JULIEJV, GENESIS OF ANTIGORITE AND TALC 35 



then known concerning the products of weathering of ferro-magriesian 

 minerals and rocks, may be briefly reviewed in connection with each 

 mineral. 



The Genesis of Talc 



In regard to talc, T. S. Hunt'* in 1860 made the following suggestion, 

 without further elaboration : 



"While steatite has been derived from a compound like sepiolite, the source 

 of serpentine was to be sought in another silicate riclier in magnesia." 



In this, his conjecture concerning talc was a happy one and was ap- 

 proved by Delesse in 1861. By neither was there ever advanced any 

 explanation or proof and the fleeting suggestion dropped from view. 



Taking for example a single mineral, olivine, as the source of talc, as 

 in the peridosteatites of Maryland and North Carolina, the following 

 genetic equation, for direct alteration of olivine into talc, has been pro- 

 posed :^ 



4(Mg, Fe), SiO,-5(Fe, Mg)0-^H,0=H,Mg3(Si03)4 

 Olivine Iron oxide and Talc 



magnesia 



So also the derivation of talc from tremolite or enstatite has been at- 

 tributed to attack by carbonated waters, as explained by the reactions^ 



CaMgaSi^Oij + H2O -f- CO, = H.MgaSi^Oi^ + CaCOj 



Tremolite Talc Caleite 



Mg4Si,0,2 4- H,0 + CO, = H,Mg3Si,0„ + MgCOs 

 Enstatite Talc Magnesite 



According to another authority :'' 



"Talc forms in the upper zone of metamorphism. In this respect it is like 

 chlorite and serpentine. It is especially likely to form under conditions of 

 weathering. ... It appears to be one of the end products of rock alteration 

 in the belt of weathering." 



Yet in the decay of olivine, for example, on weathered outcrops of 

 dunite or other peridotite, while there can be no doubt of the removal 

 of iron oxide and magnesia and of absorption of water, not a trace of 

 newly formed talc has ever been distinguished among the products of 

 decay. Furthermore, the above equations take no note of the free silica 

 which has universally separated in abundance during development of 



* Chem. and Geol. Essays. Boston, 206. 1875. 

 s J. H. Pratt and .T. V. Lewis : N. C. Geol. Survey, I. IfiO.'i. 

 8C. H. Smyth, Jr.: Sch. of Mines Quart., XVII, .^.SS. 1896. 



^C. R. Van Hisb : Treatise on Metamorphism, U. S. Geol. Surv. Monogr. 351. Wash- 

 ington. 1904. 



