4:2 Allen, etc. — Diopside and its ^Relations 



In Part I of this paper, the fact of eutectic mixtures and 

 its bearing on the present problem are shown to be of prime 

 importance. In the case of alloys eutectic textures are 

 definitely recognized and it is natural to expect such textures 

 in silicate melts. In the latter, however, power and rapidity 

 of crystallization, combined with viscosity, frequent absence of 

 stable equilibrium, and other factors, tend usually to veil 

 effectively such textures which might otherwise develop. 

 Indications of probable eutectic textures were occasionally 

 recorded in this series, but as a rule the crystallization appar- 

 ently takes place so rapidly with strong undercooling that 

 normal, theoretical conditions of equilibrium do not exist. 



It is also of interest to note that throughout this series 

 the melts of the pure compounds are, as a rule, of coarser 

 grain than the intermediate compositions and of more vitreous 

 luster. The intermediate mixtures frequently resemble porce- 

 lain in appearance. This change in aspect undoubtedly results 

 from the decrease in granularity and lack of continuity of the 

 single crystallites. 



The inversion of the a- and /3-MgSi0 3 . — With pure MgSiO s - 

 melts the thermal data show practically no heat effect at the 

 temperature of inversion of the fi- into the a-form and vice 

 versa, and only after the admixture of several per cent of 

 diopside does the thermal effect produced by the inversion 

 appear. The quenching experiments of Dr. A. L. Day, how- 

 ever, proved definitely that the a-MgSi0 3 did exist, and 

 attempts were then made to fix the temperature of inversion 

 by use of a specially constructed thermal microscope.* (Fig. 

 13, a and b.) 



* Constructed in the workshop of the Geophysical Laboratory after plans 

 by Dr. Arthur L. Day and the writer. The details of construction of the 

 electric resistance furnace are given in fig. 136, two important features of 

 which were suggested by Dr. Day, namely, the enclosing of the whole in 

 a suitable water jacket and the splitting of the thermoelement wires to serve 

 as a support for the preparations. By this latter device the purity and 

 temperature of the preparations even at high temperatures is insured. The 

 microscope is fitted with revolvable nicols ; at the base of the furnace there 

 is a thin metal slide, A 3 , by means of which part or all of the field can be 

 shaded and the characteristics of the light emitted and transmitted by the 

 body studied with respect to effects of polarization. A plate at high tem- 

 peratures may become often self-luminous and it is then necessary to 

 adopt special devices to detect transmitted polarized light and with this end 

 in view the optical system of the microscope has been arranged. With it, 

 also, the character of the emitted light alone can be examined with respect 

 to polarization effects if such exist. At high temperatures the white heat of 

 the furnace tends to veil the interference phenomena unless the transmitted 

 light be of greater intensity, and this condition has been met by using an 

 electric arc as source of light. With this furnace the birefringence of 

 quartz has been measured up to 1300° and it is proposed to study the optical 

 changes in several minerals at different temperatures in this way. — Tempera- 

 ture readings are made either roughly on a direct reading Siemens and 

 Halske voltmeter or accurately by use of the potentiometer. — The water 

 jacketing of the furnace permits its use on any microscope in which the 

 distance between the stage and the objective can be made great enough, the 

 optical system remaining thereby unchanged. 



