322 Shepherd, Rankin, Wright — Binary Systems of 



In convergent light a faint optically negative, uniaxial inter- 

 ference figure was observed. In short, the optical charac- 

 teristics of artificial corundum were, so far as determined, 

 practically identical with those of the natural mineral. 



Silicinm oxide. — In the paper on the lime-silica series,* the 

 thermal and optical behavior of silica at high temperatures 

 was described. Recent work on the silica problem, at low 

 temperatures, has shown it to be much more complicated 

 than was at first supposed. In fact several phases have now 

 been found to occur in that region which were not disclosed by 

 the first investigation. The problem as a whole is not simple and 

 has not yet been satisfactorily solved, so that in the following 

 paragraphs only a report of progress can be made. Apparently 

 six distinct phases occur : a-quartz, /3-quartz, a-tridymite, 

 /3-tridymite, a-cristobalite, and /3-cristobalite. These, will be 

 considered in the order named. 



a-quartz, or simply quartz, is the ordinary quartz of mineral- 

 ogists, and requires no further mention. On heating to 575° 

 it passes into /3-quartz, which is also hexagonal but trapezohedral- 

 hemihedral in its symmetry relations and in other respects 

 slightly different from a-quartz. The change at 575° is rever- 

 sible and is exceedingly sensitive to minute temperature varia- 

 tions, a rise or fall of 1/10° at the inversion temperature being 

 sufficient to cause the inversion. These relations have been 

 described in detail and the literature references given in a 

 recent paper in this Journal, f 



Tridymite (a-tridymite) occurs in nature in flakes of hex- 

 agonal outline. It has been made artificially by several different 

 methods but in practically every case in the presence of a flux.J 

 At ordinary temperatures tridymite is intricately twinned, 

 biaxial and apparently orthorhombic in symmetry (pseudo- 

 hexagonal). On heating, the crystals become uniaxial at about 

 130°, § and the complicated twinning disappears ; the expansion 

 coefficient also changes abruptly at this temperature. [ On cool- 

 ing the reverse process takes place slowly and the change is 

 therefore enantiotropic. This inversion of a- to /3-tridymite 

 occurs without evidence of great strain or fracturing of the 

 crystals and it is probable that the specific volumes of the two 

 phases are nearly equal. The fact that natural tridymite crystals 

 are hexagonal with respect to outline and orthorhombic in optic 

 properties, while at 130° they invert to a truly hexagonal sub- 

 stance, indicates that in all probability such hexagonal plates 



* Day, Shepherd and Wright, loc. cit. 



f Quartz as a Geologic Thermometer, this Journal (4), xxvii, 421-447, 1909. 



\ Literature references cited in Hintze, Mineralogie I, 1459-1462. 



§ Mallard, Bull. Soc. Min., xiii, 169, 1890. 



I Le Chatelier, Compt. Rendus, cxi, 123, 1890. 



