American Mineralogist, Volume 95, pages 487–494, 2010 Arsenic-rich fergusonite-beta-(Y) from Mount Cervandone (Western Alps, Italy): Crystal structure and genetic implications AlessAndro GuAstoni,1 FernAndo CámArA,2 And FAbrizio nestolA1,3,* 1 Department of Geoscience, University of Padova, via Giotto 1, 35137 Padova, Italy CNR-Institute of Geosciences and Georesources, U.O.S. Pavia, via Ferrata 1, 27100 Pavia, Italy 3 CNR-Institute of Geosciences and Georesources, U.O.S. Padova, via Giotto 1, 35137 Padova, Italy 2 AbstrACt An As-rich variety of fergusonite-beta-(Y) occurs as greenish yellow pseudo-bipyramidal crystals up to 1 mm in length in centimeter-sized secondary cavities within sub-horizontal pegmatite dikes at Mount Cervandone (Western Alps, Italy). The mineral is associated with quartz, biotite, potassium feldspar, and orange-yellow barrel-shaped hexagonal crystals of synchysite-(Ce) up to 2 mm in length. Fergusonite-beta-(Y) crystallized during the Alpine metamorphism under amphibolite-facies conditions, as a result of interaction between As-enriched hydrothermal fluids, circulating through the pegmatite dikes, and precursor accessory minerals in the pegmatites enriched in high-field-strength elements. These pegmatites are of NYF (niobium-yttrium-fluorine) geochemical type and served as the principal source of Be, Y, Nb, Ta, and rare-earth elements (REE) that were liberated and redeposited as rare Be-As-Y-REE minerals, including the As-rich fergusonite-beta-(Y). The latter mineral crystallizes with monoclinic symmetry [a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V = 293.87(14) Å3, space group I2/a] and has the empirical formula (Y0.70Dy0.07Er0.05Ca0.05Gd0.02U0.02Yb0.01 Tb0.01Th0.01Nd0.01)Σ0.95(Nb0.68As5+ 0.27W0.06Ta0.01Si0.01)Σ1.03O4. The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R1 = 6.6% for 441 observed reflections with Fo/σFo > 4. The incorporation of As in the structure of monoclinic fergusonite-type phases is discussed in the context of the data available for synthetic analogs. Keywords: Fergusonite, single crystal, X-ray diffraction, EMPA introduCtion And previous work The compound YNbO4 (fergusonite) occurs as different polymorphs depending on the temperature of crystallization (Wolten 1967): a high-T polymorph (space group I41/a, no. 88, Z = 4) isostructural with scheelite (CaWO4; Hazen et al. 1985) and powellite (CaMoO4; Crichton and Grzechnik 2004); and a low-T polymorph with the fergusonite-type structure (known also in the mineralogical literature as fergusonite-beta, space group I2/a, no. 15, Z = 4). The structures of both scheelite- and fergusonite-type polymorphs were first determined by Komkov (1959), who proposed the non-centrosymmetric space group I2 for the fergusonite type. This author observed the monoclinic polymorph after annealing a natural tetragonal fergusonite crystal at 1000 °C for 15 min. Neutron diffraction studies of synthetic YNbO4 have shown that it actually crystallizes with space group I2/a (no. 15, Weitzel and Schröcke 1980). The standard setting for space group I2/a is C2/c, which can be obtained from the former by applying the transformation matrix (001/010/101). However, to facilitate comparison with the scheelite-type structure, we will use the body-centered setting in the present work. Wolten and Chase (1967) observed in synthetic YTaO4 that the low-T fergusonite-type structure forms only as a consequence of a displacive second-order transformation of the scheelitetype structure. For the same compound, a different monoclinic * E-mail: fabrizio.nestola@unipd.it 0003-004X/10/0004–487$05.00/DOI: 10.2138/am.2010.3239 487 polymorph (space group P2/a, no. 13, Z = 2) forms at T above the tetragonal-monoclinic transition and transforms very slowly on cooling to a scheelite-type structure through a discontinuous transition. Sugitani and Nagashima (1975) claimed to have observed the P2/a monoclinic structure also in synthetic YNbO4. Recently, Hori et al. (2006) have described the natural occurrence of YTaO4, approved as the