Orthoferrosilite and other iron-rich pyroxenes in microperthite gneiss of the Mount Marcy area, Adirondack Mountains

The orthopyroxene is the first documented natural occurrence of orthoferrosilite of space group Pbca. Lattice parameters (A) and optical properties of the orthoferrosilite as compared to synthetic FeSiO₃ are a = 18.42 (18.42), b = 9.050 (9.078), c = 5.241 (5.237), γ = 1.786 (1.789), β = 1.774 (1.780), α = 1.765 (1.772), 2V_calc = 79° + (86°+). Analyses giving similar compositions have been recently reported from Norway and Canada.

Textural features of the rocks and exsolution lamellae in the pyroxenes and feldspars suggest the minerals equilibrated during the peak of Adirondack regional metamorphism and underwent exsolution and minor adjustment during subsequent cooling. Garnet in symplectic intergrowth with quartz and feldspar may have formed in part during cooling. The assemblages demonstrate great consistency in the distribution of elements between minerals, in particular, Fe/(Fe + Mg) of ilmenite = magnetite > garnet > orthopyroxene > hornblende > augite, and Mn/(Mn + Fe) of garnet > orthopyroxene > augite > hornblende ≅ ilmenite > magnetite.

The compositions of the pyroxenes in these rocks, together with analyses and optical data on fayalite-bearing assemblages from adjacent areas, permit estimates, based on experimental data, of temperature and pressure of metamorphism in the northeast Adirondacks. Absence of inverted pigeonite and compositions and distribution coefficients of coexisting augite and orthopyroxene suggest temperatures never exceeded 800°C. Exsolution features in pyroxenes and feldspars suggest the rocks cooled from above 600°C. Evaluation of the compositions of coexisting orthopyroxene-augite in terms of experimental work on the hedenbergite-ferrosilite join suggests a temperature of 760 to 790° for primary metamorphic crystallization and 600–660° for the temperature of fine exsolution. Depending on interpretation of the effects of minor constituents, the orthoferrosilite composition applied to various experimental data indicates that minimum pressures of metamorphism in the Mt. Marcy area were 7–9 kbar at 600°C or 9–11 kbar at 800°C. Fayalite-bearing assemblages from adjacent areas, evaluated using the same criteria, indicate maximum pressures of 7.5 to 9.5 kbar at 600° or 10–12 kbar at 800°C. These brackets and available experimental work on the Al₂SiO₅ polymorphs suggest metamorphism in the Mt. Marcy area took place near the high-pressure stability limit of sillimanite or in the kyanite zone.