ZINC ORES OF THE EDWARDS DISTRICT 19 



from tremolite. The nature of the change in the latter case may be 

 expressed by the following equation: 



Tremoli^ Talc 



Mg 3 CaSi 4 12 + H 2 + C0 2 = Mg 3 H 2 Si 4 O 12 + CaCO 3 . 



It is noticeable that, in the cases of both diopside and tremolite, 

 the active agents of alteration suggested by these equations are the 

 same water and carbon dioxide. 



As talc is less hydrated than serpentine, it might be expected to 

 form at a higher temperature, that is, higher up on the descending 

 portion of the curve of metamorphism ; and such is the fact, talc 

 being distinctly the older of the two minerals. This is shown by 

 the way in which serpentine eats into, or cuts squarely across, tufts 

 and plates of talc and, even more strikingly, by crystals of tremo- 

 lite or diopside, altered to talc, traversed by a network of serpen- 

 tine. In, these cases, serpentine has grown at the expense of, and 

 replaced, talc and, to this extent, talc has been an intermediate stage 

 in the production of serpentine. The same may be true of the large 

 nodules of talc surrounded by serpentine. 



The alteration of diopside to talc may be represented as follows : 



Diopside Talc 



3CaMgSi 2 O c + H 2 O + 3CO 2 =Mg s H 2 Si 4 O 12 + 3CaCO 3 + 2SiO 2 . 

 The further stage of alteration of talc to serpentine may be repre- 



Talc Serpentine 



sented as follows : H 2 Mg 3 Si 4 O 12 + H 2 O = H 4 Mg 3 Si 2 O 9 + 2SiO 2 . 



While, however, the alteration of diopside to serpentine some- 

 times passes through the intermediate stage talc, this is evidently not 

 essential to the process as, in nearly every section where the change 

 can be seen, the passage is direct from diopside to serpentine. 



Aside from the minerals briefly described, the wall rocks con- 

 tain nothing of moment except occasional scattered grains of the ore 

 minerals. As the latter increase in quantity, the rock gradually 

 passes into ore, there being, often, no sharp line of demarkation 

 between the two. In consequence, thin sections of the ores present 

 the same minerals as those of the wall rock, with the addition of the 

 metalliferous sulphides. The latter are blende (sphalerite), the 

 sulphide of zinc; pyrite, the sulphide of iron; and galena, the 

 sulphide of lead. Of these three minerals, the first two are of com- 

 mercial value in the deposits here considered. 



In thin sections, the pyrite and galena are both opaque, the former 

 being brass yellow and the latter lead gray in reflected light. The 

 blende is translucent and ranges from deep reddish brown through 

 light yellow to nearly colorless. As seen with the naked eye, these 

 minerals, excepting, perhaps, the galena, which is present in such 