new mineral iwashiroite-(Y), whose structure has the same topology as wolframite (space group P2/c, no. 13, with Z = 2). Due to the structural similarity between iwashiroite-(Y) and wolframite and the extensive Nb for Ta substitution in iwashiroite-(Y), the existence of a wolframitetype phase is expected also for YNbO4. The stoichiometry of the above-mentioned three polymorphs is of type ABX4, where the B sites are fourfold coordinated and the A sites are eightfold coordinated. However, these three polymorphs are not the only possible topologies for ABX 4 compounds with B = (Nb,Ta) and X = O. If A = Sb3+ or Bi3+, the structure is orthorhombic and can be either polar (space group Pc21n) or centrosymmetric (Pcnn), as seen in the stibiotantalitestibiocolumbite and bismutotantalite-bismutocolumbite series (Galliski et al. 2001 and references therein). Even though the charge distribution and lattice parameters of these phases are similar to fergusonite (A = R3+, B = R5+), the topologies of these minerals are completely different because in the Sb and Bi oxides, both A and B sites are sixfold coordinated. A different, yet monoclinic, polymorph has been described at high P for BaWO4 (BaWO4-II type, space group P21/n, no. 14, Z = 8), 488 GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y) which has the scheelite structure at room P (see Manjon and Errandonea 2009, for a review of high-P transformations in the ABX4 compounds). last, if Nb occupies the A site and boron the B site (as in behierite, NbBO4), the symmetry becomes I41/amd (no. 141, Z = 4) corresponding to a zircon-type structure topology (e.g. Mrose and Rose 1961). In fact, members of the solid solution between xenotime-(Y) (YPO4; Krstanović 1965) and chernovite-(Y) (YAsO4; Goldin et al. 1967), which also preserve the same stoichiometry and charge distribution, also crystallize with the zircon-type structure and can transform to a scheelitetype structure at high P (Zhang et al. 2008). Phosphates and arsenates of rare-earth elements (REE) with ABX4 stoichiometry also occur as monoclinic monazite-type structures (space group P21/n, no. 14, Z = 4). Unit-cell data for an unheated sample of fergusonite-type YNbO4 from granitic pegmatites at Batou, Inner Mongolia, were first reported by Gorzhevskaya et al. (1961). The Ce-dominant fergusonite-type CeNbO4, formerly known as brocenite, from the Chernigovskiy carbonatite in Ukraine, upon heating to 700 °C gave an X-ray pattern identical to that of synthetic CeNbO4, NdNbO4, and PrNbO4 (Chashka and Marchenko 1976). A survey of the literature shows that no crystal structure refinements of natural fergusonite-type YNbO4 have been published so far. The structure topology of the fergusonite-type structure has recently attracted interest because it is characteristic of stable high-pressure polymorphs of ABX4 compounds with the scheelite structure. Materials with the ABX4 stoichiometry are of technological importance as solid-state lasers (X = F), solid-state scintillators (X = O) (Errandonea et al. 2005, 2008 and references therein), and super-hard materials (e.g., reidite, ZrSiO4, Reid and Ringwood 1969; liu 1979; Scott et al. 2002). Recently, a fergusonite-type structure has been observed for a quenched high-P phase of (Zr,Ti)O2 (Troitzsch et al. 2007), which is used as a temperaturestable dielectric material in ceramic capacitors and as a stable oscillator at microwave frequencies in satellite communication. The present study is the first report of the crystal structure of unheated naturally occurring fergusonite-type YNbO4, as exemplified by As-rich fergusonite-beta-(Y) from Mount Cervandone (Verbano-Cusio-Ossola, Devero valley, Western Alps, Italy). We also provide the first description of the crystal chemistry of an Asrich fergusonite-group mineral. Mount Cervandone and the Swiss Wannigletscher (also referred to as Cherbadung) are among the best-known mineral localities in the Alps. A plethora of rare minerals from this locality has been investigated in detail (Graeser and Albertini 