Trof. Sollas — Olivine Converted into Serpentine. 2o9 



felspai- and quartz, some of the best of which show tolerably good 

 black crosses on rotation in polarized light, and are as well entitled, 

 or even more so, to the name of pseudo-spherulites as are those in 

 the soda rocks of Dinas Head. 



The above observations, taken together, seem to establish con- 

 clusively that spherulitic structure is not, taken alone, evidence 

 of the igneous origin of the rock exhibiting it; and, further, that 

 spherulites are sometimes the product of contact-metamorphism. 



VI. — On the Conversion of Olivine into Serpentine. 

 By Professor W. J. Sollas, D.Sc, F.R.S. 



IN the last volume of the Annals of British Geology (1893), just 

 published, Professor Blake gives an abstract of a paper by me 

 on " Variolite from Roundwood,"^ and incidentally calls attention 

 to the discordance between the formula) given by Genei-al McMahon,^ 

 Dr. Roth,^ and myself, to explain the conversioi of olivine into 

 serpentine. I hasten at once to correct a slight error which occurs 

 in my equation, but which, fortunately, does not affect the principal 

 result of the argument, as the correct formula was used in making 

 calculalions. Amended, my equation is as follows : — 



2 Si (0, Mg), + 2 H2 = Si, (0, Mg) (0 H), (0 Mg H), + Mg 



Olivine Serpentine (Tschermak) 



or, 



Ditto = Si^ (O2 Mg)2 (0 H)3 (0 Mg H) + Mg 



Serpentine (Clarke and Schneider) 



The fate of the molecule of Mg on the right is not indicated here. 

 General McMahon's equation is essentially similar to this, but less 

 detailed. Roth's may be represented thus : — 



5 Si O4 Mg2 + 4 H, = 2 Si., O9 Mgg H, + 4 Mg + Si 0, 

 Olivine Serpentine 



Since 4 Mg + Si 0, is equivalent to Si O4 Mg, + 2 Mg 0, it would 

 appear that Roth puts one molecule of olivine into the left-hand side 

 of the equation to take it out again on the right. The explanation 

 of this is, that while four molecules of olivine suffice to supply the 

 material of two molecules of serpentine, five molecules are needed to 

 furnish the space occupied by this serpentine. The fifth molecule of 

 olivine is carried away by water as well as one molecule of magnesia 

 liberated in the process of serpentinization of the other four, and 

 from this excess of material the minerals magnesite, hydrotalcite, 

 and quartz, which are found associated with serpentine, are produced. 

 In my treatment of the process I simply pointed out that the 

 hydration of olivine, by which it gives rise to serpentine, is 

 accompanied by an increase in volume amounting to 30 per cent., 



1 Sci. Proc. Eoy. Dublin Soc. vol. viii, new series, p. 110. 

 ' Proc. Geol. Assoc, vol. xi, p. 427. 



3 Abh. d. k. Akad. d. Wiss. Berlin, 1869, p. 329 (cited by Teall, British 

 Petrography, 1888, p. 105), Chemische Geologic, Bd. i, p. 116. 



