the monoelinic pyroxenes, calcium, iron, and aluminium enter 

 the molecule and are more important than magnesium. While 

 the varieties thus formed agree closely in their crystallographic 

 habit and physical properties, they differ widely in their occur- 

 rence and association. The varieties from diopside to acmite and 

 jadeite are included under the species pyroxene and they may 

 vary in their composition and pass one into the other gradually. 

 Their normal compositions and relations are given in the table, 

 page 420. 



DIOPSIDE 



Diopside. Calcium magnesium metasilicate, CaMg(Si0 3 ) 2 ; 

 Monoelinic; Type, Digonal Equatorial; & :b:c = 1.0921:1: 

 .5893 ; p = 74 10' == 100 A 001 ; 100 A 110 = 46 25' ; 001 A 101 = 

 24 21'; 001 A Oil =29 33'; Common forms, c(001), b(010), 

 a (100), m(110), s(lll), w(lll); Twinning plane 110, contact 



