2G ANNALS NEW YORK ACADEMY OF SCIENCES 



talc and has become either a prominent constituent of the resulting- 

 quartz-talc a<(<^regate, steatite, or a prominent associate in veins or seam& 

 in close vicinity to a talc-rock. 



Tin: (JEiNKsis of Antigorite 



For the purpose of this paper it will be inmecessary to discuss all the 

 hypotheses which have been devised, or to consider but one important 

 source, olivine. A review of tiie literature reveals, in my opinion, pro- 

 gressive but still imperfect recognition of the nature and conditions of 

 the genesis of antigorite, and, on the other hand, a growing consciousness 

 of the insufficiency of the tentative speculations concerning its location. 

 These comprise three methods of change of olivine directly into antig- 

 orite, viz : by weathering, by attack of deep-seated agencies and by com- 

 bination of both. 



FORMATION OF ANTIGORITE DIRECTLY FROM OLIVINE BY HYDRATION 



The hydration which antigorite represents toward olivine as the mother- 

 mineral, the visible evidences of its production by attack from the out- 

 side upon the olivine grains, the attending oxidation of ferrous iron, the 

 removal of certain bases and the release of free silica are obvious results 

 of the decay by weathering. What conclusion more simple and plausible 

 than that antigorite has been mainly produced in such instances by direct 

 hydration of olivine? 



An early writer (J. Roth,*^ 1869), although he had distinguished vari- 

 ous processes of weathering as simple and complex, discussed antigoriza- 

 tion under the former heading, thus. 



(lOMgO + SSiO^)— (4 MgO + SiO,) + 4H20 = (6MgO + 4Si02 + 4H20) 



5 molecules olivine 2 moleeiiles "sei'pentine" 



J. J. II. Teal'' (1S8S) was content to declare: 



"The alteration of olivine by surface asencies — water, carbonic acid and 

 oxygen^j^ives rise to seri)entines and other pseudoinorphs ;" 



and 



"the formation of serpentine by the. alteration and hydration of feri-o-magne- 

 sian and magnesian silicates is proved beyond all (piestion," 



with the equation: 



"2 (2MgO + SiO.,)-MgO + 2 H,0=(3 MgO + 2 Si02 + 2 HjO)" 

 Forsterite "Serpentine"' 



»Abh. d. k. Akad. d. Wiss. BorHn, 1809. 

 "British Petrograpliy, 104-106. liOndou, 1888. 



