Alumina ivith Silica, Lime and Magnesia. 319 



• The crystalline aggregates are colorless and vitreous in luster. 

 The hardness is about 6*5. The refractive indices a and 7 

 were determined on the Abbe total refractometer, while (3 was 

 measured by the immersion method. 7=1 , 661± , 002; /3= 1*654 

 ±'003; a = l*641db*002. The birefringence is fairly strong 

 and interference colors of the first to third orders are common. 

 The optic axial angle was measured by use of the double screw 

 micrometer ocular on sections showing an optic axis in the 

 field of vision. Owing to the frequent twinning some diffi- 

 culty was experienced in finding suitable sections and the 

 values obtained also varied slightly in consequence. Five 

 fairly satisfactory measurements were made and the aver- 

 age value 2V = 36°=b4° obtained. Dispersion of the optic 

 axes is very slight and ordinarily not noticeable. On one 

 section the relations seemed to be 2V p >2Y Y . Occasionally a 

 section with apparently smaller 2V than usual was observed, 

 almost uniaxial, but this was possibly due to the effect of over- 

 lapping twinning. 



These data indicate that the crystal system of the compound 

 CaO.Al 2 3 is either monoclinic or triclinic and probably the 

 former. Unfortunately, no well-developed crystals were 

 obtainable and a more definite statement in regard to the 

 symmetrv relations is not possible. 



The 3Ca0.5Al 2 0, compound: CaO 24,-78, A\0 % 7b' ( 2 ( 2.— No 

 separate crystals of this compound were obtained and the 

 evidence as to its crystal system, whether tetragonal or hex- 

 agonal, is not satisfactory. The grains are rounded and range 

 from '02 to *05 mm in diameter. Basal sections are usually with- 

 out definite outline, though occasionally there is a tendency 

 toward quadratic outline, and it is possible that the crystal 

 system is tetragonal. JSTo distinct indications of cleavage were 

 observed. Rarely rhomb-shaped to square grains were noted 

 which extinguish parallel with the diagonals and may indicate 

 poor pyramidal cleavage, or if hexagonal, rhombohedral 

 cleavage, but such grains were rare, and if cleavage be pres- 

 ent it is imperfect. The luster is vitreous and the hardness 

 about 6*5. The refractive indices were determined by the 

 immersion method: co =1-617±"002; e = l*651±*002. On a 

 section parallel with principal axis the birefringence was meas- 

 ured roughly under the microscope and the value 7 — a = '032 

 obtained. The birefringence is, therefore, fairly strong and 

 the interference colors, even in minute grains, are of the first 

 and second orders. In convergent polarized light a normal 

 uniaxial, optically positive interference figure was observed on 

 basal sections. The interference cross is well marked and on 

 thicker sections the inner edge of the first colored interference 

 ring is visible on the margin of the microscope field. In 



