318 Shepherd, Rankin, Wright — Binary Systems of 



is not strong and only rarely were interference colors as high 

 as yellow red of the first order observed, and even then the 

 color appeared slightly abnormal, due evidently to the effect of 

 overlapping fibers. The optic axial angle is large and the 

 optical character apparently negative, but not easy to deter- 

 mine satisfactorily because of the aggregate effect of superposed 

 fibers. The plane of the optic axes is parallel with the elonga- 

 tion of the fibers. The fibers show parallel extinction with the 

 ellipsoidal axis c parallel with the direction of elongation. 

 Some of the more deeply colored grains are pleochroic with 

 a = bine green, c=olive green. Absorption a> c. 



These properties indicate that this compound is probably 

 orthorhombic in crystal system. Compared with the other 

 members of the series, its chief characteristics are the refrac- 

 tive indices about 1*69, weak birefringence and tendency to 

 fibrous development. 



Evidence of solid solution of other compounds in this form 

 was looked for but was not decisive. In a preparation con- 

 taining 50 per cent A1 2 3 the unstable form was observed and 

 there appeared to have slightly higher refractive indices, but 

 the differences were only in the third decimal place and practi- 

 cally within the errors of observation. 



t The CaO.Alfi, compound: CaO 35' U, Alfi, ^tftf.—No 

 single crystals of this compound were obtained and the deter- 

 mination of the crystal system rests entirely on the optical 

 data. In one preparation of this composition from the iridium 

 furnace elongated needles and prisms were observed, but on 

 examination were found to be not single crystals but intricately 

 twinned individuals with only indications of poorly developed 

 crystal faces. 



Twinning is a characteristic feature of this compound and 

 is especially noticeable on sections nearly normal to the acute 

 bisectrix. Such sections are intricately divided into a hex- 

 agonal mesh work of interlocking sextants which extinguish 

 in different positions. On such sextants the plane of the optic 

 axes was found to be usually normal to an edge. On plates 

 cut at an angle with the acute bisectrix, polysynthetic twinning 

 lamellae were often observed and in aspect were not unlike 

 plagioclase lamellse. The general development of the mate- 

 rial from the crystallized melts is prismatic with a tendency 

 toward fibrous character. Cleavage is occasionally indicated 

 and is then parallel with the direction of elongation and appar- 

 ently prismatic or pinacoidal in character. The fibers extin- 

 guish often parallel with their elongation, the ellipsoidal axes 

 a being then parallel with the long direction ; but in many 

 sections the extinction is not parallel with the prismatic axis 

 and makes large angles with the same. 



