Minerals of the Composition MgSiO z . 431 



excess, indicates the greater stability of the former. In connec- 

 tion with this it should be noted that the two amphiboles resem- 

 ble each other as closely as do the two pyroxenes, the mono- 

 clinic forms being the stabler in both cases. The order of 

 stability of the four forms is represented in the diagram (p. 414). 

 Volume Relations. — Although theoretically there is no reason 

 requiring it, the facts show that the specific gravities, and there- 

 fore the specific volumes, of the four forms of magnesium silicate 

 lie iu the order of their stability: 



Specific gravity of glass 2*743 



" " u orthorhombic amphibole 2 857 



" " " monoclinic " * 



" " " orthorhombic pyroxene 3*175 



" " "monoclinic " 3*192 



Geological Inferences. 



Conditions of Formation of Meteorites. — It has been shown 

 above, that the monoclinic magnesian pyroxene, though not 

 generally recognized as a mineral, does occur in nature. After 

 it had been found to be the stablest form of magnesium meta- 

 silicate, its occurrence seemed altogether likely. An examina- 

 tion of the literature then developed the fact that Fouque and 

 Levy had already discovered it in meteorites.f Still their proof 

 lacked one essential point; they assumed the composition of the 

 mineral. The optical study of the material of the Bishopville 

 meteorite, which has been shown to be practically pure mag- 

 nesium silicate,^: supplied the missing link in the chain: a care- 

 ful measurement of all its important optical constants, and a 

 comparison of these with the constants of our monoclinic form, 

 established the identity of the two. Fouque and Levy state 

 that in some meteorites the monoclinic form, in others the ensta- 

 tite, is in excess. In the Bishopville and other meteorites, 

 this form is intergrown with enstatite. In the similar inter- 

 growths of enstatite with a "monoclinic pyroxene" so frequently 

 observed in rocks, it is very probable that in some cases the 

 latter has the same composition as the enstatite. 



The parallel growths of enstatite and the monoclinic pyrox- 

 ene which are characteristic of meteorites, we were able to repro- 

 duce by cooling a molten mass of pure magnesium silicate at a 

 rather rapid rate. The slower the cooling, the more of the 



* The specific gravity of this form' has not been determined, because the 

 substance has not been obtained free from other forms. Its average index of 

 refraction is very close to that of the orthorhombic amphibole. but appears 

 to be a trifle higher, so that we assume with some degree of probability that 

 the specific gravities of the two amphiboles are related just as those of the 

 two pyroxenes. 



fBull. Soc. Min., iv, 279, 1881. 



X J. Lawrence Smith, this Journal (2), xxxviii, 225, 1864. 



