Temperature-dependent single-crystal neutron diffraction study of natural chondrodite and clinohumites

The crystal structures of natural F-bearing chondrodite [Mg4.64Fe0.28Mn0.014Ti0.023(Si1.01O4)2 F1.02OH0.97] from the Tilley Foster mine (Brewster, New York), F-bearing titanian clinohumite [Mg8.805Fe0.006Ti0.214(Si0.993O4)4F0.484OH0.516] from Kukh-i-Lal (Tadjikistan) and F-free titanian hydroxylclinohumite [Mg7.378Fe1.12Mn0.052Ni0.014Ti0.453(Si0.996O4)4OH1.0] from Val Malenco (Italy) were refined in space group P21/b (unique axis a) from single-crystal neutron diffraction data, collected on a four-circle diffractometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory. Accurate H atom positions were determined at 295 K, 100 K, and 20 (10) K. Only one H position of approximately 50% occupancy was observed for each structure, which confirms a disordered H model. Time-of-flight single-crystal neutron data were also collected at 295 K and 20 K for the Val Malenco clinohumite as an additional check on space group symmetry. The crystal structure of the Kukh-i-Lal clinohumite was further investigated by X-ray single-crystal refinement at 295 K and by piezoelectric measurements. A few, very weak, symmetry-forbidden reflections were observed for each crystal at both ambient and lower temperatures. The same reflections were observed by all methods used. No temperature dependence is indicated, as no additional peaks appear at low temperature, and the intensity of the reflections are sample dependent. It appears that the real structure is made up of P21 and Pb domains so that violations are due to ordering of both H and Ti. No distinct piezoelectric effect was observed that would indicate the absence of a center of symmetry. This points to the simultaneous presence of various enantiomorphic domains, which cancels the piezoelectric effect of individual domains. The decrease in unit-cell volume with F substitution in clinohumites can be explained by the higher concentration of H-site vacancies and the coupled cationic and anionic substitution on the M3 and O/F site. al. 2000) were determined by single-crystal X-ray diffraction methods. The O-D...O bond geometry in both deuterated hydroxylchondrodite and deuterated hydroxylclinohumite has been examined using neutron powder diffraction methods (Lager et al. 2001; Berry and James 2001). The humite minerals are a homologous series of magnesium orthosilicates whose structures are based on slightly distorted hexagonal closepacked (hcp) arrays of anions (O, F, OH) with one-half of the octahedral sites occupied by M and 8–12% of the tetrahedral sites occupied by Si (Ribbe 1982). The non-standard monoclinic space group P21/b (unique axis a) for chondrodite and clinohumite is used in order to emphasize the structural relationship to olivine. The a crystallographic axis is chosen parallel to the 63 axis of an ideal hcp anion array. In terms of a polyhedral model the dominant structural units are chains of edge-sharing octahedra, running parallel to the c axis and crosslinked by SiO4 tetrahedra. The asymmetric unit of the chondrodite structure has three octahedrally coordinated M FRIEDRICH ET AL.: NEUTRON STUDY OF CHONDRODITE AND CLINOHUMITE 982 sites, one tetrahedrally coordinated Si site, five oxygen positions and one H position; the clinohumite structure has five M sites, two Si sites, eight oxygen positions, and one H position (Fig. 1). The (F, O, OH) anion is triangularly bonded to three M cations, one M2, and two M3. End-members hydroxylchondrodite (Yamamoto 1977; Lager et al. 2001) and hydroxylclinohumite (Ferraris et al. 2000; Berry and James 2001) have two symmetrically different H sites occupied. Yamamoto (1977) included the positions of the H atoms for synthetic hydroxylchondrodite as refineable parameters in the least-squares analysis, and proposed positional disorder for the protons. A statistical distribution of the OH orientation among the equilibrium positions satisfies the symmetry conditions of the space group P21/b and explains the elongated displacement parameter ellipsoid of the O atom, which serves as both acceptor and donor. This disordered model has been recently confirmed by Lager et al. (2001). Fujino