Magnesiocummingtonite-P2₁/m: A Ca- and Mn-Poor Clino-Amphibole from New South Wales

A Ca- and Mn-poor clino-amphibole with Mg/Mg+Fe_tot+Mn (atomic ratio)=0.81 is described. The structural formula is {Mathematical expression} The unit-cell constants are a₀=9.49 Å, b₀=18.00 Å, c₀=5.30 Å, β=102.0°, V₀=886 ų, the refractive indices α_Na=1.621, β_Na=1.632, and γ_Na=1.643. These values, when plotted against the Mg/Mg+Fe ratio, fit the extrapolations towards Mg₇[Si₈O₂₂/(OH)₂] from recently published determinative curves for the cummingtonite series. The clino-amphibole, or part of it, has space group P2₁/m rather than C2/m. The most magnesian cummingtonites reported thus far have Mg/Mg+Fe+Mn ratios around 0.7, but recently more magnesian Ca-poor P2₁/m clino-amphiboles have been reported. Although Ca and Mn have been claimed to stabilize cummingtonite as against anthophyllite, most magnesian cummingtonites appear to have <0.24 Ca, and <0.1 Mn per formula unit. The nomenclature of the cummingtonite series is discussed. Retaining the subdivision of the cummingtonite series at Mg/Mg+Fe=0.5, the author proposes to revive Tilley's (1939) name magnesiocummingtonite for members with Mg/Mg+Fe >0.5. Grunerite is reserved for members with Mg/Mg+Fe <0.5. The space group, C2/m or P2₁/m, may be indicated with a suffix, if known.