[Annals N. Y. Acap. Sci., Vol. XXIV, pp. 23-38, PI. VI. 25 July, 1914] 



THE GENESIS OF ANTIGOKITE AND TALC 



By Alexis A. Julien piEW 



{Presented by title before the Academy, 7 April, lOlJf) ',.. ,^r. 



CONTENTS 



Page 



Introduction 23 



The genesis of talc 25 



The genesis of antigorite 26 



Formation of antigorite from olivine by hydration 26 



Formation of antigorite directly from olivine by thermal alteration. . . 28 



Direct hydration by agencies between two belts 29 



Dual processes in genesis of talc and antigorite 30 



Genesis of chrysotile and retinalite 33 



Composite veins 36 



Conclusions 38 



Introduction 



The avowal of Delesse in 1865 — "Above all other eruptive rocks, ser- 

 pentine has hitherto remained a veritable enigma !"— still pertains to all 

 prevalent hypotheses concerning its origin. Although indeed the existence 

 of a "serpentine group," as complex as Schweizer's series (picrosmine. 

 picrophyll, substance e, antigorite, sei-pentine, chrysotile and other sub- 

 stances a. b, c, d, f) is no longer in common acceptance, there are few 

 mineralogical authorities even now who are not inclined to favor at least 

 the dual distinction, "serpentine" and chrysotile, with differentiation as 

 allomorphs in physical and optical characteristics if not in chemical com- 

 position. The solution appears to have been long delayed by ignorance 

 of certain facts : 



1). The impurity of specimens. — The term "serpentine" has been 

 indifferently applied to all forms of the mineral, and, with the same 

 freedom, to the massive rock, often designated as "ordinary serpentine," 

 in which the proportion of the mineral rarely exceeds 60 to 80 per cent, 

 and may even fall to 35 per cent, or less. In opposition to this loose 

 practice, Lacroix has long ago urged the restriction of the term "antigo- 

 rite" to all forms of the mineral proper, and of the term "serpentine" 

 exclusively to the rock occurrences. Analogy with tJie precision obtained 

 by discrimination of calcite from limestone, of dolomite (or miemite, ac- 



(23) 



