South Australian Dolonnites. 459 



and alumina enters the amphibole or pyroxene molecule which arises 

 in this case. 



So much can be gathered from an inspection of the rocks under 

 discussion. The associations tremolite-spinel, diopside-spinel are 

 not to be seen in the slides. 



That alumina is absorbed into the silicate molecule is abundantly 

 confirmed by the presence of the aluminous tremolite, edenite. 

 The presence of possible alumina in the diopside is not so easily 

 made evident by optical determinations.^ 



It seems reasonable to conclude that forsterite and spinel occur 

 together with a value of R equal to one-third, and that when this 

 value is exceeded spinel does not form, and the alumina is absorbed 

 in the metasilicate molecule in accordance with the excess silica 

 either in amphibole or in pyroxene. If spinel is found associated 

 with the metasilicate, it indicates that the original distribution of 

 the detrital materials in the rock was such that they were not 

 involved within a common sphere of difiusion, that is, part was with- 

 out the radius of diffusion of the remainder, and thus a limiting 

 value of the amplitude of diffusion in thermal metamorphism might 

 be thereby established. 



Coming now to the more complex mineral phlogopite, its formation 

 is to be ascribed to the same period of metamorphism, but the detrital 

 materials were in this case of somewhat different nature. The potash 

 and alumina are doubtless derived from detrital sericite present in 

 the original sediment. 



Accepting F. W. Clarke's views of the constitution of the micas,^ 

 we may write phlogopite as H^KMggAl (8104)3, and the following 

 equation may represent its formation, when it is accompanied by 

 spinel, as is often the case in the rocks under discussion :— 



4 CaMg (CO3), + H,KAl3 (SiOj3 = H,KMg3AU,SiOJ3 -f MgAl,0, 

 dolomite sericite phlogopite spinel 



+ 4 CaC(.)3 + 4 CO., 

 calcite 



In some rocks, however, this mineral is not accompanied by spinel, 

 and the above equation cannot represent the formation of phlogopite 

 in these cases. To produce phlogopite of this composition alone, 

 requires dolomite to interact with the oxides KgO : AI2O3 : SiOa, 

 in the ratio 1.1.6, or the proportions present in felspar. This 

 mineral, however, is unlikely to be present in any original dolomitic 

 sediment, representing the mature weathering product of pre- 

 existing rock-masses. The ultimate derivation of phlogopite in 

 these rocks must be ascribed to detrital sericite, and there is no 

 reason to infer that there has been an addition of magmatic material 

 other than the purely volatile agents, water and hydrofluoric acid. 

 By adjusting the relative amounts of the isomorphous elements, 



1 According to V. Schumoff-Deleano (Centr. Min., 1917, p. 290) diopside 

 •can take up 15 per cent AI0O3 in solid solution. 



2 F. W. Clarke, Bull. U.S.G.S., No. 616, 1916, p. 393. 