1995), including several arsenate minerals that were discovered here: asbecasite [Ca3(Ti,Sn4+)As3+ 6 Si2Be2O20] and cafarsite [Ca8(Ti,Fe2+,Fe3+,Mn)6–7(As3+O3)12·4H2O] (Graeser 1966); cervandonite-(Ce) [(Ce,Nd,la)(Fe3+,Fe2+,Ti4+,Al)3O2(Si2O7)1–x+y (AsO3)1+x–y(OH)3x–3y] (Demartin and Gramaccioli 2008); fetiasite [(Fe2+,Fe3+,Ti)3O2(As3+ 2 O5)] (Graeser et al. 1994); gasparite-(Ce) [(Ce,la,Nd)AsO4] (Graeser and Schwander 1987); and paraniite(Y) [Ca2Y(AsO4)(WO4)2] (Demartin et al. 1994). yellow color under sunlight. The mineral occurs in centimetersized secondary cavities hosted by sub-horizontal pegmatite dikes, and is associated with quartz, biotite, potassium feldspar, and orange-yellow barrel-shape hexagonal crystals of synchysite-(Ce) up to 2 mm in length. The Mount Cervandone locality was previously described as a typical example of Alpine-type quartz-bearing fissures (Graeser and Albertini 1995). However, recent studies indicate that the mineralized fissures of Mount Cervandone and Wannigletscher are mainly found in, and related to, pegmatitic dikes (Guastoni et al. 2006). These dikes are strongly deformed and contorted, exhibiting boudinage-like textures, but they are generally concordant with the foliation of fine-grained two-mica leucocratic gneiss, which can be interpreted as leucograniticaplitic rocks metamorphosed under amphibolite-facies conditions (Dal Piaz 1975). The dikes have a thickness of several decimeters and are composed of coarse vitreous and smoky quartz, potassium feldspar, and greenish micas (probably muscovite). locally, the pegmatite dikes are crosscut by Alpine-type quartz veins, which are generally subvertical, discordant with respect to the foliation of the gneissic host rocks and commonly contain open fissures lined with quartz crystals. Several accessory minerals are found in these quartz fissures, including cafarsite, agardite-(Y), asbecasite, chernovite-(Y), gasparite-(Ce), rutile, anatase, and tennantite. The latter sulfide mineral is frequently associated with several arsenates, carbonates, sulfates, and vanadates of Fe, Cu, Pb, and Zn (Guastoni et al. 2006). oCCurrenCe And desCription oF the pArAGenesis As-rich fergusonite-beta-(Y) from Mount Cervandone occurs as pseudo-bipyramidal crystals up to 1 mm in length (Fig. 1), which have a greenish yellow color under incandescent light and FiGure 1. Secondary-electron image of two pseudo-bipyramidal crystals of As-rich fergusonite-beta-(Y) from Mount Cervandone. GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y) The pegmatite dikes show a strong NYF (niobium-yttriumfluorine) geochemical signature manifested in the presence of several Be-As-Nb-Y rare-earth minerals, which include aeschynite-(Y), agardite-(Y), Nb-rich anatase, asbecasite, cervandonite-(Ce), chernovite-(Y), crichtonite-group minerals, As-rich fergusonite-beta-(Y), fluorite, gadolinite-(Y), monazite(Ce), paraniite-(Y), Nb-rich rutile, synchysite-(Ce), and xenotime-(Y). The assemblages of arsenates, carbonates, sulfates, and vanadates of Fe, Cu, Pb, and Zn are mainly hosted by quartz veins deposited by hydrothermal fluids during the Alpine event. We interpret these assemblages as products of crystallization from As-enriched hydrothermal fluids generated during the Alpine metamorphism under amphibolite-facies conditions. Interaction of the fluids circulating through the pegmatite dikes with the aforementioned accessory minerals enriched in high-fieldstrength elements resulted in the breakdown of these minerals, remobilization of Be, Y, Nb, Ta, and REE, and crystallization of several rare Be-As-Y-REE minerals, including the As-rich variety of fergusonite-beta-(Y) studied in the present work. 489 a graphite-filtered MoKα X-ray source and a CCD detector. Despite the small size of the sample, the diffraction spots showed a significant mosaicity. The intensities of 3053 reflections with –8 < h < 8, – 17 < k < 17, – 8 < l < 8 were collected to 70 °2θ with an acquisition time of 15 s per 0.2° frame. An empirical absorption correction (SADABS, Sheldrick 