322 



ANDERSEN: PROPERTIES OF PYROPHOSPHATES 



TABLE 2 

 Angles Measured on Crystals of Manganese Pyrophosphate 



* Angles used in calculation of axial ratios. 



Figure 2 shows the general appearance of a complex crystal. 

 The prism z is always the prevailing form. The 

 small faces of c terminate the narrow prisms. 

 The pinacoid a only occurs as very narrow 

 truncations of some of the edges z : z. The 

 faces were all bright, especially z, which was ob- 

 served both as crystal faces and cleavage faces 

 and gave good signals. 



The color of the crystals is brownish pink; 

 the powder is light pink. The luster is glassy. 

 Hardness about 4. Cleavage perfect along the 

 prism z and poor parallel to c. The density de- 

 termined with pycnometer is 0(4) = 3.707.^ 



Optical properties. Cleavage pieces and fine 

 powder only were examined under the micro- 

 scope. No material for more elaborate examination was avail- 

 able. The refractive indices for sodium light were determined 

 by the immersion method TNa = 1-710 ± 0.003; ^Na = 1-704 ± 

 0.003; ttNa = 1-695 ± 0.003 from which the birefringence y - a 

 = 0.015 ± 0.005 is deduced. 

 The optical character is determined as positive.'^ The axial 



3 G. W. Clarke (Amer. Journ. Sci. III. 14: 281. 1877) states G = 3.5847 (20° 

 and 3.5742 (26°). These low values may be due to impurities. 



* From the determinations of the refractive indices giving y-fi<0-a a negative 

 optical character should be inferred. It is however plainly seen that a variation 

 of the indices within the limits of error may change their relation so as to make 

 the optical character positive. 



Fig. 2. 



