224 



Allen, Crenshaw, Johnston, and Larsen — 



gated in the direction of the a-axis. The nntwinned crystals 

 are also nearly always much elongated along the a-axis, and 

 have as their principal forms {001], {010}, {111 }. Tabular 

 crystals of hexagonal shape are rare. 



Crystals A to H are from two different preparations 

 formed by the action of H 2 S on an acid solution of FeS0 4 

 at a maximum temperature of 225° C. They are about 

 0-3 X 0-15 X 0*05 mm in dimensions. Most of the crystals are 

 tabular parallel to the base and elongated along the a axis; 

 they are twinned after (023); {001}, {010}, and {110} are 



Fig. 19. 



Fig. 19. Stereographic projection of a simple crystal of a-pyrrhotite 

 showing the development of the striation and vicinal faces. 



the dominant forms. Fig. 17 shows one of these crystals. 

 Crystal H simulates a tabular hexagonal crystal ; it is not 

 twinned and is bounded by the forms {001}, {010}, {110}, 



{011 



L12[, {021}, and {111}. Crystals of this type are 



uncommon. 



A third preparation was formed at about 200° C. The 

 inside tube contained 3 g. FeCl 2 , l cc 20 per cent HC1, and 25 cc 

 boiled H 2 0. After heating at about 200° C. for several days, 

 the sealed tube was allowed to stand at room temperature from 

 June 16, 1909 to March 23, 1910. There was a considerable 



