and their Pseudomorphs. 159 



pseudomorphism, the form of the pyrrhotite alone remaining, 

 in the substance of the limonite. In such cases, the resultant 

 crystal is usually honeycombed and cavernous, though often 

 having a nearly complete external shell. Sometimes the shell 

 is quite hollow, though extremely smooth and lustrous on the 

 surface. There is a complete analogy with the pseudomorphs 

 described by Lacroix,* except in the matter of magnitude, the 

 crystals here considered being much smaller. But the analogy 

 does not cease here since Lacroix's pseudomorphs of pyrite and 

 marcasite are paralleled by instances of pseudomorphs of 

 pyrite after the pyrrhotite. In every case noted, these pseudo- 

 morphs are either hollow, consisting of a mere shell of pyrite 

 preserving the form of pyrrhotite, or are partly filled with 

 cavernous limonite. 



While the typical limonite pseudomorphs often preserve the 

 forms of the pyrrhotite crystals very precisely, even to the 

 striation of the prism faces, the pyrite pseudomorphs always 

 have a rather rough, irregular surface. Apparently, the 

 process begins with the growth of a coating of pyrite crystals 

 upon the pyrrhotite, the latter mineral being subsequently 

 more or less completely decomposed and removed, leaving a 

 hollow shell of pyrite. The- individual crystals of pyrite are 

 very minute, and their form can be seen only with the micro- 

 scope, and then imperfectly, but enough octahedral faces are 

 shown to warrant the conclusion as to the nature of the 

 mineral. In a few cases the pyrite has built up the edges of 

 the pyrrhotite, forming pronounced ridges, with the faces as 

 depressions between them. On account of the small size of 

 the secondary crystals, it is impossible to determine whether or 

 not they are definitely oriented with reference to the pyrrho- 

 tite, but apparently they are not. For the same reason, no 

 final conclusion can be reached as to the possible presence of 

 marcasite in addition to the pyrite. It might well be present 

 and escape detection. 



Evidently limonite pseudomorphs have sometimes formed 

 directly from the pyrrhotite and, again, they have followed 

 upon the conversion of the latter mineral into pyrite, the 

 former case giving more perfectly formed pseudomorphs. 



While there seems to be no doubt as to the general tendency 

 of pyrrhotite to alter into marcasite and pyrite, the conditions 

 leading to such changes are not clear, particularly in the case 

 of pyrite. The latter mineral has the greater specific gravity, 

 and higher grade of symmetry, favoring its formation under 

 deep-seated conditions, and Van Hisef concludes that " doubt- 

 less when the necessary chemical reactions can take place there 



*Loc. cit. 



f Van Hise, C. R., Treatise on Metamorphism, Monograph 44, U. S. Geol. 

 Survey, p. 216. 



