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 basedon two fundamental criteria: acidity and alkalinity. These concepts are eminentlychemical, but it must be kept in mind that the chemistry of the main elements can bededuced starting from the mineralogical proportions refining the study of each rockforming mineral. Both of Johannsen and Streckeisen classifications, and IUGS orInternational Union of Geological Sciences recommendations also, are quantitativemineralogical,which provide a strict nomenclatural reference but demonstrate difficultyin establishing the proportions of minerals. This natural difficulty, extreme invulcanites, is not present in chemicals and normative classifications, but in this lastcase the rock loses its true petrographic spirit, established by the real minerals, afterwhich the crystallization history and the rock understanding are. This preference formineralogical classifications does not invalidate the importance of other based onchemical compositions. This simple chart contains eleven boxes from the combinationof four columns (UB or ultrabasic, B or basic, M or mesosilicic, and A oracid) with three groups (I or calc-alkaline, II or subalkaline, and III or alkaline). Alleruptive rocks fit there. AIII field doesn?t exist due to foids are incompatible withquartz. At last, an addendum about potassic and sodic trends as alkali elements incommon minerals is included. The conclusion is that the difference between sodicalbite and the rest of plagioclases is as important as the difference between sodicalbite and potassic feldspar. Mineralogical changes of great petrogenetic significanceonly could represent subtle chemical changes and don?t reflect variances inthe classificatory charts.