Minerals of the Composition MgSiO z . 



391 



Olivine crystal from, rnelt 

 of MffSi0 3 in MeCl a . 



microscope. The crystal faces were well defined, giving fair 

 reflection signals on the goniometer. The measurements were 



made on a two-circle goniometer with reducing attachment, 

 and the results were found to agree fairly well with the same 

 constants for natural fosterite. The crystals measured were 

 too small to permit a very accurate measurement of the angles 



(Table I). 



The crystal represented in fig. 2 9 



measured' -15 mm in length and 0'12 mra 

 in thickness, was transparent, colorless, 

 and well developed on all sides. The 

 development was noticeably unsym- 

 metrical and the quality of the dif- 

 ferent faces varied considerably. The 

 indices of refraction were determined 

 by Schroeder van der Kolk's method 

 of refractive liquids in which the pos- 

 sible error does not exceed ±"003 ; a = 

 1-645 ± '003, /3 = 1-656 db '003, 7 = 

 1-668 ± -003. Birefringence = 0-023 

 dz '006. The interference colors are 

 bright, usually of the first and second 

 orders. The crystals extinguish parallel to the cleavage direc- 

 tions. The plane of the optic axes is perpendicular to the most 

 perfect cleavage lines after 010 and to the prismatic develop- 

 ment of the crystal and lies in the plane 001. The optic axial 

 angle is very large with c the acute bisectrix. The crystals 

 were decomposed by hydrochloric acid with the separation of 

 gelatinous silica. In one preparation fosterite crystals were 

 observed enclosing original fragments of the pyroxene which 

 had not been fully dissolved by the solvent. 



In a number of experiments with magnesium chloride we also 

 observed the formation of periclase (MgO) in well-formed 

 octahedra (l mm in length), isotropic and of characteristic cleav- 

 age and refractive index 1'73. 



Properties of the Monoclinic Pyroxene. — Though crystals 

 of monoclinic pyroxene were obtained from many fluxes and 

 under different conditions, as a general rule they were ex- 

 tremely small and could be used only in a few favorable 

 instances for crystallographic measurement. Crystalline aggre- 

 gates of this form were also readily obtained direct from 

 the molten silicate and then usually as large radiating spheru- 

 lites, the individual fibers of which were frequently 1 to 2 cm 

 long. The crystals were measured on the two-circled goniome- 

 ter with reducing attachment, were colorless, transparent, of 

 glassy luster and varied in length from -1 to l mm , and in width 

 from -05 to "5 mm . An extremely small crystal of this substance 



