Transfer and exchange equilibria in a portion of the pyroxene quadrilateral as deduced from natural and experimental data
Abstract
Differences in the chemical composition of metamorphic and igneous pyroxene minerals may be attributed to a transfer reaction, which determines the Ca content of the minerals, and an exchange reaction, which determines the relative Mg:Fe ^{2+} ratios. Natural data for associated Ca pyroxene (Cpx) and orthopyroxene (Opx) or pigeonite are combined with experimental data for Fefree pyroxenes, to produce the following equations for the Cpx slope of the solvus surface: > 1080°C: T = 1000/(0.468 + 0.246X ^{Cpx}  0.123 ln (12 [Ca])) < 1080°C: T = 1000/(0.054 + 0.608X ^{Cpx}  0.304 ln (12 [Ca])) , and the following equation for the temperaturedependence of the MgFe distribution coefficient: T = 1130/( ln K _{p} + 0.505) , where T is absolute temperature, X is Fe ^{2+}/(Mg + Fe ^{2+})) , [Ca] is Ca/(Ca + Mg + Fe ^{2+}) in Cpx, and K_{D} is the distribution coefficient, defined as X ^{Opx}/(1  X ^{Opx}) ÷ X _{Cpx}/(1  ^{Cpx}) . The transfer and exchange equations form useful temperature indicators, and when applied to 9 sets of wellstudied rocks, yield pairs of temperatures that are in good agreement. For example, temperatures obtained for the Bushveld Complex are 1020°C (solvus equation) and 980°C (exchange equation), based on 7 specimens. The uncertainty in these numbers, due to precision and accuracy errors, is estimated to be ±60°.
 Publication:

Geochimica et Cosmochimica Acta
 Pub Date:
 March 1982
 DOI:
 10.1016/00167037(82)902320
 Bibcode:
 1982GeCoA..46..411K
 Keywords:

 Chemical Equilibrium;
 Chemical Reactions;
 Igneous Rocks;
 Mineralogy;
 Petrology;
 Pyroxenes;
 Chemical Composition;
 Crystallography;
 Temperature Effects;
 Transfer Functions;
 Lunar and Planetary Exploration