Exsolution and Fe²⁺-Mg Order-Disorder in Pyroxenes

Abstract

In recent years there has been significant development in our understanding of exsolution phenomena in minerals through (a) the use of petrography (Robinson et al., 1977) and transmission electron microscopy (Champness and Lorimer, 1976; see other articles in Wenk, 1976), and (b) the use of kinetic experiments and theory (Yund and McCallister, 1970; McCallister and Yund, 1977; Lasaga and Kirkpatrick, 1981; Putnis and McConnell, 1980; McConnell, 1975). The references quoted above are only examples of the extensive literature available on exsolution phenomena. Buseck et al. (1980) have reviewed the results on pyroxenes. In all these studies there has been little mention of the Mg-Fe²⁺ order-disorder in pyroxenes that must continue simultaneously with the intercrystalline processes of ion exchanges leading to exsolution (cf. Kretz, 1982b). As discussed extensively by Putnis and McConnell (1980), exsolution, like all other reactions, is a time-dependent transformation and is best displayed on a time-temperature-transformation (TTT) diagram. These may be considered as kinetic phase diagrams. Mg-Fe²⁺ order-disorder in silicates is also a time-dependent process and the results are best displayed on TTT diagrams (Seifert and Virgo, 1975; Ganguly, 1982). Is it possible then to plot both kinds of results, i.e., percent transformation of an intercrystalline ion-exchange reaction between two crystals, e.g., the host augite and pigeonite lamellae and the Mg-Fe²⁺ order-disorder in the exsolving or other coexisting pyroxenes?