Abstract

Pigeonite crystals (Wo6-14En56-64Fs28-34) from lunar basalt 14053 were studied, using X-ray precession and Laue methods. Four distinct phases are generally present in each crystal: pigeonites P1, P2, and augites A001, A100. The orientation, relative intensity, and diffuse-streak relationships between the four phases indicate that there are two host-exsolution pairs, Pl–A001 and P2–A100, which correspond to two successive generations of exsolution at high and lower temperatures, respectively, during cooling. The presence of diffuse streaks connecting the reflections of P1–A001, P2–A100, and P1–P2 pairs suggests direct lattice interrelations of each pair.

The change of the plane of intergrowth from (001) to (100) is interpreted in terms of twodimensional misfit between host and exsolved lattices, using published high-temperature data on clinopyroxenes. The temperature dependency of the calculated misfit ratios of the a and c axes show crossing over at a “critical temperature,” above which the a axis yields the better fit, whereas the c axis fit is preferable below that point. The temperature dependency of the misfit ratios is consistent with two stages of exsolution of A001 and A100 above and below the critical temperature.

Because of the two generations of exsolution, the chemical compositions of the P1 and P2 hosts are expected to be somewhat different. The small differences in the lattice constants produce an apparent rotation of the c* and a* axes of the two hosts which can be recognized in precession photographs. The sense and magnitude of the rotation seems to depend on the change in chemical composition of the host resulting from the exsolved phase.

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