Two geothermometers have been devised for determining exsolution temperatures in monoclinic pyroxene based on (1) the orientation of exsolution boundaries and (2) the differential changes in unit-cell sizes for host and lamellar phases during cooling.

Lamellar orientations are selected to minimize strain energy between host and lamellae. The orientations in clinopyroxene at different exsolution temperatures have been modeled under the condition that the b dimensions of both phases are identical, thus the orientations are a function of only the differences Δa, Δc, and Δβ between host and lamellae. By slight lattice rotations, lamellar boundaries can be found that achieve perfect dimensional fit. These are termed “exact phase boundaries.” For constant Δβ, variation of Δa by 0.20A can result in 18° variation in the angle of “001” lamellae and variation of Δc by 0.20A can result in 32° variation in the angle of “100” lamellae. The orientations of exsolution lamellae vary with temperature. In the 50°C interval near the C2/cP21/c transition this variation is particularly marked, due to the abrupt contraction of pigeonite lattice parameters.

The model has been used to determine the exsolution temperature of four different sets of pigeonite lamellae in augite from the Bushveld Main Zone Gabbro: 5 μm thick “001” pigeonite lamellae (Pig I, “001” ∧c = 103°), formed near 1000°C, well above the P-C transition; 0.5 μm thick “100” pigeonite lamellae (“100”∧c = −6°), formed near 850°C, above the P-C transition; 1 μm thick “001” pigeonite lamellae (Pig II, “001” ∧c; = 106°), formed near 800°C, just above the P-C transition; and 0.8 μm thick “001” pigeonite lamellae (Pig III, “001’ ∧c = 112°), formed near 560°, below the transition, as in most metamorphic augites.

Below the temperature of exsolution, additional lattice rotation occurs because β of pigeonite decreases more rapidly than β of augite, thus severely straining the confined pigeonite lamellae. The strain is relieved by formation of stacking faults on (100) of pigeonite. Both the stacking fault density observed on electron micrographs and the amount of β decrease determined by X-ray and electron diffraction are potential independent geothermometers. The stacking faults may inhibit growth of lamellae, and this factor should be considered by those researchers attempting to model lamellae growth by bulk diffusion rates alone.

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