Acidity and Alkalinity as Foundational Parameters in the Ordering of Eruptive Rocks and Some Fuegian Examples

Abstract

It is feasible to obtain a comprehensive ordering of eruptive rocks based on two fundamental criteria: acidity and alkalinity. These concepts are eminently chemical, but it must be kept in mind that the chemistry of the main elements can be deduced starting from the mineralogical proportions refining the study of each rock forming mineral. Both of Johannsen and Streckeisen classifications, and IUGS or International Union of Geological Sciences recommendations also, are quantitative-mineralogical, which provide a strict nomenclatural reference but demonstrate difficulty in establishing the proportions of minerals. This natural difficulty, extreme in vulcanites, is not present in chemicals and normative classifications, but in this last case the rock loses its true petrographic spirit, established by the real minerals, after which the crystallization history and the rock understanding are. This preference for mineralogical classifications does not invalidate the importance of other based on chemical compositions. This simple chart contains eleven boxes from the combination of four columns (UB or ultrabasic, B or basic, M or mesosilicic, and A or acid) with three groups (I or calc-alkaline, II or subalkaline, and III or alkaline). All eruptive rocks fit there. AIII field doesn’t exist due to foids are incompatible with quartz. At last, an addendum about potassic and sodic trends as alkali elements in common minerals is included. The conclusion is that the difference between sodic albite and the rest of plagioclases is as important as the difference between sodic albite and potassic feldspar. Mineralogical changes of great petrogenetic significance only could represent subtle chemical changes and don’t reflect variances in the classificatory charts.