Orientation of Pigeonite Exsolution Lamellae in Metamorphic Augite: Correlation with Composition and Calculated Optimal Phase Boundaries Available to Purchase

Optical examination of metamorphic augites coexisting with orthopyroxenes ranging in composition from Fs₁₅ to Fs₉₅ from the Adirondacks, the Hudson Highlands, and the Cortlandt Complex, New York, and the Belchertown Complex, Massachusetts, shows three sets of exsolution lamellae. X-ray single crystal photographs show these are orthopyroxene lamellae on (100), and pigeonite lamellae, termed “001” and “l00”, oriented on irrational planes near (001) and (100). Optically observed angles of the phase boundaries of “001” and “100” lamellae with respect to the c axis of host augite vary with iron-magnesium ratio determined from electron probe analyses of 29 pyroxenes, supplemented by 76 measurements of gamma index of refraction. The angles are largest in magnesian specimens and are in near agreement with angles of optimal phase boundaries calculated from measured lattice parameters of host and one or both sets of pigeonite lamellae. Compositional control of lamellae orientation is related to compositional control of lattice parameters, which appear not to have changed significantly since exsolution in the range 800-500°C.