298 Day and Shepherd — Lime-Silica Series of Minerals. 



The an gle between the prism faces and the rhombohedrons 

 (1010 : 1011) measured 37° 48' and differs appreciably from 

 that of pure quartz, which is given by Dana, 38° 13'. The 

 three rhombohedron faces from which this angle was obtained 

 gave perfectly sharp reflection signals, and although the obser- 

 vations on the goniometer were made with the reducing attach- 

 ment, the difference no doubt exists and is due probably to a 

 solid solution of quartz with some other ingredient of the 

 original mixture. The hardness of the crystals is 7 ; their 

 specific gravity, 2*650 determined in Thoulet solution, and 

 their refractive indices e = r654±'002, to co = l-644±'002; 

 measured by the method of refractive liquids, their birefring- 

 ence is abont *01. The crystals are unaxial and optically posi- 

 tive, and extinguish parallel to the prism edge, which was 

 found to be the direction of the least ellipsoidal axis c. 



Quartz could not be produced by direct crystallization from 

 silica glass, since at low temperatures at which it is stable the 

 viscosity of the glass is too great to allow sufficient molecular 

 mobility for the rearrangement. Fluxes or solutions were 

 therefore used to increase this mobility, and with satisfactory 

 results. Quartz crystallites were formed at temperatures 

 below 760° and tridymite above that point in. the same flux. 



The chief effect of such crystallizing fluxes or crystallizers 

 seems to be that they tend to increase the molecular mobility 

 of the crystallizing material and thus procure greater freedom 

 and power for crystallization. In many experiments per- 

 formed in this laboratory it has been noted that crystals 

 obtained directly from silicate melts are usually minute and 

 ill-formed ; while the addition of a few per cent of a second 

 substance improves both size and quality of the crystals to a 

 remarkable degree, even though the added substance may 

 solidify as glass and enter apparently in no wise into the com- 

 position of the crystals. 



(b) Tridymite. — In the experiments, well developed crys- 

 tals of tridymite of sufficient size for goniometric measure- 

 ments were not obtained, and optical properties alone were 

 relied upon for its identification. The refractive index 7 was 

 measured 1*485±'003 on one preparation and 1"483±*003 on 

 a second by the method of refractive liquids. These values 

 are slightly higher than that given for natural tridymite, 

 which is 1'478. No reason has yet been suggested to explain 

 this discrepancy. The birefringence is extremely low, appar- 

 ently not over *002» The crystallites are biaxial with an 

 optical axial angle so large and indistinct that the optical char- 

 acter could not be determined satisfactorily. In some of the 

 crystals an elongation in a direction diagonal to the positions 

 of extinction was noticeable. The best preparations of tridy- 