1998) was applied to the acquired data. The refined unit-cell parameters (Table 2) were obtained from 928 reflections. On the basis of 441 unique observed reflections (Fo > 4σF), the crystal structure was refined in space group I2/a (no. 15, setting b3), starting from the coordinates of Santoro et al. (1980), transformed for this setting with the SHElXl 97 program (Sheldrick 1997) using the WinGX integrated system (Farrugia 1999) to R1 = 6.6% and a goodness-of-fit value of 1.16. Scattering curves for ionized atoms were taken from the International Tables for Crystallography (Wilson 1992). The R indices and all other pertinent details of the refinement are summarized in Table 2. Site occupancies for the A and B sites were refined with the scattering curves of Dy and Nb, respectively. The chemical analysis (Table 1) obtained from the same crystal indicates that X-ray scattering at the A site is significantly higher than that from Y. Accordingly, we decided to use the scattering factor of the stronger scatterer Dy, which has an ionic radius similar to that of Y. Agreement between observed (X-ray) and calculated (i.e., based on the electron microprobe analysis) scattering per site is satisfactory (Table 1). Final atomic parameters are given in Table 3, and selected atomic distances and angles in Table 4. A table of observed and calculated structure factors is available from the authors upon request. results And disCussion AnAlytiCAl methods The chemical composition of As-rich fergusonite-beta-(Y) was determined using a Cameca-Camebax SX50 electron microprobe operating in wavelengthdispersive mode with a focused beam (~1 µm in diameter), an acceleration voltage of 20 kV, and a beam current of 20 nA, with 10 and 5 s counting times for peak and background, respectively. X-ray counts were converted to oxide wt% using the PAP correction program supplied by Cameca (Pouchou and Pichoir 1985). The following natural and synthetic standards, spectral lines and detector types were used in the analysis: wollastonite (SiKα, CaKα, TAP), corundum (AlKα, TAP), apatite (PKα, TAP), Fe2O3 (FeKα, liF), AsGa (AsLα, TAP), Zr-Y-REEsilicates (ZrLα, YLα, REELα, and NdLβ, liF), synthetic UO2 and ThO2 (UMα and ThMα, PET), metallic Nb (NbLα, PET), and Ta and W (TaLα and WLα, liF). The concentrations of Al, P, Fe, and Zr were found to be below the detection limit of the electron microprobe (ca. 0.05 wt%). The average composition and formula of fergusonite-beta-(Y) are reported in Table 1. Diffraction data were obtained from a very small single crystal (0.13 × 0.10 × 0.07 mm3) mounted on a Bruker AXS SMART APEX diffractometer equipped with TABLE 1. Oxide SiO2 As2O5 Ta2O5 Nb2O5 WO3 Ce2O3 Nd2O3 Sm2O3 Gd2O3 Tb2O3 Dy2O3 Er2O3 Yb2O3 ThO2 UO2 Y2O3 CaO Total Average composition of As-rich fergusonite-beta-(Y) from Mount Cervandone and formula calculated on the basis of 4 O atoms wt% 0.17 12.05 0.71 35.23 5.04 0.21 0.41 0.16 1.66 0.59 5.01 3.61 0.90 0.58 2.28 30.79 1.01 100.41 Range 0.09–0.25 10.19–16.15 30.31–37.38 0.51–1.18 4.24–6.70 0.06–0.38 0.17–0.51 0.10–0.45 1.52–1.91 0.38–0.73 4.73–5.20 3.32–3.83 0.55–1.30 0.45–0.66 1.51–3.23 29.45–31.50 0.81–1.15 Element Si W6+ As5+ Ta5+ Nb5+ ΣB apfu 0.007 0.056 0.270 0.008 0.683 1.024 Y Ca Ce Nd Sm Gd Tb Dy Er Yb Th U ΣA 0.702 0.046 0.003 0.006 0.002 0.024 0.008 0.069 0.049 0.012 0.006 0.022 0.949 s.s. Bcalc 41.75 s.s. Acalc 42.29 s.s. Bobs 41.82 ∆B (%) –0.1 s.s. Aobs 41.51 ∆A (%) 1.8 Note: s.s. obs is the observed site scattering obtained from the refinement of the crystal structure, here reported in electrons per formula unit (epfu); s.s. calc is the site scattering calculated from the chemical composition obtained by electron-microprobe analysis. Composition Back-scattered electron images (BSE) of polished fragments of the As-rich fergusonite-beta-(Y) crystal used for the structure refinement show that it is devoid of zoning and does not contain any exsolved phases (Fig. 2a). However, the electron microprobe analyses (Table 1) indicate significant variations in the occupancy of the