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The role of Fe and cation order in the crystal chemistry of surinamite, (Mg,Fe²⁺)₃(Al,Fe³⁺)₃O[AlBeSi₃O₁₅]: A crystal structure, Mössbauer spectroscopic, and optical spectroscopic study

¹ Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4M1, Canada
² Department of Geological Sciences, University of Maine, 5790 Bryand Center, Orono, Maine 04469, U.S.A.
³ SGAB Analytica, Box 511, SE-183 25 Täby, Sweden
⁴ Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

Correspondence: ^* E-mail:barbier{at}mcmaster.ca

The crystal structure of surinamite from "Christmas Point," Enderby Land, Antarctica, has been newly refined by single-crystal X-ray diffraction: ^VI((Mg_2.26Fe²⁺ _0.74Fe³⁺ _0.39Al_2.61)O^IV (Al_1.00Be_1.00Si_3.00)O₁₅ (simplified formula), space group P2/n, a = 9.915(2), b = 11.368(2), c = 9.617(2), b = 109.30(2)°, Z = 4, wR(F²) = 0.074 for 4876 independent reflections. The refined site occupancies agree well with the chemical composition determined by electron microprobe analysis and with the Fe³⁺/Fe²⁺ ratio estimated from stoichiometry. The surinamite structure is characterized by an ordered Al/Be/Si distribution on the tetrahedral sites and by charge ordering with extensive Mg²⁺-Fe²⁺ and Al³⁺-Fe³⁺ exchange on the octahedral sites. This cation ordering is distinct from that observed in related phases of the sapphirine series and aenigmatite groups, and the difference is linked to the unique structural topology of the tetrahedral chains in surinamite. Optical and Mössbauer spectra of surinamite have been fully interpreted in terms of an octahedral distribution of Fe²⁺ and Fe³⁺ cations that agrees very well with the X-ray site populations. Both the X-ray and Mössbauer data establish the absence of significant tetrahedral Fe and the non-uniform distribution of octahedral Fe. Intense and strongly polarized absorption bands caused by IVCT-processes in clusters of Fe³⁺ and Fe²⁺ iron in edge-sharing octahedral sites produce the unusual color and pleochroism observed in surinamite.

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