GEOPHYSICAL LABORATORY. 145 



The importance is emphasized of bringing all minerals that behave in this 

 way to a definite maximum water-content before analyzing them and of fol- 

 lowing carefully the course of dehydration under prescribed conditions. De- 

 tailed directions are given for such tests and for avoiding several sources of error. 



Attention is also called to two fairly constant associates of metahewettite. 

 One of these (also a constituent of carnotite ores) is a gray hydrous silicate 

 of aluminum, trivalent vanadium, and potassium. The other is elemental 

 selenium, the existence of which as a mineral species seems now for the first 

 time established. 



(13) Hewettit, Metahewettit, und Pascoit, Calcium Hydro vanadate. W. F. Hillebrand, 



H. E. Merwin, und Fred. E. Wright. Z. Kryst. (In press.) 



A German translation of "Hewettite, metahewettite, and pascoite, hydrous 

 calcium vanadates" (Proc. Am. Phil. Soc, 53, 31-54, 1914). Reviewed under 

 No. 12 above. 



(14) The ternary system: diopside-forsterite-silica. N. L. Bowen. Am. Jom-. Sci. (4), 38 



207-264 (1914). 



The results obtained in the investigation of the three binary systems 

 involved are first presented. The system diopside-silica shows the simple 

 eutectic relation, as does also the system diopside-forsterite. The system 

 forsterite-silica shows one intermediate compound, MgSiOs (clino-enstatite), 

 unstable at its melting-point. 



In the ternary system it is found that clino-enstatite and diopside form a 

 complete series of solid solutions (monoclinic pyroxenes) and therefore have 

 a common field. As a further consequence of this unbroken series of solid 

 solutions there is no ternary eutectic, the lowest point of formation of liquid 

 in the system being the binary eutectic diopside-silica. 



A brief theoretical discussion of solid solution in ternary systems is given. 

 The course of crystallization in typical mixtures of the present ternary system 

 is described, and the value of certain lines termed three-phase-boundaries is 

 pointed out, especially their usefulness in determining the composition of mix- 

 crystals separating at any temperature. In considering crystallization it is 

 shown that crystallization may proceed according to two different methods: 

 first, that in which the liquid is, at any temperature, in equilibrium with all 

 the crystals and all parts of the crystals occurring in it, and second, that in 

 which the liquid is in equilibrium at any temperature only with the crystals 

 separating at that temperature. In a general way, crystallization of the 

 second type is favored by quick cooling. 



The importance of distinguishing between the two types of crystallization 

 is great in the present system. It is shown that the difference between the 

 conclusions arrived at in the present work and those arrived at in an earlier 

 investigation of the pyroxene series carried out at this Laboratory is due largely 

 to the fact that in the earlier work crystallization of the second type occurred 

 in many of the mixtures, whereas, if equilibrium is to be studied, crystallization 

 of the first type must be obtained. 



In the optical part of the paper the optical properties of the various crystal- 

 line phases are given. The properties of the series of monoclinic pyroxenes 

 extending from diopside to enstatite vary continuously with composition. 

 The artificial pyroxenes are compared with the enstatite-augites of Wahl. 



In considering the bearing of the results on petrologic problems attention is 

 called to the resorption of the olivine forsterite in the artificial mixtures and 

 its probable relation to resorption of olivines in natural rocks. The conse- 

 quences of the possible settling of crystals in a fluid magma (crystallization- 

 differentiation) is discussed in the light of the facts known concerning the 

 artificial mixtures. 



