190 Allen, Crenshaw, Johnston, and Larsen — 



experiment was repeated several times with similar results. 

 Under these Conditions, there is a plain evolution of heat 

 accompanying the change of marcasite into pyrite. This 

 shows, of course, that marcasite possesses the more energy* of 

 the two and is a monotropic form. This condition of instability 

 is in accord with the more rapid oxidation of marcasite in 

 nature, and in it is probably to be found the reason for the dif- 

 ference in behavior between marcasite and pyrite toward other 

 oxidizing agents. Monotropic forms often crystallize from 

 some particular solvent or within a limited temperature range. 

 The formation of marcasite from acid solutions is in accord 

 with this, though as yet we do not understand the reason for 

 it. A rise in temperature doubtless increases the velocity of 

 the change marcasite -> pyrite. At low temperatures this 

 becomes infinitesimal or zero ; above 450° it becomes measur- 

 able. This irreversible relation has a bearing on the question 

 of paramorphs of iron disulphide, for it is impossible to see 

 how pyrite could change to marcasite without first passing into 

 solution, while the opposite change is experimentally estab- 

 lished. Paramorphs of pyrite after marcasite are certainly 

 possible, but paramorphs of marcasite after pyrite are evidently 

 impossible. 



The agency of organic matter in the formation of natural 

 pyrite and marcasite. — The fact that pyrite is sometimes found 

 in nature replacing wood has been alluded to. Liversidgef gives 

 an example of recent pyrite which is found on twigs in a hot 

 spring at Tampo, N. Z. The sulphides of southwestern Mis- 

 souri, including pyrite and marcasite, are frequently associated 

 with asphaltic matter, and in Oklahoma this is sometimes so 

 great in quantity as to interfere with the concentration of the 

 ores. (Lindgren.) It is pretty generally believed by geolog- 

 ists that the organic matter of certain shales acted as a precip- 

 itant of the pyrite they contain. Such a shale underlies the 

 sulphide deposits of Wisconsin. We learn from Mr. W. H. 

 Emmons of the IT. S. Geological Survey that this shale con- 

 tains a small quantity of hydrogen sulphide, which naturally 

 may have been the precipitating agent. Coals, also, in which 

 pyrite is commonly found, are frequently permeated with 

 hydrogen sulphide. 



The role of micro-organisms in the formation of iron di- 

 sulphide. — The question naturally arises whether there is any 

 connection between organic matter and the formation of 



* Cavazzi (Rend. Accad., Bologna, N. S., ii, 205, 1898) states that the 

 heats of combustion of pyrite and marcasite are identical (1550 cal.). This is 

 certainly incorrect. 



f J. Royal Soc. N. S. Wales, xi, 262, 1877. 