and Takéuchi (1978) have located the centrosymmetrically related, partially occupied H sites in a titanian clinohumite from difference-Fourier maps. In nature only Ti-rich chondrodite and clinohumite are F-free among humite group minerals. Both Ti-free hydroxylchondrodite and hydroxylclinohumite have been synthesized at high pressures and temperatures (Yamamoto and Akimoto 1977; Akaogi and Akimoto 1980). There is renewed interest in chondrodite and clinohumite because they may be constituents of the Earth’s lower crust and upper mantle, and therefore could provide a means of storage for water in this region. The close structural relationship to olivine, a major constituent of the upper mantle, and the structurally bound H make them interesting materials for creating models for the incorporation of water in olivine and the mechanisms of transport of water into the Earth’s upper mantle (Thompson 1992). For the above reasons, several papers have been published that deal with chondrodites and clinohumites in the past ten years. Spectroscopic data have been used to extract elastic properties (Fritzel and Bass 1997; Sinogeikin and Bass 1999) and the pressure and/or temperature dependence of OH vibrational frequencies (Williams 1992; Lin et al. 1999; Mernagh et al. 1999; Lin et al. 2000). The influence of the F(OH)–1 substitution on the H-atom environment has also been investigated by spectroscopic methods (Akaogi and Akimoto 1986; Cynn et al. 1996; Phillips et al. 1997). Diffraction data have been used to study the effect of the OH/F ratio on the compressional behavior. Bulk moduli have been reported for F-bearing chondrodite (Faust and Knittle 1994; Kuribayashi et al. 1998; Friedrich et al. 2000) and for hydroxylchondrodite and hydroxylclinohumite (Ross and Crichton 2000; Lager et al. 1999). FIGURE 1. The crystal structure of clinohumite projected on the (100) plane with bond distances from the Val Malenco sample. Open circles represent H sites; filled circles M sites. The inset highlights the symmetrically distinct bonds and angles around the O/F site (see also text). The O/F atoms associated with the two labeled M3-O/F bonds are located in the polyhedral sheet below the one illustrated, i.e., the O/F-O/F edge is not shared between M3 octahedra. Distances are given in angstroms. FRIEDRICH ET AL.: NEUTRON STUDY OF CHONDRODITE AND CLINOHUMITE 983 TABLE 1. Electron microprobe analyses of the samples of this work. Unit formulae calculated on the basis of 18 (clinohumite) and 10 (chondrodite) O atoms (Ribbe 1982) Val Malenco Kukh-i-Lal Tilley Foster wt% oxides SiO2 35.6(3) 37.8(2) 34.7(8) TiO2 5.4(2) 2.71(7) 0.5(1) Al2O3 0.004(6) 0.01(1) 0.009(8) Cr2O3 0.011(1) 0.01(1) 0.01(1) FeO 11.9(2) 0.07(2) 5.7(7) MnO 0.55(3) 0.02(2) 0.28(4) NiO 0.16(4) 0.014(15) 0.02(2) MgO 44.3(3) 56.3 (3) 53.5(8) F 0 2.30(7) 5.5(8) H2O* 1.47(4) 1.17(2) 2.5(4) O=F 0 –0.97(3) –2.3(3) Total 99.4(5) 99.5(4) 100.4(7) unit formulae Si 3.98(2) 3.97(1) 2.02(4) Ti 0.45(1) 0.214(5) 0.023(6) Al 0.0005(8) 0.001(1) 0.0005(5) Cr 0.0010(9) 0.0009(9) 0.0003(4) Fe2+ 1.1(2) 0.006(2) 0.28(4) Mn 0.052(2) 0.001(1) 0.014(2) Ni 0.014(4) 0.001(1) 0.0008(7) Mg 7.38(2) 8.81(2) 4.64(7) F 0 0.76(2) 1.0(2) H 1.10(3) 0.82(2) 1.0 (2) X* XFe 0.131(2) 0.0007(2) 0.056(2) XTi 0.45(1) 0.214(5) 0.023(6) XF 0 0.48(1) 0.52(1) XCl 0 0 0 XOH 1.00(0) 0.52(1) 0.48(1) * Calculated on the basis of 13 (clinohumite) and 7 (chondrodite) cations. In this work, a neutron single-crystal diffraction study was undertaken to determine accurate hydrogen positions of naturally occurring F-free titanian-hydroxylclinohumite, F-bearing clinohumite and F-bearing chondrodite. The goal of this study is to present data on the structural environment of H in chemically diverse (OH/F ratio) natural humites at both ambient and lower temperatures. EXPERIMENTAL METHODS Titanian hydroxylclinohumite from Val Malenco (Italy) occurs as megacrysts in ultramafic serpentinite veins (Weiss 1997). The quality of an opaque, dark red-brown crystal was checked with several X-ray photographs produced by the Laue back-scattering technique to exclude twinning and domains. A 3.7 × 4.0 × 3.0 mm crystal was cut from the sample for the neutron experiment. The specimen from