Minerals of the Composition MgSi0 3 . 



401 



better methods cannot be applied — a point which was realized 

 and emphasized by Becke in the original paper. The values 

 for the optic axial angle in air thus obtained were much lower 

 than those given for natural enstatite, ranging between 44° and 

 70°, — a fact for which no explanation has yet been found. 

 Similar low angles were also measured on the pure enstatite 

 from the Bishopville, 8. C, meteorite, which will be discussed 

 in a later paragraph. 



The specific gravity at 25° is lower than that for the mono- 

 clinic variety : 



Preparation II (free 

 from monoclinic). 



3-176 



3-174 



3-175 



Preparation I (not entirely 

 free from monoclinic). • 



3-176 

 3-174 

 Average 



Intergrowth of enstatite and mono- 

 clinic pyroxene. — Both from the molten 

 silicate and from solutions of it, though 

 only rarely from the latter, enstatite 

 and the monoclinic form were obtained 

 together, sometimes in parallel inter- 

 growth. In a product of magnesium 

 metasilicate in molten magnesium chlor- 

 ide and also in a preparation of amorph- 

 ous silica heated with magnesium chlor- 

 ide, the intergrowth of the two phases 

 was clearly marked on a section after 

 the clinopinacoid (fig. 6). In the figure 

 the arrows represent the positions at 

 which the different lamellae extinguish 

 between crossed nicols. Lamellae 1 and 



-,. . , . ... served on (010), Enstatite — 



3, monoclinic, are m twinning position Mg-pyroxene. 



and extinguish at an angle c: c — 22° ; 



2 extinguishes parallel and is enstatite. On such sections the 



difference in refractive indices between the two forms was very 



slight ; the birefringence of the monoclinic lamellae appeared 



somewhat stronger. 



Parallel intergrowths of enstatite and a monoclinic pyroxene 

 are not uncommon in nature, and although they have been 

 frequently described, no one, apparently, has suspected that the 

 two forms may have the same composition. 



In scanning the literature on this subject, it soon became 

 evident that natural enstatite is rarely pure magnesiurn silicate, 

 but nearly always contains some ferrous silicate. The purest 

 enstatite is probably that of the well-known Bishopville, S. C, 

 meteorite, specimens of which are preserved in the National 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 131.— November, 1906. 



28 



Intergrowths ob- 



