GEOPHYSICAL LABORATORY. 



143 



single set of central blocks. This latter arrangement is easily secured in the 

 "gridiron" potentiometer, by means of a new and simple type of switch con- 

 struction. 



Instruments now in use in this laboratory are described which illustrate 

 the above types of design and construction. 



(8) The binary system MgO-Si02. 

 37," 487-500 (1914.) 



N. L. Bowen and Olaf Andersen. Am. Jour. Sci. (4), 



Equilibrium in the binary system MgO-Si02 was studied by applying the 

 method of quenching. 



There are two compounds, the orthosilicate Mg2Si04 and the metasilicate 

 MgSiOs, capable of existing in contact with liquid in the binary system. The 

 former crystallizes in a form corresponding with the mineral forsterite and the 

 latter forms crystals similar to enstatite in most properties, but of monoclinic 

 symmetry, clino-enstatite. 



Clino-enstatite is the only stable form of MgSiOs encountered. It has no 

 true melting-point, but breaks up at 1557° C. (formerly considered the melting- 

 point) into forsterite and liquid, and the temperature must be raised to 1577° C. 

 before complete solution of the forsterite takes place. 



In an earlier publication from this Laboratory, crystals termed a-MgSiOs 

 were described as a high-temperature form of magnesium metasilicate. They 

 were considered to be the product of inversion of clino-enstatite (iS-MgSiOs), 

 but the crystals described have now been proved to be a product of the dis- 

 sociation at 1557° C. and to be the orthosilicate forsterite, not a form of the 

 metasilicate. 



On account of the break-up of clino-enstatite into forsterite and liquid 

 there is no eutectic between the two compounds, and the liquids show, on 

 cooling, the partial or complete re-solution of forsterite at the reaction-point, 

 1557° C, the liquid reacting with the forsterite crystals to give clino-enstatite. 



A discussion is given of the geological significance of this resorption of the 

 olivine forsterite, by reaction with the liquid to give the pyroxene clino- 

 enstatite. 



Summary of invariant -points} 



Solid phases. 



Periclase 



Periclase 

 Forsterite 



Forsterite 



Forsterite 

 Clino-enstatite 



Clino-enstatite 

 Cristobalite 



(MgO) 



(MgO) 

 (Mg2Si04) 



(Mg2Si04) 

 (Mg2Si04) 



(MgSiOs) 

 (MgSiOs) 



(Si02) 



Liquid phase. 



MgO 100% 



<14% MgO 

 >86% Mg2Si04) 



Temperature. 



2800° (Kanolt) 

 1850° ±20° 

 Mg2SiO4l00 % 1890° ±20° 



/ MgSiOs 87.5% 1 

 I SiOa 12.5%/ 



1543° 



Cristobalite (Si02) 



SiOo 



100 % 1625° (Fenner) 



^These points are, of course, invariant only when the system is consid- 

 ered as a condensed system. 



(9) Das binare System Magnesiumoxyd-Silicium-2-oxyd. Olaf Andersen und N. L. Bowen. 

 Z. anorg. Chem^ 87, 283-299 (1914.) 



A German translation of "The binary system MgO-Si02" (Am. Jour. Sci. 

 (4), 37,' 487-500: 1914). Reviewed under No. 8 above. 



