VI.—Saturation of Minerals

Abstract

In a recent paper Professor Shand put forward a pleafor the more careful consideration, in rock classification, of the saturated or unsaturated state of the constitnent minerals. The criterion of saturation which is assumed is the capability of co-existence with some form of free 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.