South Australian Dolomites. 497 



The conversion of diopside in the diopside rock into actinolite in 

 vugs, along transverse veins and joints, is paralleled in the pyroxenites 

 (altered limestones) of Ontario as described by Adams and Barlow.^ 

 It is stated (p. 91), " a deep green hornblende is also seen occasionally 

 running through the pyroxenite in narrow veins, as if it had developed 

 along cracks." 



The impurer dolomitic sediments on metamorphism have 

 ultimately been converted into silicate rocks, in which the minerals 

 diopside and microcline are prime constituents. This association 

 of pyroxene and felspar is one of considerable importance, especially 

 when the nature of the detrital minerals in the original sediment is 

 considered. The development of these minerals can be represented 

 by the following equation, in which sericite and quartz are the 

 detrital materials involved : — 



(a) CaMg (CO,), + H^KAl, (SiOJs + 3 SiO^ 



= CaMg (SiOg), + KAlSigOg + Al^SiOj + H^O + 2 CO., 



(&) Al^SiO, + CaCOg + SiO^ = CaAl^SiA + CO2, 

 and this doubtless represents the case where the diopside and" 

 microcline are intimately associated with |)lagioclase. How far the 

 reaction has proceeded by absorption of excess alumina in the 

 pyroxene is not evident, thus :— 



(c) 4 MgC03 + 3 CaCOg + H^KAlg (SiOJg + 7 SiO, 



_/3CaMg(Sib3)2\ -^.,0. ^ I 7 CO 4- HO 

 -^ MgAl,SiOs j+-»^Albi308 + 7LO,+ M2U, 



but the very characteristic association of these two minerals, often 

 to the exclusion of other aluminous types in which excess alumina 

 could be absorbed, would seem to demand this equation. 



We have already interpreted the formation of phlogopite in the 

 dolomite marbles as a result of an interaction between dolomite 

 and detrital sericite, or sericite and quartz, and it would appear that 

 with an excess of silica this reaction in turn would be further 

 advanced, the resultant phlogopite giving place to diopside and 

 microcline. Using the minerals of the right-hand side of this 

 equation, we have with excess of silica {vide p. 459) — 



{d) H^KMggAl (SiOJs + MgAl^O, + 5 CaCOg + 10 SiO^ 



= 4 CaMg (SiOs)^ + KAlSigOg + CaAl^SiaOg + 5 CO, + H^O. 



Diopside microcline rocks may thus arise, by simple residuary 

 crystallization or the metamorphism may possibly take place in 

 two stages. Firstly residuary crystallization may be only sufficient 

 to produce the phlogopite marble stage, and by a later additive 

 metamorphism ^ the silication of this rock to a diopside microcline 

 type. For the rocks under discussion the view that residuary 

 crystallization is solely responsible for the mineralogical changes 



^ Loc. cit. supra, p. 91. 



- Cf. J. E. Spurr, Econ. Geol, vol. vii, 1912, p. 455. This term would be 

 embraced under V. M. Goldschmidt's term " pneumatolytic contact 

 metamorphism ". 



VOL. LVir. — NO. XI. 32 



