ANDERSEN; PROPERTIES OF PYROPHOSPHATES 319 



Complete crystallization always resulted, no matter how quickly 

 the cooling took place and from the cavities some measurable 

 crystals could be picked out. 



MAGNESIUM PYROPHOSPHATE 



The magnesium pyi-ophosphate (Mg2P207) forms tabular crys- 

 tals of variable size, the tables generally having an orientation 

 perpendicular to the surface of the melt and roughly parallel 

 to each other or radiating from a few centers near the sides of 

 the crucible. The measm'able crystals were of the average size 

 3x2x1 mm. They were, however, always formed by parallel 

 (or subparallel) intergrowths of a number of small individuals 

 and the signals reflected from the faces of such crystals were 

 multiple and indistinct. Fairly sharp signals could be obtained 

 by stopping off the light from all the subfaces except the two 

 joining in the little edge selected for measurement. In this way 

 one crystal often yielded a number of measurable edges between 

 the same two faces. The two circle goniometer (Goldschmidt's) 

 was accordingly used as a one circle instrument. 



The crystals proved to be monochnic belonging to the pris- 

 matic class. From the average of the measurements the axial 

 ratios were calculated as follows: a ib : c = 0.7947 : 1 : 1.0880; 

 /3 = 75°49'. Referred to these axial ratios the following forms 

 were observed: c (001); (110); r (lOl); a; (112). 



The results of the measurements are listed in Table 1 contain- 

 ing the values obtained from 14 crystals, some of which gave 

 only one measm'able edge, while from others several' edges for 

 each pair of faces were obtained. 



Figure 1 reproduces the general shape of a composite crystal. 

 The predominating forms are always c and z, the crystals being ■ 

 tabular parallel to c or sometimes short prismatic along z,. while 

 the other two forms r and x only occur as narrow truncations of 

 the edges; r, however, is sometimes broad enough to give fairly 

 good signals. The prism z is not observed except as bright and 

 smooth cleavage faces. The other forms occur as natural crys- 

 tal faces always fairly bright and without striation. 



The subindividuals of the larger crystals are arranged so as 

 to make the bases form steps descending in the direction of the 



