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. Alumina 
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 21, 
and remarks that the different varieties of Pyroxene, which 
are considered different species, all offer crystals jDroper 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. Perihexsedral 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-. 2.^. 
t Traite 3, p. 83, fig. 139. 
Vol. I. 8 
