Abstract
A Mn-rich orthopyroxene Mg1.41Mn0.56Ca0.03Si2O6, occurs in a manganiferous pod in the marble units near Balmat, N. Y. The pyroxene coexists with triodite, tourmaline, ferrian braunite, manganoan dolomite, and hedyphane. Refinement of the crystal structure of the pyroxene (space group Pbca) shows that as expected virtually all of the Mn is located in the M2 site and the general formula is therefore (Mn,Mg)MgSi2O6. Because Mn is completely ordered into the M2 site and comprises over 50 percent of that site, the mineral is a new orthopyroxene. It has been named donpeacorite after Donald R. Peacor in recognition of his work on pyroxenes and manganese minerals. The unit cell parameters are a = 18.384(11), b = 8.878(7), c = 5.226(3), Z = 8, D (meas.) = 3.36(1). Donpeacorite is biaxial negative with 2VX = 88°, Z||c, α = 1.677(2), β = 1.684(2), γ = 1.692(2). It is yellow-orange with a vitreous luster and has an approximate hardness of 5–6 with perfect (110) cleavages.
The occurrence of donpeacorite and published data for metamorphic pyroxenes and pyroxenoids permit modeling of a phase diagram for the system MnSiO3–MgSiO3–FeSiO3. The diagram is dominated by extensive fields for solid solutions of orthopyroxene and of pyroxmangite, with a small field inferred for kanoite (MnMgSi2O6, C2/c clinopyroxene) and a three-phase field for ferroan kanoite, manganoan hypersthene, and magnesian pyroxmangite.