128 The Monoclinic Pyroxe,nes of New York State. 



General Characters of Pyroxene. 



The Pyroxene group includes minerals which are for the most 

 part normal metasilicates of the general formula RSiOg. They 

 are simple silicates involving a single base (as Wollastonite) as 

 well as isomorphous mixtures of several. They also sometimes 

 depart from the normal metasilicate ty^e in that they often con- 

 tain an excess of iron oxide or alumina. All the monoclinic 

 pyroxenes are isomorphous mixtures of more than one base. 

 Hintze makes the following varieties :* 



Diopside. MgCa SiaOg 



Sahlite. (MgFe) CaSiaOg 



Hedenbergite. CaFeSijOg 



ScheflEerite. (MgFe) (CaMn) SiaOg 



Jeffersouite. (MgFeZn) (CaMn) Si^Og 



Au-ite r(MgFe) CaSi,Oe+ 1 



^ L(MgFe) (AlFe)^ Si OgJ 



Pyroxene crystallizes in the monoclinic system, and the crys- 

 tallographic relations of the different members vary but little. 

 The members are theoretically of different composition, although 

 they nevertheless grade into each other. 



The axial ratios upon which measurements are usually reckoned 

 are those calculated by G. Y. Kath on augite from Vesuvius 

 (Pogg. Ann. 18T3, Erg. Bd. YI, p. 340.) 



a : b : c==l. 09213 : 1. : 0.58931. 

 /3=74° 10^ 9^'' 



The habit of pyroxene crystals is usually columnar parallel to 

 c (001), those of augite tending towards a short thick form, while 

 those of diopside are often slender. In cross-section they ' are 

 frequently rectangular or square from the great development of 

 a (100) and h (010) but they are sometimes tabular, and if so the 

 flattening is parallel to a (100)."}" (The New York pyroxenes are 

 an exception to this rule). Hemihedi'ism is known to occur, but 

 it is very rare. Twinning is commonest parallel to a (100) and 

 sometimes parallel to c (100). Hintze also mentions twins par- 

 alel to y (101) and W (122). The twinning line often runs 

 through the middle of the crystal and in this case gives no reen- 



* Handbunch der Mineralogie, p. 1016. 

 t Hintze, Handbucli der Mineralogie, p. 1019. 



