A lexander Scott — 8a turation of Minerals. 319 



VI. — Saturation of Minerals. 



By Alexander Scott, M.A., B.Sc, 

 Carnegie Kesearch Scholar of the University of Glasgow. 



IN a recent paper ^ Professor Shand put forward a plea for the more 

 careful consideration, in rock classification, of the saturated or 

 unsaturated state of the constituent minerals. The criterion of 

 saturation which is assumed is the capability of co-existence with 

 some form of fi-ee silica, as shown by the "observed facts of 

 distribution ". By means of this criterion, the rock-forming minerals 

 are divided into two groups — the saturated minerals and the unsaturated 

 ones. The former group includes the felspars, amphiboles, pyroxenes, 

 micas, and most of the so-called accessory minerals, as well as those of 

 pneumatolytic origin ; while the latter group comprises the felspathoids, 

 olivine, the spinels, corundum, and the garnets (except spessartite, which 

 is put into the first class). Rocks are divided into three classes, 

 according as they are made up wholly of saturated minerals, partly of 

 saturated and partly of unsaturated ones, or wholly of unsaturated ones. 



The criterion is so obvious that it has been implicitly used, at 

 least in part, in most of the classifications proposed since Zirkel's^ in 

 1866. In the American Quantitative Classification, for example, the 

 ratio of felspar to lenad (felspathoid) takes second place only to the 

 ratio of salic to femic minerals in determining the position of a rock.' 

 The ' class ' is determined by the latter ratio and the ' order' by the 

 former. In practically all cases, however, the application of this 

 idea has not extended beyond the discrimination of the felspar and 

 felspathoid-bearing rocks, and what Professor Shand advocates is the 

 extension of the principle to the other minerals of igneous rocks. 

 There are, however, certain inherent difficulties in such a general 

 application, and it is now proposed to consider, very shortly, some of 

 these difficulties. 



The first question which arises is whether the method of 

 determining Avhich minerals are saturated and which are not, is 

 sufficiently exact. The distribution of minerals in igneous rocks, 

 apart from all consideration of the history of the rocks, seems to be 

 rather too empirical to be used as a satisfactory criterion. For 

 example, olivine must be included in the list of unsaturated minerals, 

 and yet it occurs along with quartz in a number of rocks. In addition 

 to those mentioned by Professor Shand, several rhyolites in America* 

 and elsewhere^ contain olivine and tridymite, while the author has 

 described pitchstone-porphyries from Arran ® containing phenocrysts 

 of olivine and quartz. In the former case the minerals occur in 

 lithophysa3 and may have had a hydrotherraal origin after the 

 consolidation of the rock, while in the latter the olivine is probably 

 xenocrystic. Quartz -dolerites are usually supposed to have caught up 



^ " On Saturated and Unsaturated Rocks " : Geol. Mag., Dec. V, Vol. X, 

 pp. 508-14, 1913. 



^ Lehrbuch der Petrographie, Bd. i, p. 450, 1866. 



■ Quant. Class, of Igneous Rocks, 1903, p. 123 et seq. 



^ e.g. Iddings, Amer. Journ. Sci., ser. Ill, vol. xxx, pp. 58-60, 1885. 



'•> Iddings & Penfield, ibid., ser. ill, vol. xl, pp. 75-8, 1890. 



" " Pitchstones of South Arran": Trans. Geol. Soc. Glasgow, vol. xv, 

 pp. 19-37, 1913. 