Kukh-i-Lal (Pamir, Tadjikistan) is a light orange-yellow, F-bearing gem-quality titanian clinohumite from marbles. The F-bearing chondrodite sample from Tilley Foster mine (Brewster, New York) was obtained from the Smithsonian Museum (no. C3200-10). It is a transparent, orange-brown crystal that was cut into a prism of approximate dimensions 1.50 × 2.35 × 3.60 mm. The chemical composition of the chondrodite (8 analyses) and clinohumite (22 analyses) samples was determined using a Cameca SX 50 electron microprobe (EMA) (15 kV, 20 nA, 5 μm electron beam diameter) (Table 1). Standards used were forsterite (Mg, Si), rutile (Ti), corundum (Al), chromite (Cr), fayalite (Fe), tephroite and rhodonite (Mn), NiO (Ni), and F-rich phlogopite (F). For the F-analysis a PC1 analyzer multi-layer crystal with a 2dvalue of ~60 Å was used. Data collection time was 20 s. Peakbackground ratio was used to determine elemental concentrations. The OH content was calculated from stoichiometric constraints based on 13 and 7 cations, respectively. Neutron single-crystal diffraction data were collected at 295, 100, and 20 K (10 K for the chondrodite) on the HB2a fourcircle diffractometer at the high-flux isotope reactor (HFIR) at Oak Ridge National Laboratory (ORNL). A wavelength of 1.0037(2) Å was obtained using a (331)Ge monochromator at a take-off angle of 45°. The crystals were glued to an aluminium pin and mounted on the cold-tip of a closed-cycle He refrigerator, which is mounted on the diffractometer for low-temperature measurements. A set of 31 to 40 reflections for the clinohumites and 72 to 102 reflections for chondrodite were used for refining the unit-cell parameters (Table 2) and determining the orientation matrix. Intensity-data collection was carried out by radially scanning through the Ewald sphere. At the limits of 2θ = 0° and 180° the radial scan is a pure ω scan and a pure θ-2θ scan, respectively. A simple trigonometric relationship controls the relative speeds of the ω and 2θ motors at intermediate values. A 1/4-sphere of Bragg reflections was measured to sin θ/λ = 0.763 Å. Three reflections were monitored to correct the intensities for variations in the neutron flux, which did not change by more than 1% for the duration of each data collection. The intensities were integrated using the Lehman-Larsen algorithm and corrected for the Lorentz effect with the UCLA Crystallographic Computing Package (1994, personal communication). The intensities of equivalent monoclinic reflections were then averaged, giving approximately 2750 reflections for each data set. Due to shadowing effects caused by the mounting brackets of the refrigerator, reflections at angles in a range of about 2θ > 100° and 92° ≤ 2θ ≤ 100° with –77° ≤ χ ≤ –61° were excluded from refinements. Details on the data collection are summarized in Table 3. The structures were refined in the space group P21/b (unique axis a) using the General Structure Analysis System (GSAS), (Larson and Von Dreele 1994). Starting values in the refinements for chondrodite and clinohumite were those reported by Gibbs et al. (1970) and Robinson et al. (1973, ICSD no. 4569), respectively. The initial values for the H positions for TABLE 2. Unit-cell parameters and volumes of chondrodite and clinohumites studied Val Malenco clinohumite Kukh-i-Lal clinohumite Tilley Foster chondrodite 295 K 100 K 20 K 295 K 100 K 20 K 295 K 100 K 10 K a (Å) 4.7344(9) 4.7282(9) 4.7313(9) 4.7404(4) 4.7366(5) 4.7362(5) 4.7401(3) 4.7345(2) 4.7321(3) b (Å) 10.286(1) 10.273(1) 10.274(1) 10.2380(9) 10.226(1) 10.226(1) 10.2843(7) 10.2674(5) 10.2641(5) c (Å) 13.713(2) 13.702(2) 13.695(1) 13.651(1) 13.636(2) 13.635(1) 7.8831(5) 7.8716(3) 7.8673(4) α (°) 101.042(8) 101.004(9) 101.029(8) 100.909(8) 100.904(9) 100.904(8) 109.097(2) 109.060(2) 109.052(2) V (Å) 655.5(2) 653.3(2) 653.4(2) 650.5(1) 648.5(1) 648.4(1) 363.14(4) 361.67(3) 361.19(3) FRIEDRICH ET AL.: NEUTRON STUDY OF CHONDRODITE AND CLINOHUMITE 984 TABLE 3. Details on data reduction and results of the temperature-dependent structure refinements clinohumite Val Malenco* clinohumite Kukh-i-Lal† chondrodite Tilley Foster‡ RT 100 K 20 K RT 100 K 20 K RT 100 K 10 K Total |F| 2791 2746 2742 2765 2756 2755 1576 157