Experimentelle Gesteinsmetamorphose—VI Einfluss von Anionen auf metamorphe Mineralreaktionen

Abstract

Experiments at a total pressure of 2000 bars in the temperature range from 230–750°C have been carried out with mixtures of natural illite, quartz and 3–5 per cent of various Na-, K-, Mg- and Ca-salts to which 10 per cent of H 2O had been added. The experiments served to study the influence of anions on metamorphic mineral reactions. The results are summarized on page 174; the most interesting of them are given here: 1. (1) The acidity of a system depends sensitively on the anions present. Whenever anions partake in a reaction, as is the case in those studied, the kind and concentration of an anion influences strongly the reactions. 2. (2) At constant pressure and temperature and constant concentration of electrolyte the acidity is so much higher the stronger the acid formed with the anion. 3. (3) The acidity increases with pressure. 4. (4) Temperature and pressure remaining constant, the acidity required for the formation of mica must be higher than for the formation of the corresponding feldspar. This applies to the following pairs of minerals: muscovite-potassium feldspar; margarite-anorthite; paragonite-albite. 5. (5) The temperature for the reaction mica → feldspar increases with rise in acidity. Fig. 1. summarizes qualitatively the influences of the different anions on the stability of phyllo- and tectosilicates, respectively. Some conclusions bearing on metamorphic petrology have been drawn (p. 176). It should be mentioned here that occurrence of margarite and paragonite is a definitive proof for a high acidity having been active during rock formation.