496 G. E. Tilley — Metamorphison of 



The Chemistry of the Development op the Calc-magnesian 

 Silicate Rocks. 



We have already referred to the order of development of meta- 

 morphic minerals in an impure dolomite. The minerals which first 

 make their ajDpearance are predominantly silicates or aluminates 

 of magnesia, and it is in accordance with this greater chemical 

 activity of MgO that the dominant lime-bearing minerals, as 

 scapolite, plagioclase, etc., are only develoj)ed when magnesia has 

 been satisfied. ^ The formation of diopside occurs in the presence 

 of abundant silica, and the occurrence of extensive beds of practically 

 pure diopside is eloquent evidence that the original sediment had 

 the lime-magnesia ratio of a true dolomite. The explanation of the 

 formation of such large masses of diopside rock, however, by 

 metamorphism, without the addition of material, i.e. by residuary' 

 crystallization, demands just that amount of silica uniformly 

 distributed through the mass to satisfy the diopside reaction. The 

 conditions yielding such a distribution are, however, not likely to 

 have been attained, and the field evidence strongly favours the view 

 that addition of material in the form of the numerous quartz veins 

 was in part responsible for the diopsidization. The rock as now 

 constituted is to be regarded as a hybrid type, in which silication 

 was partly effected by residuary crystallization and completed by 

 silica solutions from the neighbouring granite, now represented by 

 the numerous quartz veins traversing the diopside rock. Moreover, 

 in the adjoining calc-magnesian silicate rocks, there is abundant 

 evidence of veining of these rocks by quartz strings. For the 

 same reason it is doubtless to the addition of material from the 

 magma, rather than to detrital material present in situ, that we 

 must ascribe the expulsion of the last remnants of carbon dioxide 

 from some of the more aluminous beds. 



In the calc-magnesian silicates at Sleaford, and in the Hutchison 

 area, however, there is no reason — with certain exceptions to be 

 referred to below — to invoke the aid of magmatic material other 

 than the purely volatile reagents and silica. The metamorphism, 

 by addition to these rocks, is at most a silication. 



In the rifted diopside block in the igneous gneisses of Fishery 

 Bay, the mass is penetrated by felspathic stringers, and this is to be 

 ascribed to the presence of the surrounding fluid magma, 



^ Cordierite would appear to be an exception to this rule. Its absence in 

 metamorphosed limestones is remarkable. A development in accordance 

 with the equation 

 2CaMg(C08)o + 2H4Al2Si20q + SiOs 



= (2 MgO . 2 AI.2O3 . 5 SiOo^ + 2 CaCOs + 2 CO2 + 4 H2O 

 cordierite 

 might be exjjected, but it would appear that the alumina is absorbed in 

 anorthite, and diopside forms instead. 



CaMg(C03)o + CaCOg + H4Al2Sio09 + 2Si02 



= CaMg(Si03)2 + CaAl2Si208 -h 2H2O + SCOa 



