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Cummingtonite from central Massachusetts, which has experienced pervasive exsolution of hornblende, contains numerous hornblende precipitates (lamellae and discs) ranging in thickness from 2.5 μm to 4 nm that represent multiple generations of exsolution. The smallest precipitates were investigated using transmission electron microscopy and analytical electron microscopy to characterize the coherent cummingtonite-hornblende solvus at low temperature. In contrast to older (coarser) generations of exsolution lamellae, which are 0.5-2.5 μm in width, planar, and extend across entire grains, the youngest generation consists of nanometer-scale discs (4-80 nm thick and 10-1000 nm long) that represent exsolution at approximately 300 °C (Klein et al. 1996). Quantitative energy-dispersive X-ray analyses were obtained from discs as small as 20 nm in thickness, providing compositional data for coherent exsolution of hornblende at ~ 300 °C. Surprisingly, these tiny discs have a lower Ca content and a higher Al content than the coarser, higher temperature lamellae. These compositions appear to lie well within miscibility gaps of the equilibrium and coherent solvi for cummingtonite-hornblende (in terms of Ca ↔ Fe 2+  exchange) at low temperatures, and may represent a more tschermakitic hornblende coexisting with cummingtonite with excess Al relative to Ca. The composition of the cummingtonite host between nanometer-scale discs is very low in both Ca and Al relative to bulk cummingtonite analyses that were obtained from electron microprobe analyses of 1-2 μm areas.

Analytical electron microscopy of nanometer-scale hornblende lamellae: Low-temperature exsolution in cummingtonite