653 * Current address: Department of Natural History, Mineralogy, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario M5S 2C6, Canada § Corresponding author: frank_hawthorne@umanitoba.ca The Canadian Mineralogist Vol. 51, pp. 653-662 (2013) DOI : 10.3749/canmin.51.4.653 THE CRYSTAL CHEMISTRY OF THE GRAFTONITE-BEUSITE MINERALS Kimberly T. TAiT* And FrAnK C. HAWTHOrne § Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 miCHAel A. WiSe Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560, U.S.A. AbSTrACT The crystal structures of seven members of the graftonite-beusite series, ideally (Fe 2+ ,Mn 2+ ,Ca) 3 (PO 4 ) 2 , monoclinic P2 1 /c, a 8.77–8.81, b 11.43–11.58, c 6.13–6.17 Å, β 99.19–99.32°, V 607.5–617.7 Å 3 , have been refned to R 1 indices of 2.1–3.7% using ~1300–1600 unique observed refections (|F o | > 5σF) collected using a single-crystal diffractometer equipped with MoKα X-radiation. The crystals used in the collection of the X-ray data were subsequently analyzed with an electron microprobe and the structural and microprobe results were used to assign site populations. The refned site-scattering values and linear variation in mean bond-length as a function of aggregate-cation radius indicate that Ca is completely ordered at the M(1) site. Similarly, Mn is ordered at the M(1) and M(3) sites, with any excess Mn occurring at M(2), and Mg is completely ordered at M(2). Detailed consideration of incident bond-valence sums at the three M sites indicates that the coordination numbers of the M(1), M(2), and M(3) sites are [8], [5], and [6], respectively, although the differences between these and [7], [5], and [5] are very small. Ca is dominant at the M(1) site in a previously refned beusite structure, and there are compositions reported here and elsewhere in which Ca is dominant at M(1) in graftonite-like compositions, indicating the potential for new mineral species in this group. Keywords: Graftonite, beusite, crystal-structure refnement, electron-microprobe data. inTrOduCTiOn Graftonite, ideally [Fe 2+ 3 (PO 4 ) 2 ], was described by Penfeld (1900) from a granitic pegmatite in New Hampshire. Beusite, ideally [Mn 2+ 3 (PO 4 ) 2 ], was frst noted by Beus (1950) and Brooks & Shipway (1960) as a graftonite-like mineral with Mn 2+ dominant over Fe 2+ , and was formally described as a distinct species from the pegmatites of the San Luis area, Argentina, by Hurlbut & Aristarain (1968). Both minerals form a solid-solution series and occur commonly as late-stage accessory minerals in complex granitic pegmatites (e.g., Fransolet 1977, Fransolet et al. 1986, Lahti 1981, Wise & Černý 1990, Wise et al. 1990, Černý et al. 1998, Smeds et al. 1998, Pieczka 2007, Guastoni et al. 2007, Vignola et al. 2008, Galliski et al. 2009, Ercit et al. 2010). These minerals also occur as constituents of phosphate-oxide inclusions in IIIAB iron meteorites (Bild 1974, Olsen et al. 1999), and graftonite has been reported as a primary phase in a phosphorous-rich iron formation (Stalder & Rozendaal 2002). The graftonite-beusite structure is a dense frame- work of polyhedra, with extensive edge- and corner- sharing (Hawthorne 1998, Huminicki & Hawthorne 2002) between phosphate tetrahedra and [5]- to [8]-coordinated divalent-metal-oxide polyhedra. The crystal structure of graftonite was solved by Calvo (1968) and beusite by Hurlbut & Aristarain (1968). The chemical composition of this series may be written as (Fe 2+ ,Mn 2+ ,Ca) 3 (PO 4 ) 2 , and Ca plays an important role in the structure of these minerals. In this structure, there are three distinct divalent-metal sites: M(1), M(2), and M(3). Calvo (1968) described the coordination of these sites as [7], [5], and [5], respectively, with Ca preferentially occupying the M(1) site. With decreasing Ca content, it has been suggested that the coordination of the M(1) site tends to decrease from [7] to [6] to [5] (Calvo 1968, Wise et al. 1990, Steele et al. 1991). The M(2) site has been reported as [5]-coordinated and tends to be Fe 2+ -dominant (Nord & Ericsson 1982); M(3) has been described as both [5]- and [6]-coordinated, and tends to be Mn 2+ -dominant. Wise et al. (1990) reported