Andersen — System Anorthite-Forsterite- Silica. 421 



A small charge of each was heated for one-half hour at 

 1500-1600° and quenched. In the first two mixtures the result 

 was glass and periclase, the last one showed forsterite, periclase 

 and glass. The periclase crystals were alike in all mixtures, 

 forming rounded grains without faces. Distinct cleavage lines 

 were frequently observed. Refractive index approximately 

 1*73. These properties are distinctly different from those of 

 the regular crystals of the anorthite-forsterite-silica system, 

 which crystals, therefore, cannot be periclase. 



A conclusive proof was finally obtained by testing the solu- 

 bility in the following manner : A mixture An 80 4- Fo 20 was 

 quenched after one and one-half hours heating at 1316°. It 

 then contained glass with included crystals of forsterite and 

 well-developed regular octahedrons. The octahedrons showed 

 numerous glass inclusions. This quenching product was heated 

 for one-half hour in a mixture of H 2 S0 4 and HF. After care- 

 fully decanting the acid there was left a little residuum which 

 was examined under the microscope. It consisted of nothing 

 but isocropic crystals with glass inclusions. The refractive 

 index of the crystals was : n = 1*715 ± 0'003. 



Remembering that periclase is comparatively easily soluble* 

 in acids and spinel practically insoluble, this test finally proved 

 that the phase considered was spinel, even if the validity of the 

 optical distinctions might be doubted. 



III. The Binary Systems. 



The System Forsterite- Silica. 



The true binary system forsterite-silica has been described 

 in a previous f paper as a part of the MgO-Si0 2 system. This 

 system contains a binary compound, MgSi0 3 , which is unstable 

 at its melting point. In consequence there is no eutectic 

 Mg 2 Si0 4 : MgSiG 3 (forsterite : clinoenstatite), but an alteration 

 point (point F, fig. 1) where forsterite, clinoenstatite and liquid 

 can exist together in equilibrium. This point has a composition 

 Si0 2 2*5, En 97*5 and the temperature is 1557°. The eutectic 

 clinoenstatite : cristobalite is located at a composition Si0 2 12*5, 

 En 87*5. The eutectic temperature is 1543°. 



The crystallization in the different parts of the system will 

 be briefly described : X In mixtures of composition between 

 D and E (fig. 1), i. e., mixtures between pure forsterite and pure 

 MgSi0 3 , forsterite is the primary phase. In cooling the melt 

 forsterite crystallizes out until the temperature of the point 

 F, the alteration point, is reached. Here forsterite begins to 



* The mixture containing glass, forsterite, and periclase treated in exactly 

 the same way as the spinel-bearing mixture left no residuum whatever. 

 fN. L. Bowen and Olaf Andersen, loc. cit., p. 495. 

 X -For details see N. L. Bowen and Olaf Andersen, loc. cit. 



