552 Wright — Three Contact Minerals. 



lites, the individual libers of which are difficult to separate 

 satisfactorily, and rarely measures '5 mm in length. In the 

 hand specimen, especially when examined with a lens, these 

 libers tend to produce a faint silky luster on the otherwise vit- 

 reous to porcelain-like mass. Cleavage, so far as could be 

 observed, prismatic. Brittle. Hardness between 5 and 6, 

 about 5'5. Color, pure porcelain white, often with faint tinge 

 of pale green. Translucent in small chips. Streak, white. 



finder the microscope the optic properties are those of 

 aggregates of libers, often in approximately parallel orienta- 

 tion, rather than of a single liber. As a result the optical 

 data are not easy to determine with great accuracy, although 

 certain features of the mineral are so characteristic that its 

 determination as such is a relatively simple matter. 



The refractive indices <y and a were measured in sodium 

 light on an Abbe-Fulfrich total refractometer on a polished 

 plate of the mineral. It has been found by experience that 

 even in the case of such fine-grained masses as hillebrandite, 

 the phenomena in sodium light on the refractometer are suffici- 

 ently distinct, when reducing attachment is used, to permit a 

 fairly accurate determination of the two limiting curves 7 and 

 a, although in the flood of light from the different grains, the 

 medium refractive index line does not appear with sufficient 

 distinctness to allow of its determination. On such a plate 

 the refractive indices were found to be 



y = 1-612 ± '003 

 a = 1-605 ± -005 



The birefringence is medium to weak, but difficult to deter- 

 mine directly because of interweaving of overlapping libers. 



The extinction is parallel, the ellipsoidal axis (c) being 

 invariably parallel with the liber direction which at the same 

 time is the cleavage direction. The optic axial angle is not 

 very large, 2Ep being possibly between 60° to 80°, while the 

 dispersion of the optic axes is unusually strong and gives rise 

 to peculiar, abnormal blue interference colors resembling those 

 in certain epidotes and characteristic of hillebrandite. The 

 optic character is negative with 2E V > 2Ep. The plane of 

 the optic axes was found to vary, being in the one plate 

 parallel with the liber direction and in the next perpendicular 

 to the same, an abnormal phenomenon which may be due in 

 whole or in part to the disturbing influence of the interlacing 

 libers which tend to veil the optic phenomena and often most 

 effectively. On a section normal to the acute bisectrix the 

 plane of the optic axes was parallel with the cleavage and 

 direction of elongation. 



