320 



ANDERSEN: PROPERTIES OF PYROPHOSPHATES 



Fig. 1. 



negative a-axis. By this arrangement the composite crystals 

 often approximate an orthorhombic shape. Sometimes the 



crystals are grown together 

 in a way indicative of poly- 

 synthetic twinning. No defi- 

 nite twinning law however 

 could be inferred from these 

 intergrowths. The crystals 

 are colorless and of a glassj^ 

 luster. The hardness is a 

 little less than 4. Cleavage 

 perfect after the faces of the 

 prism z and also good parallel 

 to c. Density determina- 

 tions with pycnometer gave G(/) = 3.058.- 



Optical properties. Because of the good cleavages most of the 

 attempts to grind oriented sections of the small crystals failed. 

 Only one thick slice approximately parallel to (010) was ob- 

 tained. The optical properties have therefore been studied 

 chiefly on fine powder or small cleavage pieces under the micro- 

 scope. The results are accordingly incomplete and only of ap- 

 proximate accuracy. 



The refractive indices were determined by the immersion 

 method in sodium hght with the following results :7Na = 1-615 

 ± 0.003; /3n, = 1.604 ± 0.003; «n, = 1.602 ± 0.003. 



According to these determinations the maximum birefringence 

 is: 7 - a = 0.013 ± 0.005 The optical character is positive. 

 The axial angle was determined under the microscope on small 

 gi-ains showing both axes near the center of the field. Average 

 *of 4 determination of reasonable sharpness gave 2V = 20^° 

 (2E =-33°). The optical orientation is the following: Bxa = 7 

 approximately parallel to a-axis. (8 parallel to 5-axis. Bxo = a 

 forming approximately 15° with c-axis in obtuse angle (i.e., being 

 nearly perpendicular to c (001). 



2 G. W. Clarke (Amer. Journ. Sci. III. 14 : 281. 1877) gives G = 2.598 (22°) and 

 2.559 (18°). These values, however, can not refer to the crystallized substance, 

 but may represent the density of the amorphous powder. 



