Mutual exsolution in hornblende and cummingtonite; compositions, lamellar orientations, and exsolution temperatures

Exsolved pairs of hornblende and cummingtonite from two localities in southern New England, U.S.A., have been studied using transmission electron microscopy (TEM) to measure the crystallographic orientation of the exsolution lamellae. Both the cummingtonite and the hornblende show multiple generations or stages of exsolution. Electron microprobe analyses gave compositions of the pre-exsolved (averages) and the coarsest exsolved amphiboles.

In amphiboles, two orientations of lamellae are usually present and nearly parallel to {100} and {101} of the host. The observed lamellar orientations are consistent with orientations predicted by the optimal phase-boundary (OPB) theory and are a function of small differences in a, c, and β of the host and exsolved amphiboles (Δa = a_host - a_lamella, Δβ = β_host - β_lamella, and Δc = c_host - c_lamella). Because these lattice parameters vary with T and composition, the precise orientations of the lamellae are controlled by these variables. The widths of determined exsolution lamellae varied from microscopic (micrometer size) to submicroscopic (nanometer size). The TEM images show, in some cases, multiple generations of lamellae with lamellar orientations near {101} differing up to 4.1°, whereas orientations near {100} differed up to 4°. For two pairs of coexisting amphiboles, the T dependence of the lattice parameters from 25 to 600 °C was measured using a Guinier camera. The OPB calculations indicate that for all the samples the different lamellar generations formed between about 780 and 300 °C (±80 °C, on the basis of maximum errors of the lattice-parameter determinations). Lamellae of the same generation all showed nearly the same width. Exsolution temperatures could not be derived from the “100” lamellae because relatively small variations in Δc, which controls exact orientation of the “100” lamellae, could not be measured accurately enough.