Minerals of the Composition Mg Si O r 433 



2 42 grams of magnesium silicate and about 4 grams of labrador- 

 ite (Ab AnJ were melted and cooled under similar condi- 

 tions. The magnesium silicate crystallized chiefly as mono- 

 clinic pyroxene with some enstatite present. Small amounts 

 of glass, probably plagioclase glass, filled the interstices 

 between the pyroxene laths. 



3. To solution 1, 5 grams of orthoclase were added ; the whole 



was then fused and crystallized as before. This time a quan- 

 tity of coarsely crystalline enstatite was produced. The ortho- 

 clase remained as glass. 



4. 50 grams of magnesium silicate and 5 grams of albite were 



fused and cooled as before. The magnesium silicate crystal- 

 lized mostly as enstatite, with the characteristic properties: 

 paralled extinction, c = c, cleavage prismatic with an angle 

 of about 90°, birefringence not strong, optically positive; 2E 

 apparently larger than usual. The section consisted of clear 

 enstatite laths and intercalated patches of a cryptocrystal- 

 line, dust-like aggregate which appears to contain albite 

 and enstatite — perhaps a eutectic mixture of the two. Parts 

 of the aggregate had a refractive index apparently equal to 

 that of albite. 



5. 46 grams magnesium silicate, 4 grams sodium metasilicate, and 



1 gram potassium metasilicate were fused and cooled under the 

 usual conditions. The cake was filled with beautiful prismatic 

 crystals, the longest of which was 23 mm in length. Most of the 

 magnesium silicate had ciystallized as enstatite, though there 

 was some of the monoclinic form. The optic axial angle of the 

 enstatite was apparently larger than usual. 



6. Preparation No. 4, consisting of the metasilicate with 10 per 



cent albite, was remelted and cooled more rapidly than the 

 previous'solutions. The product consisted chiefly of the low 

 refracting, orthorhombic amphibole form of the metasilicate. 

 Many of the fragments of the product were clearer and showed 

 brighter interference colors than the amphiboles; they con- 

 sisted of finely intergrown fibers of refractive index slightly 

 less than that of the amphiboles (about 1 -560) which may 

 have been due to a solid solution of albite in the amphibole. 



7. 4-6 grams of albite were added to 46 grams of the magma de- 



scribed under 6, giving a solution containing about 20 per cent 

 as much albite as silicate of magnesium. The fusion was 

 cooled like 6 and the low refracting amphibole formed. The 

 grains w T ere not clearly transparent and exhibited the dusty 

 appearance observed in the albite-enstatite mixture of pro- 

 duct No. 4. The refractive index and other optic properties 

 coincided closely with those of the magnesium amphibole 

 from pure melts. 



Silicate solutions can probably be prepared from which the 

 amphibole will crystallize by a process of slow cooling such as 

 prevails in nature, but thus far we have not hit upon a compo- 

 sition which is effective. 



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

 30 



