Abstract
An investigation of exsolution microstructures in 17 orthoamphibole samples (anthophyllite and gedrite) has been carried out using transmission and analytical electron microscopy (TEM and AEM). All the amphiboles studied, even those appearing to be optically homogeneous, contain exsolution lamellae. TEM observations show a wide variety of exsolution microstructures, ranging from extremely coarse lamellae over 200 nm in thickness to very fine scale, homogeneously distributed Guinier Preston (GP) zones. Many samples show evidence of progressive exsolution during slow cooling of the samples.
Evidence for heterogeneous nucleation and growth of lamellae is abundant, with (100) stacking faults and (010) chain-width errors (CWEs) as common nucleation sites. Heterogeneous nucleation has also been observed at grain boundaries, dislocations, and micro-fractures and along the interfaces of oxide inclusions. TEM images suggest that the incoherent terminations of (010) CWEs and (100) stacking faults are regions of significant structural distortion and lattice strain. These areas of high local strain energy appear to be responsible for unusual lamellar morphologies consisting of embayments in the lamellae caused by boundary pinning of the lamellar interfaces. In addition, there is considerable bulging of lamellae in the vicinity of (010) CWE terminations, which may result from enhanced chemical diffusion along the structural tunnels associated with the defect terminations, as well as from strain efects. In many samples, heterogeneous nucleation and growth was followed by homogeneous nucleation of smaller platelets in the solute-depleted regions between the larger lamellae.
The most common lamellar orientation was found to be (010). Other lamellar orientations have also been observed, including curved lamellae straddling (010), (140), (130), and (120). Calculations using the three-dimensional lattice fitting program, EPLAG, for intergrown anthophyllite and gedrite indicate that the combination of Δb and Δa between the amphiboles controls the actual lamellar orientations. If Δb dominates over Δa, then a (010) optimal phase boundary is predicted. As Δb decreases and Δa increases, the optimal phase boundary gradually shifts from (010) to (120). AEM of the observed (120) exsolution microstructures suggests that increased Ca content is largely responsible for this orientation. The diferences in lattice parameters are also sensitive to the Na content and Fe/Mg ratio.
AEM analyses of exsolved pairs of anthophyllite and gedrite indicate that edenite and tschermakite substitutions are of central importance in controlling exsolution. The exsolved pairs define the widest gap found to date for coexisting orthoamphiboles. Plotting the AEM data on the T-X solvus diagrams of Spear (1980) suggests that exsolution typically took place between 460 and 520 °C.
