On spinodal decomposition in Fe-free pyroxenes

Theoretical treatments of spinodal decomposition are kinetic models of the decomposition mechanism. Use of morphologic or textural arguments to determine mechanistic changes during the solidification of alloy, ceramic, or mineralogic systems can be misleading. The literature on spinodal mechanisms in Fe-free pyroxene systems forms a consistent picture when the experimental data for iron-free and iron-enriched pyroxenes are compared and related to the more recent mathematical treatments of late stage spinodal decomposition. Recognition of spinodal coarsening via a tl/3 rate law and coarsening of coherent nucleation at tl/3 may elucidate positioning of the coherent spinodal and the coherent solvus in the iron-free pyroxene system. Decompositon morphologies Morphologic considerations have often been employed to delineate spinodal decomposition. A specific repetitive spacing or mutual connectivity between the conjugate product phases has been cited as evidence for the operation ofthe spinodal process. Clearly, the repetitive spacing of exsolution lamellae fall into this category and spinodal decomposition has been invoked as one possible mechanism for exsolution in pyroxenes (McCallister and Yund, 1975). Phase connectivity and periodicity cannot be considered to be unequivocal evidence of spinodal decomposition (Ardell et al., 1966; Seward et al., 1968; Herman and MacCrone, l97l; McCallister and Yund, 1977; Jantzen and Herman, 1978). Anisotropic strain fields can give rise to periodicity (Ardell et al., 1966), discontinuous cellular structures can give rise to lamellar structures (Fine, 1964; Herman and MacCrone, l97l; Tarshis et al., 1973), while eutectic solidification (Kraft, 1973; Shewmon, 1969), eutectoid decomposition (Mangonon et al., 1973; Mack, 1973) and discontinuous precipitation (Newkirk, 1973) can all give rise to lamellar structures. Systems in which the volume fraction of each of the conjugate phases are comparable can also exhibit connectivity based on geometric arguments (Herman and MacCrone, l97l). Indeed, work on two systems in which the origin of modulated structures were investigated has demonstrated conclusively that spinodal composition was not the operational mechanism (Ardell et al., 1966; Seward et al., 1968). Exsolution morphologies observed in both synthetic iron-rich and lunar pyroxenes have recently been as0003-004)v84l0304-0277$02.N 277 cribed to a variety of mechanisms which include two homogeneous exsolution mechanisms and heteroge.neous nucleation, e.9. , Iarge heterogeneously distributed lamellae nucleate on dislocations or subgrain boundaries (Grove, 1982). Tweed structures in rapidly cooled pyroxenes have been attributed to decompositon by a spinodal mechanism while slower cooling rates (<10"C/hr) produce coarsening and cooling rates of 0.5"C/hr and 0.2"C/hr form large lamellae by homogeneous nucleation (Grove, 1982). Moreover, on slow cooling, heterogeneously nucleated lamellae on the equilibrium solvus may grow at elevated temperatures and subsequently decompose by a spinodal or nucleation and growth mechanism. Complex microscopic textures can, thereby, be produced over small temperature intervals (Grove, 1982). The textural variations of the pyroxenes, as observed by McCallister and Yund (1975, 1977), should not be quoted as unequivocal evidence for an operative spinodal mechanism in Fe-free pyroxenes, as in, e.g., a recent review of pyroxene crystal chemistry by Cameron and Papike (1981). Morphologic studies are ambiguous unless kinetic studies of the evolution of the concentration profiles of the emerging phases with time are examined by the "microstructural sequence method" (Laughlin and Cahn, 1975) or by small-angle scattering techniques (Rundman and Hilliard, 1967). Spinodal decomposition vis-a-vis nucleation and growth The spinodal theories developed by Cahn (1962) and co-workers (Huston et al., 1966) are kinetic models. The use of morphologic arguments alone is, therefore, mis-