222 Allen, Crenshaw, Johnston, and Larsen — 



usually prominent in the natural mineral, was not observed on 

 the synthetic crystals, while the prism zone is often more 

 prominent on the artificial crystals. The natural mineral is 

 described as having the brachydomes and pinacoids deeply 

 striated parallel to the edge (010) (001) ; the synthetic mineral 

 is similarly striated, the pyramid and prism zones are even 

 more prominently striated parallel to the base, and the zone 

 h + I = Jc on some of the crystals is a more or less continuous 

 series of faces due to striations. 



Pyrrhotite. 



The experiments of Kinne and Boeke* show an inversion in 

 FeS with 7 per cent, the maximum amount, of dissolved iron 

 at 138°. On decreasing the amount of dissolved iron to 5 per 

 cent the absorption of heat takes place between 90° and 98°. 

 Although they were not able to observe a heat absorption in 

 FeS with less than 5 per cent of dissolved iron or in natural 

 pyrrhotite, they state that the inversion takes place but is 

 too sluggish to be observed. Ferrous sulphide with a little 

 carbon showed the inversion at 134*5°, while meteoric troilite 

 with a little carbon inverted at about 143°. 



The present crystallographic study tends strongly to confirm 

 the work of Rinne and Boeke. The low temperature form or 

 /3-pyrrhotite appears to be hexagonal, while the high temper- 

 ature form or a-pyrrhotite appears to be orthorhombic. The 

 measured interfacial angles for the two forms are near together 

 and the fact that these angles vary with the amount of sulphur 

 in excess of that required for FeS, together with the impos- 

 sibility of determining the chemical composition of the meas- 

 ured crystals, makes a comparison of the crystal constants of 

 the two forms impossible. However, the crystal habit, the 

 twinning, and the development of the faces on the two forms 

 afford good evidence for the view that they belong to different 

 crystal systems. 



The color of the synthetic, pyrrhotite is similar to that of the 

 natural mineral. That prepared by melting iron and sulphur 

 together is a little darker and has more of a grayish cast than 

 the natural mineral. The crystals from some preparations of 

 a-pyrrhotite are only slightly magnetic while those from others 

 are strongly so. The crystals always show polarity with the 

 poles along the &-axis. The poles for /3-pyrrhotite are along 

 the c-axis in the one crystal observed. 



a- Pyrrhotite. 



Twenty-two fairly satisfactory crystals of a-pyrrhotite, repre- 

 senting four different preparations, were measured on the 



* Zs. anorg. Chesiie, liii, 338-343, 1907. 



