168 The Monoclinic Pyroxenes of New York State. 



This pyroxene also approximates sahlite in composition givil|g 

 the formula 



MgaoFejoCa^AlaSiiooOaos+Fe^Os 

 DiOPSIDE. 



DeKalb, St. Lawrence Go. — The crystals from this locality are 

 usually transparent and light green to colorless. They vary in 

 size from a half inch to an inch and a half long. No doubly 

 terminated ones have been seen. The faces of the prismatic zone 

 are mostly smooth and bright, but sometimes have longitudinal 

 striations. The terminal planes are invariably pitted, those on 

 It (111) and A (311) being more like deep interrupted strise. A 

 tabular habit parallel to 6 (010) is frequent. Several combina- 

 tions of forms occur, but the most general is a (100), 6(010), 

 mfllO), c(OOl), i6(lll),s (111), 0(221), A ("311) and sometimes 

 2)(T01)(P1. Xiri,Fig.9). Other combinations are a (100), & (010), 

 m(llO), c(OOl), 1^(111) (PI. XVI, Pig. 4); 6(010), m (110) and 

 w(lll) (PL XVI, Fig. 8);and w(lll), 0(221), c (001), a (100), 

 6 (010) and m (110) (PI. XVI, Fig. 3). The plane v is present on a 

 few crystals, but is always very narrow. The clinodome e(Oll) 

 was seen on two crystals, one in the private collection of Prof. 

 Egleston, and the other in the collection of Prof. A. H. Chester. 

 It is a small triangular face. A common habit of the DeKalb 

 diopsides is the unequal development of w(lll), the left-hand 

 face being often large, while the right-hand one is so narrow as to 

 appear wanting. 



No basal parting has been observed in the fresh crystals, but is 

 universally present in the altered ones, which are of a greenish 

 white color and opaque. 



The DeKalb diopsides have been described by V. Rath (Ref. 38), 

 who noted the forms a (100), 6 (010), m (110), c (001), m (111) and 

 u(221). He states that they are often intergrown with horn- 

 blende and also mentions a twinning parallel to a (100). The 

 pyramid tj(221) was first recorded by him from this locality. 



The two forms of diopside figured by Prof. Penfield in Dana's 

 System of Mineralogy are the same as the first two mentioned 

 above by the writer. 



This diopside is among the purest of pyroxenes found with the 

 State, as the following analysis shows. 



