• journal_title：The Canadian Mineralogist
• Contributor：Paula C. Piilonen ; André E. Lalonde ; Andrew M. McDonald ; Robert A. Gault ; Alf Olav Larsen
• Publisher：Mineralogical Association of Canada
• Date：2003-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.41.1.1
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：41
• issue：1
• firstpage：1
• section：Articles

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)₄X_0–1, 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⁺/Fe_tot 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(1)²⁺ + M(2,3)²⁺ + (Zr,Ti) + F M⁽¹⁾Na + M^(2,3)Fe³⁺ + Nb + O.