105 



i. n. in. 



SiO, ... ... 41-32 ... 42-72 ... 43-48 



Al.O, ... ... 0-28 ... 0-06 



FeO ... ... 2-39 ... 2-25 



CrO ... ... 0-05 ... tr. 



MgO ... 54-69 ... 42-52 ... 43.48 



H 2 0-20 13-39 13-04 



!)8-9;' 100-94 100-00 



I. Olivine. Snarum. 



II. Serpentine derived from the abuve. 



III. Theoretical composition of serpentine. 



Of the two molecules of water which are present in serpentine, one is 

 driven off at a lower temperature than the other, and hence RAMMELSBEKG (1) 

 concludes that the molecular constitution of serpentine may be expressed 

 as follows : H, Mg 3 Si 2 O 8 + H 2 0. 



. The general question of the formation of serpentine by the alteration of 

 the magnesian and ferro-magnesian silicates, has been discussed in an able 

 memoir by J. ROTH. tlber den Serpentin und die genetischen Beziehungen 

 desselben. <2) In this memoir the author points out that the ordinary agents 

 of weathering water, carbonic acid and oxygen are powerless to remove 

 alumina ; and he draws the general conclusion that if a non-aluminous 

 silicate rock occurs as an alteration product, it must arise from the modifica- 

 tion of an original rock free from alumina. Powerful chemical reagents, 

 such as sulphuric acid which may arise from the oxidation of pyrites, capable 

 of removing alumina, (8) would of course also remove the other bases 

 including the magnesia necessary for the formation of serpentine. Now the 

 rock, serpentine, is practically non-aluminous, and it is also in most if not in 

 all cases, unquestionably an alteration product. It becomes important 

 therefore to consider what common rock-forming minerals are capable of 

 yielding serpentine. The above considerations point to the conclusion that 

 these minerals are olivine, the rhombic pyroxenes, diallage and 11011- aluminous 

 hornblende and augite. According to the author the process of making serpentine 

 commences by the taking up of water and by the oxidation of the iron if the 

 infiltrating water contains free oxygen. It proceeds by the removal of bases, 

 especially lime if present, mainly as carbonates ; and a portion of the 

 precipitated and therefore soluble silica. If we take the case of olivine ; then 

 assuming that no change of volume occurs. 



(1) Z.D.G.G., Vol. XXI., p. 98 (1869). 



(2) Abh. d. k. Akad. d. Wiss. Berlin. 1869. 



(3) A spring at Trefriw, between Conway and Llanwrst contains -136 p.c. of sulphate of 

 aluminum. The removal of alumina is illustrated by the occurence of pscudomorphs of quartz 

 after felspar. (See Blake Q.J.G.S.. Vol. XL., p. 308 ) 



