ANDERSEN: PROPERTIES OF PYROPHOSPHATES 



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TABLE 1 

 Angles Measured on Crystals of Magnesium Pyrophosphate 



* Angles used in calculation of axial ratios. 



The plane of symmetry (010) is accordingly the plane of the 

 optic axes. As the acute bisectrix 7 is nearly parallel to (001) 

 and the obtuse axial angle is very large, basal sections show no 

 axial interference figures when examined in convergent light. 

 Sections parallel to (010) extinguish nearly parallel to the cleav- 

 age lines of (001) and at an angle of about 15° with the c-axis 

 (i.e., cleavage lines of (110). 



MANGANESE PYROPHOSPHATE 



The manganese pyrophosphate (Mn2P207) forms small pris- 

 matic crystals composed of a great number of thin prisms in 

 parallel intergrowth. The crystals are monoclinic and, as far 

 as the measurements go, show very nearly the same crystallo- 

 graphic properties as the magnesium pyrophosphate. Only two 

 independent angles could be measured, however, and the com- 

 plete axial ratios could not be established. The following incom- 

 plete axial ratios were calculated: a -.b :c = 0.7834 : 1 :?; /3 = 74°9'. 



The forms c (001); a (100), and z (110) were observed on 4 

 measured crystals. The angles are given in Table 2. 



