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MPM-01 General contributions to mineralogy

Re-examination of the crystal-chemistry of aenigmatite: Electron microprobe analyses, structure refinement, and Mössbauer spectroscopy of aenigmatite from Vesterøya, Sandefjord, Oslo Igneous province

Edward Grew, University of Maine (United States)
Ulf Halenius, Swedish Museum of Natural History (Sweden)
Marco Pasero, Universita di Pisa (Italy)
 

Aenigmatite, end member Na₄Fe²⁺₁₀Ti₂Si₁₂O₄₀, is a widespread chain silicate of alkaline volcanic and plutonic rocks; for example, it is found in masses up to 20 cm across in syenite pegmatites cutting larvikite at several localities on the peninsula Vesterøya, Vestfold, Norway (Andersen, Berge & Burvald, 1996, Mineralien-Welt, 7(4), 20-100). Electron microprobe analyses of two crystals from an aggregate 2.5 x 2 x 2 cm across from Vesterøya gave (Na_3.73Ca_0.27)_=4.00 (Fe²⁺_8.55Ti_2.10Mg_0.46Fe³⁺_0.40Mn_0.40Ca_0.06Zn_0.01Zr_0.01)_=11.99(Si_11.10Al_0.64Fe³⁺_0.26)_=12.00O₄₀ and (Na_3.92Ca_0.06K_0.02)_=4.00(Fe²⁺_8.05Ti_2.05Fe³⁺_0.76Mg_0.55Mn_0.37Ca_0.19Zn_0.02Zr_0.01Nb_0.01)_=12.01(Si_11.07Al_0.59Fe³⁺_0.34)_=12.00O₄₀ (Fe³⁺/Fe²⁺ ratio calculated by assuming 28 cations and 40 oxygens). Cr₂O₃ was also sought, but did not exceed 0.01 wt%. The structure of the first crystal was refined in the centrosymmetric triclinic space group P-1 to give R₁ = 0.0277 for 5515 reflections with F_o > 4σ(F_o) and 0.0324 for all 6145 data; GooF 1.124. Cell parameters are a 10.415(1), b 10.840(1), c 8.931(1) Å, α 105.107(4), β 96.610(5), γ 125.398(4)°, V 746.8(1) ų. Absence of twinning was ascertained by a series of accurate intensity measurements with the four-circle diffractometer. The recorded Mössbauer spectrum of a bulk sample is characterised by a broad and strong absorption doublet and by a weaker absorption region centred at 0.9 mm/s. These features are attributed to overlapping quadrupole doublets caused by Fe²⁺ at several different sites and to the high-energy quadrupole components from Fe³⁺, respectively. Relative areas of the doublets give Fe³⁺/Fe_tot = 0.12(1) whatever the initial fitting model, and ^IVFe³⁺/Fe_tot = 0.04(2), which is consistent with ^IVFe³⁺/Fe_tot = 0.018(1) obtained from occupancy of T3, the only T site to found have Fe in the X-ray refinement. Ti is fully ordered at M7, the site with the most shared edges, whereas ^VIFe³⁺ is mostly ordered at the smallest M sites (M1 and M2), consistent with what is reported in several isostructural minerals in the sapphirine group. Ca is ordered at M8, one of two 8-coordinated sites. Bond lengths and bond valence sums suggest that only Si occupies T2 and T4, and that T1 and T3 have the highest proportion of Al. Thus, one of four of the substitutions proposed by L. M. Larsen, (1977, Lithos, 10, 257-270) for aenigmatite, Ca + ^IVAl Na + ^IVSi, could be operating at a local scale. The compositional and Mössbauer data are consistent with two other substitutions proposed by Larsen, ^VIFe²⁺ + ^IVSi ^VIFe³⁺ + ^IVAl and ^VIFe²⁺ + ^IVSi ^VIFe³⁺ + ^IVFe³⁺ for incorporating Al and Fe³⁺, but the Ti content of 2 per 28 cations rules out the fourth, viz., incorporation of ^VIFe³⁺ in a wilkinsonite component, Na₄Fe²⁺₈Fe³⁺₄Si₁₂O₄₀.