234 Allen, Crenshaw, Johnston, and Zarsen — 



forms and the unknown limiting values would introduce a 

 greater ambiguity, yet crystals for which. p Q is considerably 

 less than 1*0000 are likely to be a-pyrrhotite while those for 

 which p Q is considerably greater than 1*000 are probably /3- 

 pyrrhotite. 



Natural Pyrrhotite. 



Pyrrhotite has generally been considered hexagonal, but as 

 early as 1878 Streng* suggested that it was orthorhombic and 

 isomorphous with sternbergite. However, in 1882 hef con- 

 cluded that the mineral was hexagonal from a study of the crystal 

 form, etch figures, heating curve, and magnetic properties. 

 Yrba,$ Frenzel,§ and Dom Pedro von Sachsen-Coburgj| 

 measured crystals whose angles indicated orthorhombic sym- 

 metry. Crystals associated with limestone containing garnets 

 are described by Koth^f as elongated along a horizontal axis 

 and hence orthorhombic in habit. ISTicoP* described crystals 

 with a "decided orthorhombic appearance." Recently Weissff 

 studied the magnetic properties of pyrrhotite and concluded 

 that it was probably monoclinic but that it could not have a 

 higher symmetry than orthorhombic. Kaiser*):*): concluded, 

 from a study of magnetic and other properties, that the mineral 

 occurred in twinned orthorhombic crystals. 



There has also been a lack of agreement in the measured 

 angles and length of the vertical axis. Table XIII gives the 

 important crystal data for the eleven measured crystals 

 of natural pyrrhotite which seemed to be most reliable. 

 The first column contains the name of the author ; the second, 

 the observed faces in the order of their prominence on the 

 crystal giving the hexagonal symbol assigned by Dana ; 

 the third column gives the habit of the crystal; the 

 fourth column, the most probable value of p . The occurrence 

 and associated minerals are listed in the fifth column. The 

 values for the constant p Q are reliable only for the crystals of 

 Busz, Nicol, Seligmann, Kenngott, Rose, and perhaps Shep- 

 hard. The crystal of D'Achiardigave values of p Q ranging from 

 0*9658 to 1*0240 depending on which pyramid was used in the 

 calculation. That of Dom Pedro gave no good measurements. 

 The two crystals of Streng and the one of Dana afforded 

 measurements on only one pyramid and the value of p Q de- 

 pends upon the symbol assigned to it. 



* Jahrb. Min., p. 797, 1878. f Jahrb. Min., i, p, 183, 1882. 

 \ Zs. Kryst. Min., iii, p. 190, 1879. § Min. Petr. Mitt., iii, 297, 1881. 

 I Min. Petr. Mitt,, x, 451, 1888. *fZ. Kryst. Min., ix, 309. 1884. 

 **Zs. Kryst. Min., xxxi, 53, 1899. 



ft Jour, de Phys., pp. 469 and 829, 1905; Centralbl. Min., p. 338, 1906. 

 XX Centralbl. Min., p. 261, 1906. 



