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The crystal-chemistry of aenigmatite revisited: electron microprobe data, structure refinement and Mössbauer spectroscopy of aenigmatite from Vesterøya (Norway)

¹ Department of Geological Sciences, University of Maine, 5790 Bryand Research Center, Orono, ME 04469, USA
² Department of Mineralogy, Swedish Museum of Natural History, Box 50007, 104 05 Stockholm, Sweden
³ Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy

^* Corresponding author, e-mail: esgrew{at}maine.edu

The structure of an untwinned aenigmatite crystal from Vesterøya, Vestfold district, Norway, having a composition (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)_=12.00 O₄₀ was refined in space group P to give R₁ = 0.0277 for 5515 unique reflections with F_o > 4(F_o) and 0.0324 for all 6145 unique data; GooF 1.124. Cell parameters are a = 10.415(1), b = 10.840(1), c = 8.931(1) Å, = 105.107(4), β = 96.610(5), = 125.398(4)° and V = 746.8(1) ų. Mössbauer spectroscopy of a bulk sample gave Fe³⁺/Fe_tot = 0.12(1) whatever was 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 have Fe. 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 related minerals of the sapphirine group. Ca is ordered at M8, one of the two 8-coordinated sites. Bond lengths and bond-valence sums suggest that T2 and T4 are occupied by Si only, and that T1 and T3 have the highest proportion of Al. Ordering of Al³⁺ and Fe³⁺ at T3 could be related to Ca being ordered at M8, because M8 is linked with T3 through four oxygen anions, more than M8 with any other T site. Thus, one of the four substitutions proposed for aenigmatite, Ca + ^IVAl Na + ^IVSi, could be operating at a local scale. Since (^IVAl + ^IVFe³⁺) exceeds Ca, there must be other substitutions for incorporating Al and Fe³⁺. Two of the four proposed substitutions, ^VIFe²⁺ + ^IVSi ^VIFe³⁺ + ^IVAl and ^VIFe²⁺ + ^IVSi ^VIFe³⁺ + ^IVFe³⁺, are consistent with the compositional and Mössbauer data, but the Ti content of 2 per 28 cations rules out incorporation of ^VIFe³⁺ in a wilkinsonite component, Na₄Fe²⁺₈Fe³⁺₄Si₁₂O₄₀.

Key-words: aenigmatite, sapphirine group, crystal structure, electron microprobe, chemical analyses, Mössbauer spectroscopy, Vesterøya (Norway).

This article has been cited by other articles:

				E. S. Grew, U. Halenius, M. Pasero, and J. Barbier Recommended nomenclature for the sapphirine and surinamite groups (sapphirine supergroup) Mineralogical Magazine, December 23, 2008; 72(4): 839 - 876. [Abstract] [Full Text] [PDF]