416 Andersen — System Anorthite-Forsterite-Silica. 



Anorthite. 



Anorthite (CaAl 2 Si 2 8 ) melts at 1550° ± 2° C* The crys- 

 tals are triclinic. The optical properties are :f a = 1*576; 

 £ = 1-583; 7 = 1-589; 2 V = 80° db 3°. Optical character 

 negative. Extinction angle on (001) = 35°, crystals always 

 showing polysynthetic twinning. 



In the mixtures of the present system, the anorthite crystals 

 were small and never showed distinct faces. The best crystals 

 separated from melts near anorthite in composition. They were 

 generally lath-shaped, or sometimes tabular, always elongated 

 after the <x-axis, forming the usual polysynthetic twins. Mean 

 refractive index approximately 1*585 ; extinction angle deter- 

 mined from 27° to 42°, average 34*5 (20 separate determina- 

 tions). Sometimes the crystals formed thin needles after the 

 a-axis, without twinning, but otherwise showed the usual 

 properties. In very viscous melts, for instance in (or near) the 

 anorthite-silica series, the crystals became extremely small, and 

 all attempts to determine their properties with any degree of 

 accuracy failed. Even in such cases, however, the identifica- 

 tion of the crystals could be considered certain. It was seen, 

 for instance, in some mixtures that glass of refractive index 

 1*57 enclosed minute crystals which had a slightly higher 

 refractive index than the glass and a low double refraction. 

 This single observation proved at once that the crystals could 

 not be forsterite, clinoenstatite, spinel or any of the silica 

 modifications, and made the conclusion that they were anorthite 

 at least very probable. The conclusion was fully corroborated 

 by observations on other mixtures, determining points on the 

 liquidus in cases where the properties of the solid phase could 

 be ascertained. By bringing together these points it was 

 plainly seen that no reasonable course of the liquidus could be 

 obtained if the phase had been anything but anorthite. 



Similar considerations can, of course, be equally well applied 

 to other solid phases, when they form crystals included in 

 glass, and are too small or otherwise unfit for measurement of 

 optical properties. 



Forsterite. 



Forsterite (Mg 2 Si0 4 ) melts at about 1890°4 It is found to 

 be stable at the melting point.§ Formed in fluxes it appears 

 in well-developed, orthorhombic crystals with (001) (010) (021) 

 (101) and others as common forms. || Cleavage after (001) and 



* Arthur L. Day and Eobert B. Sosinan, this Journal (4), xxxi, 341, 1911. 

 fH. E. Merwin cited by N. L. Bowen, this Journal (4), xxxiii, 564, 1912. 

 ^N. L. Bowen and Olaf Andersen, loc. cit., p. 491. 

 § See, however, Deleano, Zs. anorg. Chem., lxxxiv, 423, 1914. 

 || E. T. Allen, F. E. Wright and J. K. Clement, this Journal (4), xxii, 391, 

 1906. 



