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

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8 Scopus citations


A Ca- and Mn-poor clino-amphibole with Mg/Mg+Fetot+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 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 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.

Original languageEnglish
Pages (from-to)319-331
Number of pages13
JournalContributions to Mineralogy and Petrology
Issue number4
StatePublished - 1 Apr 1969
Externally publishedYes

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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