150 



MAGNESIA. 



2. Anhydrous Silicates of Magnesia, and Compounds 

 Isormorphous with tliem. 



PYROXENE. ^ 



Monoclinic. In modified oblique rhombic prisms ; M : 

 M = 87° 5'. Cleavage perfect parallel with the sides of the 



prisms, and also distinct parallel with the diagonals. 



Usually in thick and stout prisms, of 6 or 8 sides, 

 i terminating in two faces meeting at an edge ; a ; 



a= 120° 32', M: e = 133°33, M: e = 136° 27'. 



Occurs also in oblique octahedrons, much modified. 



Massive varieties of a coarse lamellar structure ; 

 also fibrous, usually very fine and often long capillary ; also 

 granular, usually in coarse angular grains and friable, some- 

 times round ; sometimes fine and compact. 



Colors green of various shades, verging to white on one 

 side and brown and black on the other, passing through blue 

 shades, but not yellow. Luster vitreous, inclining to resin- 

 ous or pearly; the latter especially in fibrous varieties. 

 Transparent to opaque. H = 5 — 6. Brittle. Gr = 3'2 — 

 3-5. 



Pyroxene consists of silica and magnesia, combined with 

 one or more of the bases, lime, protoxyd of iron, or protoxyd 

 of manganese. These bases replace one another in a com- 

 pound without changing the crystalline form, and have the 

 same form nearly in their own crystallizations, as explained 

 on page 74. The varieties of pyroxene arise from the va- 

 riations in composition dependent on this isomorphism, and 

 they differ much in appearance. 



Varieties and Composition. The varieties may be divided 

 into three sections — the light colored, the dark colored, and 

 the thin foliated. 



I. White malacolite or white augite — includes white or 

 grayish-white crystals or crystalline masses. Diopside , in 

 greenish-white or grayish-green crystals, and cleavable 

 masses cleaving with a bright smooth surface. Sahlite ; of 

 a more dingy green color, less luster and coarser structure 

 than diopside, but otherwise similar ; named from the place 



What is the character of the crystals of pyroxene 1 What is a com" 

 mo'., form? What is said of its massive varieties? its colors and lua» 

 ter 1 What are the constituents of pyroxene 1 