B site: 30.31–37.38 wt% Nb2O5, 10.19–16.15 wt% As2O5, 4.24–6.70 wt% WO3, and 0.51–1.18 wt% Ta2O5. From Figure 2b, it is clear that the highest (Nb+Ta) contents (0.704–0.748 atoms per formula unit, apfu) are found at the core, whereas the highest As contents (0.300–0.394 apfu) are confined to the rim of the crystal. The W content is almost constant across the crystal, and As is never a dominant species at the B site. At the A site, the mineral shows a more homogeneous composition (Table 1) dominated by Y and heavy REE. TABLE 2. Crystal data and structure refinement details for As-rich fergusonite-beta-(Y) Formula weight Temperature Wavelength Crystal system Space group Unit-cell dimensions Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 35.15° Absorption correction Max. and min. transmission Refinement method Data / restraints / parameters Goodness-of-fit on F2 Final R indices [I > 2σ(I)] R indices (all data) Largest diff. peak and hole 960.34 298(2) K 0.71073 Å Monoclinic I2/a a = 5.1794(14) Å α = 90° b = 11.089(3) Å β = 91.282(8)° c = 5.1176(14) Å γ = 90° 293.87(14) Å3 8 5.427 Mg/m3 26.250 mm–1 432 0.13 × 0.10 × 0.07 mm3 3.67 to 35.15° –8 ← h ← 8, –17 ← k ← 17, –8 ← l ← 8 3053 655 (Rint = 0.0469) 99.7% Semi-empirical from equivalents 0.160 and 0.080 Full-matrix least-squares on F2 655 / 0 / 31 1.157 R1 = 0.0663, wR2 = 0.2019 R1 = 0.0874, wR2 = 0.2189 5.139 and –1.495 e·Å–3 490 TABLE 3. GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y) Atomic coordinates and equivalent isotropic displacement parameters (Å2 × 103) for As-rich fergusonite-beta-(Y) Atom Wyck Occ.* x y z Ueq U11 U22 U33 U23 U13 U12 A 4e 41.51 ¼ 0.1244(1) 0 23(1) 27(1) 14(1) 27(1) 0 –1(1) 0 B 4e 41.82 ¼ 0.6302(1) 0 25(1) 21(1) 33(1) 20(1) 0 1(1) 0 O1 8f 0.0039(16) 0.7108(7) 0.1760(20) 43(3) 39(5) 25(4) 66(6) 10(4) 2(4) 4(3) O2 8f 0.9105(16) 0.4580(6) 0.2508(15) 32(2) 35(4) 21(3) 41(4) 1(3) –4(3) 4(3) Notes: Ueq is defined as one third of the trace of the orthogonalized Uij tensor. The anisotropic displacement factor exponent takes the form: –2 π2[ h2a*2U11 + ... + 2hka*b*U12]. * Occupancies obtained using the scattering curves of Dy and Nb at the A and B sites, respectively. TABLE 4. Selected distances and angles for As-rich fergusonitebeta-(Y) A-O2 (×2) 2.335(8) A-O2 (×2) 2.402(7) A-O1 (×2) 2.344(9) A-O1 (×2) 2.414(8) <A-O> 2.374 V A (Å3) 23.56 A-B 3.600(1) A-A 3.783(1) B-O2 (×2) 1.802(8) B-O1 (×2) 1.813(9) <B-O> 1.807 3 V B (Å ) 2.98 AV* 46.8 QE* 1.0123 O1-B-O1 120.9(5) O2-B-O2 114.2(4) O1-B-O2 (×2) 107.6(4) O1-B-O2 (×2) 103.4(4) O2-A-O2 133.9(4) O1-A-O1 131.8(4) O1-A-O1 81.6(4) O2-A-O2 79.6(4) O2-A-O1 (×2) 101.6(3) O2-A-O1 (×2) 96.8(3) O2-A-O2 (×2) 71.02(17) O2-A-O2 (×2) 74.0(3) O1-A-O2 (×2) 153.6(3) O1-A-O2 (×2) 74.5(3) O2-A-O1 (×2) 153.8(3) O2-A-O1 (×2) 72.3(2) O1-A-O1 (×2) 74.7(3) O1-A-O1 (×2) 69.21(19) O2-A-O1 (×2) 126.0(3) O2-A-O1 (×2) 125.1(3) *AV = angle variance, QE = quadratic elongation (Robinson et al. 1971). Crystal structure The observed site scattering value at the B site agrees well with the average value obtained from the chemical composition. The substitution of As for Nb in the B site reduces the average observed <B-O> distance from 1.883 Å in synthetic fergusonitetype REENbO4 compounds to 1.808 Å in the Mount Cervandone sample (Table 5). Complete substitution of Nb with As in the B site would lead to a <B-O> distance of 1.740 Å observed in chernovite-(Y) (Strada and Schwendimann 1934). At the same time, this substitution reduces deformation of the B-site geometry from an angle-variance (AV, Robinson et al. 1971) value of 141.4 to 46.9 for fergusonite-type ErNbO4 (Keller 1962) and the studied sample, respectively. In the scheelite-type structure, AV values are in the range 22.4–26.14 (see Table 5). Nonetheless, in the zircon-type structure of chernovite-(Y), the AsO4 tetrahedron is highly deformed. An increase in As content is probably also responsible for the reduction of the β angle from 94.6 to 91.3° from REENbO4 compounds to As-rich fergusonite-beta-(Y), respectively (see Table 