A PYRRHOTITIC PERI DOT ITE 133 



composition of the original minerals involved. The feldspar of the 

 rock, andesine-labradorite, has the composition Na^O • 2CaO • 

 3AI2O3 • loSiO^ and the olivine 2(MgFe)0 • SiO^. In the former 

 the molecular proportion of soda to lime is as 1:2. In any reaction 

 in which andesine-labradorite and olivine were involved without 

 outside gain or loss, the resulting product should show a similar pro- 

 portion between Na^O and CaO. As shown by the microscopic 

 studies, the resulting mineral is hornblende, but an examination of 

 the available hornblende analyses shows that in nearly every case 

 the soda is greatly subordinate to the lime, the molecular proportions 

 ranging from soda i and lime 12 to soda i and lime 3, but in no case 

 approaching the proportion of 1:2. The reaction probably involved, 

 therefore, either a loss of soda, or a notable addition of lime, or both. 

 In the more altered portions of the rock the hornblende, as already 

 stated, occurs in larger, massive crystals which occur not alone between 

 feldspar and the femic minerals but in contact with other constituents. 

 In some places areas originally occupied by plagioclase now show 

 only a few feldspar remnants entirely surrounded by hornblende. 

 Alterations of the type just described are probably of somewhat 

 common occurrence among peridotites though readily traceable 

 only in rocks which show the initial stages of alteration. Few 

 instances are described in the literature. Van Hise' makes the follow- 

 ing statement in regard to a reaction between olivine and anorthite : 



In some instances the alteration into actinolite is described as taking place 

 in connection with feldspar as a reaction rim. In this case the calcium may be 

 supposed to be derived from anorthite, as calcium silicate. The aluminum may 

 be supposed to pass into common spinel and hercynite (isometric; sp.gr. 3.93) 

 which are well known to be alteration products of olivine. The reaction may be 

 4Mg3FeSi208 + 4Ca AUSi^Og = Mg9Fe3Ca4Sii6048 + 3MgAl,04 + FeAL04 • 



Olivine Anorthite Actinolite Common Hercynite 



spinel 



Tornebohm in a description of some of the diabases and gabbros 

 of Sweden describes an alteration to hornblende almost identical 

 with that observed in the Maine rock. 



The alteration of the olivine, as usually exhibited in the rocks under discussion, 

 proceeds exclusively from the borders of the grains. There forms next to it a 



I C. R. Van Hise, Treatise on Metamorphism, Mon. XLVII, U. S. Geological 

 Survey, pp. 310, 311. 



