INSIGHTS INTO ASTROPHYLLITE-GROUP MINERALS. I. NOMENCLATURE, COMPOSITION AND DEVELOPMENT OF A STANDARDIZED GENERAL FORMULA

Abstract

The composition of 135 samples of astrophyllite-group minerals from 15 localities has been established by EMPA, ICP-AES, FTIR, TGA, thermal decomposition, NRA and Mössbauer spectroscopy. A standardized general formula has been developed and is of the form A2BC7D2T8O26(OH 4X0-1, where [10]-[13]A = K, Rb, Cs, H3O+, H2O, Na or □; [10]B = Na or Ca; [6]C = Mn, Fe2+, Fe3+, Na, Mg, or Zn; [6]D = Ti, Nb, or Zr; [4]T = Si or Al, X = Φ = F, OH, O, or □. Data acquired by Mössbauer spectroscopy, thermodynamic approximations, and EMP analyses have been used to demonstrate that F orders at the Φ(16) site and does not occur at the two general OH sites within the O sheet. On this basis, formulas of the eight species of astrophyllite-group minerals have been redefined. Results from Mössbauer spectroscopy indicate Fe3+/Fetot values in the range from 0.01 to 0.21, corresponding to 0.05 to 0.56 apfu Fe3+, confirming that Fe2+ is the dominant valence state for iron in the structure. Minerals from silica-oversaturated and -undersaturated alkaline intrusions are distinct in chemical composition. In oversaturated rocks, the dominant member of the group is astrophyllite sensu stricto, which occurs as a late-stage postmagmatic phase, enriched in Rb, Fe2+, Ti, Si and F. In contrast, undersaturated intrusions, in particular Mont Saint-Hilaire, Quebec, show the greatest diversity in species and range in chemical composition. Kupletskite-subgroup samples are enriched in Na, Mn, Fe3+, Zn, Zr and Nb, whereas astrophyllite-subgroup samples are enriched in K, Ca, Fe2+, Ti, Zr and Al. Enrichment of kupletskite-subgroup samples in Fe3+, Mn and Nb suggests crystallization under more oxidizing conditions than those of the astrophyllite subgroup. Incorporation of Nb into the structure and the formation of Nb-bearing kupletskite and niobokupletskite are the result of the substitution M(I)2+ + M(2.3)2+ + (Zr, Ti) + F ⇔ M(1)Na + M(2.3)Fe3+ + Nb + O.