Graphical Representation of Metamorphic Mineral Parageneses

Abstract

A correct graphical representation of a mineral paragenesis is possible only if the number of components constituting the minerals does not exceed four, because only four components can be represented in space at the corners of a tetrahedron. A two-dimensional representation is, of course, greatly desired and, in some special investigations, suitable projections of points within a tetrahedron onto some plane may be developed. For instance, such method can be employed very advantageously in the study of pelitic schists, as shown by THOMPSON (1957) (a detailed discussion is given in Section 5.4). However, in order to represent mineral assemblages in rocks of diverse composition and metamorphic grade, a triangular representation developed by ESKOLA is used extensively. This method is necessarily a compromise, because only three components can be represented in a plane, yet the rocks contain more than three components. Nevertheless, “by means of suitable selections and restrictions” his method “allows the representation of most rocks of not too unusual composition and having an excess of silica.” He continues: “If silica is present in excess (quartz is a constituent of many metamorphic rocks) only those minerals with the highest possible Si02 content can be formed; consequently, the amount of SiO2 has no influence on the mineral assemblages and need not be represented graphically. At one corner of the triangle, designated as A, that portion of Al2O3 (more exactly, Al2O3 + Fe2O3, because Fe³+ and Ala⁺ can substitute for each other) is plotted, which is not combined with Na and K. The second corner is defined as C = CaO and the third one as F = (Fe, Mg, Mn)O. Accessory constituents are disregarded in the graphical representation; however, before calculating the A, C, and F values, the amounts of (Al, Fe)2O3, CaO, and (Fe, Mg) O contained in the accessories are subtracted (from the chemical analysis). In this manner, the more important silicate minerals can be represented, with the exception of K and Na silicates and silica-undersaturated silicates, like olivine.”