American Mineralogist, Volume 98, pages 361–366, 2013 0003-004X/13/0203–361$05.00/DOI: http://dx.doi.org/10.2138/am.2013.4259 361 Superstructure, crystal chemistry, and cation distribution in ilipstadite, a Sb 5+ -bearing, spinel-related mineral Paola Bonazzi,* laura Chelazzi, and luCa Bindi Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy aBstraCt The crystal structure of the rare, spinel-related Sb mineral filipstadite from Långban, Filipstad district, Värmland, Sweden, has been solved and refined in the space group Fd3 m [a = 25.9300(6) Å, V = 17434.4(5) Å 3 , and Z = 216] and refined to R = 4.41% for 681 F o > 4σ(F o ) using MoKα X-ray data. The structure of filipstadite is topologically identical to the spinel-type structure with cations occupying 1/8 of the tetrahedral (T) and 1/2 of the octahedral (M) interstices of a cubic close-packing of oxygen atoms. Due to the cation ordering, which leads to the tripling of the unit-cell edge, the M and T sites of the spinel-type structure split into six and five independent sites, respectively. Chemical composition was determined by electron microprobe. The fractions of major cations obtained from chemical analysis were distributed between T and M sites taking into account the weighted electron number at both T and M sites, and minimizing the discrepancy between the calculated and the observed overall <M-O> distance. Cations present in minor amounts were assigned on the basis of their known site preference. The obtained populations (2M = Mn 2+ 0.56 Mg 0.76 Fe 3+ 0.16 Al 0.02 Sb 5+ 0.50 ; T = Mn 2+ 0.60 Mg 0.07 Fe 3+ 0.30 Zn 0.02 Si 4+ 0.01 ) were then tentatively distributed among the individual M and T sites on the basis of crystal chemical considerations. Keywords: Filipstadite, spinel, crystal structure determination, superstructure, cation distribution, Långban (Sweden) introduCtion Filipstadite, ideally (Mn 2+ ,Mg) 2 (Fe 3+ 0.5 Sb 5+ 0.5 )O 4 , was first de- scribed by Dunn et al. (1988) as a new spinel-related mineral having orthorhombic symmetry and unit-cell parameters a = 36.7, b = 36.7, c = 25.9 Å. Using Weissenberg and precession meth- ods, Dunn et al. (1988) showed a substructure-superstructure relationship with intense reflections corresponding to those of spinel-type structure (a = 3√2a s , b = 3√2a s , and c = 3a s , where a s is the unit-cell translation of a spinel-type structure) and supposed the superstructure to be due to cation ordering at the octahedral sites. This reasonable hypothesis, however, remained to be proven by a structure analysis. At the type-locality (Långban, Filipstad district, Värmland, Sweden) filipstadite occurs intimately associated with jacobsite, ingersonite and calcite. In particular, filipstadite was described to replace jacobsite, (Mn 2+ ,Fe 2+ ,Mg)(Fe 3+ ,Mn 3+ ) 2 O 4 , and many filipstadite crystals were reported as having residual, irregular cores of jacobsite. Two other occurrences of filipstadite were later reported at two other “Långban-type” Mn-Fe deposits, namely Jakobsberg and Nordmark, in south-central Sweden (Holtstam 1993; Holtstam et al. 1998). At Jakobsberg the min- eral coexists with hausmannite, calcite, forsterite, phlogopite, and another spinel-related phase exhibiting close similarities with filipstadite (Holtstam 1993), which was later approved as a new mineral species with the name tegengrenite (Holtstam and Larsson 2000). At Nordmark filipstadite occurs in fine-grained rock samples, including arsenates of svabite-johnbaumite and adelite-tilasite series, forsterite, phlogopite, manganoan calcite, plumbiam stibarsen, and plumbian romeite (Holtstam et al. 1998). Interestingly, in the Nordmark samples described by Holtstam et al. (1998), Sb-poor filipstadite and Sb-rich jacobsite occur, possibly indicating solid solution between jacobsite- magnesioferrite spinel and filipstadite. As pointed out by these authors, however, the crystallographic features of filipstadite involving lower symmetry and superstructure should inhibit complete solid solution and some intermediate composition might reasonably represent disordered metastable phases and/ or sub-optical topotactic intergrowths. Much attention has been devoted to the cation distribution in spinel-type oxides in the last decades (Lavina et al. 2002 and ref- erences therein), but only few crystal-chemical relationships are known for spinels containing Sb 5+ or other pentavalent cations. According to Tarte and Rulmont (1988), the cation distribution over tetrahedral and octahedral sites in spinels containing high- valence cations is found to be in contradiction with the well known relative site preferences, which are based on their size and/or, in the case of cations subjected to Jahn-Teller distortion, on their crystal field stabilization energies. To contribute to the scientific debate on the spinel crystal chemistry, we present the crystal structure determination of filipstadite from the type material (sample 163012, Smithsonian Institution, Washington, D.C.). Crystal chemical considerations led us to propose a tentative distribution of cations among octa- hedral and tetrahedral sites. exPerimental methods X-ray diffraction A crystal was selected and preliminarily examined with a Bruker P4 single- crystal diffractometer using graphite-monochromatized MoKα radiation. The * E-mail: paola.bonazzi@unifi.it