5). Interestingly, the compound BiAsO4, FiGure 2. Back-scattered electron image of the studied crystal (a) showing the location of electron-microprobe analyses plotted in b, in terms of the main constituents at the B site. which can crystallize with both the scheelite-type structure (tetrarooseveltite, Mooney 1948) and the monazite-type structure (rooseveltite, Bedlivy and Mereiter 1982), show smaller <As-O> distances of 1.619 and 1.682 Å, respectively. The structure of synthetic SmAsO4 has been recently reported to also have a scheelite-type structure (Kang and Schleid 2006) and shows <As-O> = 1.697 Å. The difference in the observed <As-O> distances between SmAsO4 and tetrarooseveltite may be due to the lower quality of structural data available for the latter. The AV value of rooseveltite (54.68) is closer to that observed in the studied crystal. In the literature, there is a significant volume of crystallo- GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y) TABLE 5. 491 Selected average bond distances, distortion parameters, average ionic radii (<ri>), and lattice parameters of ABX4 compounds with A = REE, B = Nb or As, and X = O A Structure type ABX4 <A-O> (Å) <B-O> (Å) AV* QE* <ri> (Å) B<ri> (Å) a (Å) b (Å) c† (Å) β (°) (a+c)/2 (Å) Ref. Fergusonite-type LuNbO4 2.321 1.886 120.91 1.033 0.977 0.48 5.229(3) 10.822(8) 5.042(2) 94.38(2) 5.136 1 Fergusonite-type YbNbO4 2.329 1.877 128.43 1.034 0.985 0.48 5.239(1) 10.834(1) 5.044(1) 94.47(1) 5.142 2 Fergusonite-type TmNbO4 2.331 1.894 119.83 1.033 0.994 0.48 5.262(2) 10.876(5) 5.049(2) 94.52(5) 5.156 1 Fergusonite-type ErNbO4 2.346 1.888 141.40 1.036 1.004 0.48 5.280(1) 10.916(5) 5.064(2) 94.48(8) 5.172 1 Fergusonite-type HoNbO4 2.358 1.880 129.47 1.035 1.015 0.48 5.299(1) 10.947(1) 5.072(1) 94.53(1) 5.185 2 Fergusonite-type YNbO4 2.363 1.894 121.79 1.035 1.019 0.48 5.317(1) 10.999(1) 5.090(1) 94.53(1) 5.204 3 Fergusonite-type DyNbO4 2.360 1.899 139.40 1.036 1.027 0.48 5.320(1) 11.000(8) 5.074(4) 94.57(7) 5.197 1 Fergusonite-type TbNbO4 2.382 1.881 136.27 1.035 1.040 0.48 5.349(1) 11.036(9) 5.085(1) 94.62(3) 5.217 1 Fergusonite-type GdNbO4 2.394 1.889 135.06 1.035 1.053 0.48 5.374(3) 11.095(9) 5.106(5) 94.58 5.240 4 Fergusonite-type EuNbO4 2.407 1.884 134.89 1.036 1.066 0.48 5.395(1) 11.135(1) 5.116(1) 94.62(1) 5.253 1 Fergusonite-type SmNbO4 2.417 1.888 134.45 1.036 1.079 0.48 5.422(5) 11.178(9) 5.121(5) 94.69 5.272 4 Fergusonite-type PrNbO4 2.466 1.875 134.62 1.035 1.093 0.48 5.499(2) 11.342(7) 5.157(2) 94.57(5) 5.328 1 Fergusonite-type NdNbO4 2.447 1.879 129.09 1.034 1.109 0.48 5.467(1) 11.279(1) 5.146(1) 94.50(1) 5.307 2 Fergusonite-type CeNbO4 2.476 1.883 132.79 1.035 1.143 0.48 5.535(1) 11.399(1) 5.159(1) 94.60(1) 5.347 5 Fergusonite-type LaNbO4 2.505 1.874 121.41 1.032 1.160 0.48 5.565(1) 11.519(1) 5.202(1) 94.10(1) 5.383 2 Fergusonite-type (As-rich) YNbO4 2.374 1.808 46.94 1.012 1.024 0.39 5.179(1) 11.090(3) 5.118(1) 91.28(1) 5.149 6 Scheelite-type SmAsO4 2.427 1.697 26.14 1.007 1.079 0.335 5.066(1) 11.461(1) 5.066(1) 90.00 5.066 7 Scheelite-type CeNbO4 2.497 1.854 22.43 1.006 1.143 0.48 5.377(1) 11.595(1) 5.377(1) 90.00 5.377 8 Scheelite-type YNbO4 2.500 1.892 25.41 1.007 1.014 0.48 5.160 10.890 5.160 90.00 5.160 9 Note: References: [1] Keller (1962), [2] Tsunekawa et al. (1993), [3] Weitzel and Schröcke (1980), [4] Trunov and Kinzhibalo (1982), [5] Santoro et al. (1980), [6] this study, [7] Kang and Schleid (2006), [8] Skinner et al. (2004), [9] Komkov (1959). * AV = angle variance and QE = quadratic elongation (Robinson et al. 1971). † c refers to b in scheelite-type structures. graphic data on synthetic fergusonite-type niobates with different cations in the A site (REE and Y). Using these data, we confirm a linear correlation between the average distance <A-O> and the ionic radius of the A-site cation A<ri>, as first noted by Kinzhibalo et al. (1982): A<ri> = 0.943*<A-O> (Å) – 1.2057 (R² = 0.97). The sample examined in the present study and published data on scheelite-type structures (with As or Nb at the B site) plot on the same