160 Smyth, Jr. — X w Locality of Pyrrhotite Crystals. 



is a tendency in the Lower zone for pyrrhotite to alter to 

 l»\ rite." 



It is a fact that both minerals form readily under deep- 

 seated conditions, but it is also true that pyrite often is deposited 

 near, or at, the surface, being a common constituent of sedi- 

 mentary rocks; while on the other hand pyrrhotite is distinctly 

 rare in such environment, and apparently tends to pass over 

 into pyrite as it" conies within the influence of surface condi- 

 tions. Thus while it may be that pyrrhotite changes into 

 pyrite at great depth, it certainly does so nearer the surface, 

 although the comparatively rare occurrence of crystals causes 

 a correspondingly limited number of determinable examples of 

 the change. 



M area site, on the other hand, is a mineral of moderate depth, 

 changing under the deeper seated conditions into the denser 

 and more symmetrical pyrite. As a pseudomorph after pyr- 

 rhotite, it would ordinarily represent more superficial condi- 

 tions, and might well result from a second stage of alteration, 

 the first being the alteration of pyrrhotite to pyrite, the second 

 the alteration of pyrite to marcasite. Thus pyrrhotite, formed 

 originally at considerable depth, when brought under the influ- 

 ence of more superficial conditions becomes unstable and tends 

 to pass over into such compounds as would naturally form 

 under these conditions, and we have pseudomorphs of pyrite 

 and marcasite, sulphides that readily form at moderate depths. 

 This is in harmony with the molecular volume relations of the 

 three minerals, the molecular volume of pyrrhotite, pyrite and 

 marcasite being respectively, 19*1, 23"2 and 25'4, as quoted by 

 Grubenmann* from Becke. 



Finally, under distinctly superficial conditions, with abund- 

 ance of oxygen, the last stage of the series is the formation of 

 limonite, a most stable product under these conditions. 



While, in some cases, all of these stages of alteration might 

 be passed through, in other cases only part of them would be 

 represented, as determined by local conditions, and any stage 

 might be partial or complete. In the particular instance here 

 described, it is quite clear that there has been a partial altera- 

 tion of some crystals to pyrite followed by a more or less com- 

 plete change of the pyrite to limonite. In other crystals the 

 pyrite stage is apparently lacking, the passage from pyrrhotite 

 to limonite being direct. Finally, many crystals are quite 

 fresh and unchanged. As before stated, it is impossible to 

 determine whether or not any marcasite has been formed. 



The unusual feature of the ease lies in the fact that the first 

 stage in the whole series of operations, that is the formation of 

 the pyrrhotite, represents geological conditions quite different 

 from those under which this mineral is usually developed. 



Princeton tJniversity. 



*Grubenmann, U., Die Kristallinen Schiefer, I, p. 37. 



