Calcic amphiboles in carbonate rocks at the same metamorphic grade from the Waits River Formation, northern Vermont, contain 2.29–19.06 wt% Al2O3, (0.38–3.30 Al atoms per formula unit, pfu). These Al-rich amphibole samples are among the most aluminous examples of hornblende ever analyzed. The amphibole-bearing metacarbonates are interbedded with andalusite-bearing pelitic schists and therefore crystallized at P < 3800 bars. These results demonstrate that factors in addition to pressure must control Al content in hornblende. We have identified temperature, mineral assemblage, mineral composition, and rock chemistry [especially Fe/(Fe + Mg)] as other important factors.
To explore semiquantitatively the dependence of the Al content of calcic amphibole on P, T, and coexisting mineral assemblage, a simple thermodynamic model was developed for mineral equilibria involving tremolite-tschermakite ([Ca2Mg5Si8O22(OH)2]-[Ca2Mg3Al4Si6O22(OH)2]) amphibole solutions. The model uses the thermodynamic data base of Berman (1988), with the addition of new values for standard-state enthalpy and entropy for pure end-member tschermakite derived from experimental and field data on the Al content in tremolite coexisting with diopside, anorthite, and quartz. Calculated phase equilibria lead to three conclusions: (1) At a specified P and T, the Al content of calcic amphibole is strongly dependent on the coexisting mineral assemblage. (2) No universal relationship exists between the Al content of amphibole and P. (3) The Al content of amphibole may change dramatically wilh changes in P and 7. Maximum Al contents calculated by the model are ~ 1.2 Al atoms pfu. These values are far short of the 3.30 Al atoms pfu measured in some amphibole samples from Vermont. The principal shortcoming of the model is its failure to consider both total Fe in amphibole and the partitioning of Fe and Mg among the Ml, M2, and M3 crystallographic sites.