trend (Fig. 3a). The only outliers are synthetic PrNbO4 studied by Keller (1962) and scheelite-type fergusonite studied by Komkov (1959), who reported a rather unusual A-site occupancy for this natural sample, Y0.85Yb0.15NbO4. With the above equation, Komkov’s data give a calculated ionic radius of 1.15 Å, which is much closer to that of la. It is clear that deviation of this sample from the above trend results from the inadequacy of its chemical analysis. Another interesting relationship was observed between the a lattice parameter and the mean ionic radius of the A-site cation. Fergusonite-type compounds define a linear trend with A<ri> = 0.521*a (Å) – 1.7468 (R² = 0.98) because changes in A-site occupancy (with fixed B-site occupancy) have a very limited effect on the β angle. Scheelite-type polymorphs plot outside that trend and so does the As-rich monoclinic sample examined in this study (Fig. 3b). If we ignore the monoclinic distortion by averaging the a and c lattice parameters, the scheelite-type compounds with Nb in their B site plot along the trend (Fig. 3c), but not the As-rich sample. This deviation implies that the incorporation of As reduces the length of the a lattice parameter, or has an important effect on the β angle, thus reducing the magnitude of monoclinic distortion. This can be also seen in Figure 3d, where the AV value of Robinson et al. (1971) is plotted vs. A <ri>: while monoclinic phases with light to medium rare-earth cations at the A site show a progressive decrease of the AV value with increasing A<ri>, tetragonal phases show a much smaller distortion of the B tetrahedron, and so does the As-rich sample. Considering the fact that only about 25% of the B positions are occupied by As and that fergusonite-type structures undergo a phase transition to the scheelite-type structure with decreasing T, it is plausible that the presence of As reduces the temperature of the tetragonal-to-monoclinic transition. Therefore, in principle, it would be possible to expect that an increase in As content would lead to a change in symmetry from I2/a to I41/a, the distortion being proportional to the As content. Clearly, experimental data on intermediate Nb-As compositions will be necessary to explore the compositional dependence of the monoclinic-to-tetragonal phase transition in fergusonite. It is well known that increasing P leads to phase transitions from scheelite-type to fergusonite-type structures (Grzechnik et al. 2005) and from zircon-type to scheelite-type structures (Stubican and Roy 1963; Zhang et al. 2008). It has been also proposed that a wolframite-type phase is the post-fergusonitetype structure with increasing pressure (li et al. 2004), whereas the scheelite to wolframite high-P phase transition has been predicted for CaMoO4 and CaWO4 by Nicol and Durana (1971) and confirmed by Errandonea et al. (2003). These phase transitions have been modeled by Fukunaga and Yamaoka (1979) and Bastide (1987) as a function of the ratio of the ionic radii of cations at both A and B sites and of the ionic radius of the anion. Recently, transformation from a zircon-to-scheelite structure and, at higher P, to a fergusonite-type structure has been described for luVO4 (Mittal et al. 2008). Kolitsch and Holtstam (2004) reviewed the crystal chemistry of REEXO4 compounds (with X = As, V, and P), which show both the monazite- and zircon-type topologies. Depending on the ionic radius of the lanthanide considered, for a fixed composition of the B site, the larger the REE3+ cation, the more stable the monazite-type structure. At the same time, an increase in size of the XO4 group destabilizes the monazite structure in favor of the zircon-type structure. loskutov et al. (1977) even proposed that an increase in ionic radius of the X cation ultimately leads to the stabilization of a scheelite structure and, for very large X, a wolframite-type structure. Interestingly, SmAsO4 falls close to the region in which the structural topology of REE arsenates changes from that of monazite (REE = la, Ce, Pr, and Nd and REE = Pm-lu, Y, and Sc; see Kolitsch and Holtstam 2004 and 492 GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y) FiGure 3. Relations between the average ionic radius at the A site and (a) the average bond distance at the A site, (b) a lattice parameter, (c) average a+c lattice parameter, and (d) anglevariance value (Robinson et al. 1971) for the studied sample (open circle), SmAsO4 of Kang and Schleid (2006) (gray circle), T-fergusonite(Ce) of Skinner et al. (2004) (gray diamond), T-fergusonite-(Y) of Komkov (1959) (cross) and synthetic fergusonitetype REENbO 4 with variying composition of the A site (references as in Table 5) (black circles). references therein). According to Kang and Schleid (2006), scheelite-type structures are stabilized at high P, and become metastable at room P. However, scheelite-type NdAsO4 has also been reported from room-P synthesis at 550 °C (Mazhenov et al. 1988). In Figure 4, three ABO 4 structure types with different topologies but the same coordination for A and B cations (fergusonite-type, scheelite-type, and zircon-type) are compared. From a descriptive point of view, a shift along the c axis within (110) planes would change the zircon-type topology [as found in chernovite-(Y)] to the scheelite topology. The latter structure type can easily undergo a symmetry reduction to monoclinic by softer distortion. Figure 4 shows only the topologies stable at room P and T; the actual mechanism of the zircon-scheelite transformation at high pressure is much more complex and involves a change in coordination of the O atoms, which undergo a displacement of ca. 1 Å (Smirnov et al. 2008). Structural changes are required to effect a transition to the fergusonite topology, as can be seen from the results of Mittal et al. (2008), who observed a negligible volume change during the scheelite-to-fergusonite phase transition, and proposed that it is a second-order displacive phase transition. In addition, Kolitsch and Holtstam (2004) proposed that the presence of impurities (Ca, Th, U, and Si) can stabilize one or the other FiGure 4. Sketch comparing the structures of fergusonite topology (a), scheelite topology in two different orientations (b), and zircon topology (c). The open squares in the fergusonite and scheelite topologies help in visualizing the tetragonal-to-monoclinic deformation. White spheres are A sites. Tetrahedra are B sites. The arrows in c show how a differential shift along [001](110) relates the zircon and scheelite topologies. GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y) polymorph. The sample studied in the present work contains all of the above elements, which may explain the stability of the fergusonite structure relative to the scheelite structure. To conclude, although we cannot rule out the possibility that a complete series of scheelite-type compounds involving the As-for-Nb substitution exists, it is highly probable that there is at least one change in topology across that series, controlled mainly by atomic substitutions in the B site. Paraniite-(Y) [Ca2Y(AsO4)(WO4)2], which is also present at Mount Cervandone and may be viewed as an intermediate member of the solid solution between scheelite and chernovite-(Y), has an overall scheelite topology, but a tripled lattice periodicity along [001] owing to cation ordering in the symmetrically non-equivalent B sites along the c axis. We are conducting a detailed study of chernovite-(Y) samples from the same locality to constrain the chemical variations and possible changes in symmetry in naturally occurring Y arsenates. ACknowledGments We thank the fruitful comments of W. Crichton, an anonymous reviewer, and Associate Editor A. Chakhmouradian that helped to improve the manuscript. We also thank R. Carampin of CNR-IGG Padova for access to the WDS electron microprobe facility and R. Gastoni of CNR-IGG Pavia for sample preparation. Fernando Cámara was supported by funding by CNR-IGG through the project TAP01.004.002. 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