Magnesiocummingtonite-P21/m: A Ca- and Mn-Poor Clino-Amphibole from New South Wales |
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| Authors: | Hanan J Kisch |
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| Institution: | 1. Dept. of Petrology, Mineralogy, and Crystallography Rijksuniversiteit Leiden, Garenmarkt 1 B, Leiden, The Netherlands
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| Abstract: | A Ca- and Mn-poor clino-amphibole with Mg/Mg+Fetot+Mn (atomic ratio)=0.81 is described. The structural formula is $$Na_{0.09} (Ca_{0.19} Mg_{5.45} Fe_{1.23}^{2 + } Mn_{0.04} Fe_{0.00}^{3 + } Ti_{0.01} Al_{0.07} )_{6.99} (Si_{7.83} Al_{0.17} )_{8.00} O_{22} /(OH)_2 ].$$ The unit-cell constants area 0=9.49 Å,b 0=18.00 Å,c 0=5.30 Å, β=102.0°,V 0=886 Å3, the refractive indices αNa=1.621, βNa=1.632, and γNa=1.643. These values, when plotted against the Mg/Mg+Fe ratio, fit the extrapolations towards Mg7Si8O22/(OH)2] from recently published determinative curves for the cummingtonite series. The clino-amphibole, or part of it, has space groupP21/m rather thanC2/m. The most magnesian cummingtonites reported thus far have Mg/Mg+Fe+Mn ratios around 0.7, but recently more magnesian Ca-poorP21/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 reviveTilley’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 orP21/m, may be indicated with a suffix, if known. |
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