197 



observations are correct; e.g. ,,Des aiguilles d'eiistalite _y soiit 

 fréqiietniiient disséminées on en reconvient la surface" (i. e. of" (he 

 olivine, obtained iliroiigli smelting of serpentine witli the addition 

 of magnesia) and again in case lie did not add magnesia: ,,le 

 (peridot) se montre en moindre proportion que dans les fusions 

 faites en presence de la niagnésie. 



Now since most peridotites (with the exception of dunites) consist 

 of olivine and pyroxene, consequently of orthosilicate and metasili- 

 cate, we may venture to bring the primary and the secondary 

 serpentinization into one focus. Foi' a general theoretical treatment 

 of the case the knowledge of the tliermal pressure-diagram of the 

 system H,0 — MgO — SiO^ would be a first step. ') Needless to say, 

 that tiiis diagram will become very complicate owing to the great 

 difl'erence in volability of the components. 



From the foregoing it is evident, however, that under a pressure 

 of one atmosphere serpentine is unstable; it would be worth while 

 to repeat the experiment of Daubkék in watervapours of various 

 tensions in order to establish the limit of stability of serpentine. Now 

 if we are right in supposing that olivine and pyroxene are not 

 stable at high pressure and in the presence of watervaponr, but 

 tliat they are transformed into ser[)entine, the former with liberation 

 of MgO') the latter with precipitation of silica, serpentinization 

 may be accounted for as follows: 



1. If the pressure is high enough serpentine crystallizes first from 

 a magma, which is composed of x Mg^SiO,, y MgSiO,, z H^O ; at a 

 lower pressure the crystallization begins with olivine. 



2. When olivine and (or) |)yroxene are segregrated, the volatile 

 components congregate in the Lipper zone of the balholite, which 

 nuxy give rise to a high tension, in case they have no opportunity to 

 escape; thus the field of stability of the olivine and (or) the pyroxene 

 is abandoned, and that of serpentine is attained, aftei- which ser- 

 pentinization of olivine and pyroxene commences, occasionally with 

 a residue of MgO (FejO,) or (and) silica; wliile in most cases MgO 

 is present as magnesite. 



Already Daübrèe acknowledged: ,,Rien ne prouve d'allenrs que 

 riiydratation qui s'est produite dans la transformation des roclies 

 de peridot en serpentine ait été opérée |)ar les agents de la surface 

 du globe". 



') See e.g. H. E. Boeke, Grundlagen d. phys. chem. Petiogiaphie, p. 179. 

 1) E.g. as magnesite, because the component carbondioxyde is always presenl. 

 Many serpentine deposits in fact contain magnesite and quartz. 



