Mechanisms of exsolution in sodic pyroxenes |
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| Authors: | Michael A Carpenter |
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| Institution: | (1) Department of Geological Sciences, Hoffman Laboratory, 20 Oxford St., 02138 Cambridge, Massachusetts, USA |
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| Abstract: | The free energy curves for simple binary solid solutions with limited miscibility or atomic ordering have been combined to
predict the phase relations and exsolution mechanisms for a system in which both ordering and exsolution are possible. The
nature of the ordering process affects which exsolution mechanisms may be used. If the ordering is second (or higher) order
in character then continuous mechanisms predominate and a ‘conditional spinodal’ (Alien and Cahn, 1976) can be described which
operates between ordered and disordered end members. For a first order case, the ordered phase can only precipitate a disordered
phase by nucleation and growth.
Microstructures in omphacites observed by transmission electron microscopy include exsolution lamellae and antiphase domains
and the relations between them in selected specimens have been used to interpret the exsolution mechanisms which operated
under geological conditions. It appears that most omphacites undergo cation ordering, and then remain homogeneous or exsolve
a disordered pyroxene by spinodal decomposition. The predominance of continuous mechanisms has been used to indicate that
the C2/c→P2/n transformation may be second (or higher) order in character.
A possible phase diagram for jadeite-augite is presented. It is based on the idea that there should be limited miscibility
between the disordered end members at low temperatures and that the cation ordering at intermediate compositions (omphacite)
is superimposed on a solvus. It is adequate to explain many of the observed microstructures and fits with petrographic evidence
of broad two phase fields between impure jadeite and omphacite and between omphacite and sodic augite. The effect of adding
acmite is analogous to increasing temperature so that the phase relations for jadeite-acmite-augite can also be predicted. |
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