Pctrologn atid Geolojjy of Qacnii^totCH. 129 



gular shapL^d grain of pyi'rliotite, suggesting an intergrowtli of the 

 two niinerals. However, ai'ound the t'dges of the pyrrhotite. some 

 leucoxene is seen, strongly suggesting that the pyrrhotite is re- 

 placing the leucoxene, and that tlie bars ai'e relics of a former 

 ihuenite grain. The iron, set free from the ilmenite in the change 

 to leucoxene, lias apparently united with sulphur from hydrogen 

 sulphide in the gases or solutions, causing pi'opylitisation, and 

 pyrrhotite has formed. 



Of late, much attention has been devoted in the laboratory to 

 the study of the iron sulphides, and we are particularly indebted 

 to American chemists for valuable syntheses of them. Of parti- 

 cular interest are the results of Allen, Crenshaw, and Johnston at 

 the Carnegie Institute, Wasliington.i They have artificially pre- 

 pared pyrrhotite by the direct union of iron and sulphur at 700o, 

 and have shown that pyrite dissociates into pyrrhotite and sulphur 

 in an atmosphere of sulphuretted hydrogen, at temperatures above 

 5650. The action is represented by the following equation : — 

 FeS2^FeS(S)„ + (l-n)s 

 Pyrite Pyrrhotite Sulphur. 

 Above 565° the forward action proceeds, and below 565^ pyrrhotite, 

 in an atmosphere of hydrogen sulphide, passes into pyrites. The 

 formation of pyrrhotite, and not pyrites in the Queenstown diorite, 

 apparently by the attack of hydrogen sulphide on the ilmenite, 

 should therefore have taken place at high temperature; however, 

 not necessarily as high as 565, o as the dissociation temperature is 

 dependent on the pressure. Pyrrhotite has also been foi-med synthe- 

 tically by these authors in an acid environment, at temperatures 

 between 80o and 225, o but the syntheses ai-e undoubtedly inap- 

 plicable in our case, since the solutions causing propylitisation 

 must have been alkaline. 



Recapitulating, it is seen that tlie minerals formed during the 

 propylitisation of the diorite, are, in their geological occurrence, 

 usually fairly high temperature minerals. Microscopic and field 

 evidence show that the propylitisation and sericitisation are inde- 

 pendent, and that the effects of the latter alteration are superim- 

 posed upon those of the former. The chemical analysis of the rock 

 shows, further, that the gases or solutions causing the alteration 

 must have contained water, carbon dioxide, and sulphides. The 

 composition of the original rock does not appear to have been 



1 "The Mineral Sulphides of Iron." Anier. Jour. Sci., 4th sen, vol. xxxii., 1911. 



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