Troost on the Pyroxene. 53 



to little better than a tenth for every one, particularly as to 

 the lime, which amounts sometimes to a fourth. Alumine 

 is always present, though in small quantity. Iron varies 

 from 1 to 14 hundredths. Manganese is also found, and 

 even traces of potash and chrome. Of these analyses we 

 shall speak more particularly hereafter. 



Pyroxene is nearly always crystallized; its forms are 

 sometimes very much complicated and difficult to deter- 

 mine. They are prisms whose summits, mostly obtuse, of- 

 fer faces which by the obliquity of the primitive nucleus, 

 and the extent which some of the faces acquire at the ex- 

 pense of the others, seems unequally disposed. The difficulty 

 of determining these forms is besides sometimes augmented 

 by the property which these crystals have to form various 

 hemitropic varieties, by which are formed crystals with one 

 projecting and a re-entering pyramid. The forms of Py- 

 roxene are varied and numerous. M. Hauy describes 27, 

 and remarks that the different varieties of Pyroxene, which 

 are considered different species, all offer crystals proper to 

 the family of Pyroxene ; and the different aspects given to 

 these varieties have assisted to distinguish them. The sur- 

 face of these crystals is generally smooth and shining in 

 case they are translucent or transparent, but dull and even 

 rough to the touch when they are opaque. The following 

 is an enumeration of some of the secondary forms, which 

 are most common, according to Hauy. 



1. Primitive Pyroxene.* The crystals having this form 

 belong to the variety known by the name Mussit. 



2. Perihexaedral P.t A hexaedral prism with an oblique 

 base; it is the first mentioned form having the two obtuse 



* Hauy, tab. comp. et Mem. Mus. 1, p. 283, pi. 14, fig. 23. 

 t Traite 3, p. 83, fig. 139. 

 Vol. I. S 



