G. N&#233

G.D. Gatta

P. Vignola

G. Nénert

AIR Universita degli studi di Milano

  CONTRIBUTIONS TO THE 6th INTERNATIONAL SYMPOSIUM  ON GRANITIC PEGMATITES  EDITORS WILLIAM B. SIMMONS KAREN L. WEBBER ALEXANDER U. FALSTER ENCARNACIÓN RODA-ROBLES SARAH L. HANSON MARÍA FLORENCIA MÁRQUEZ-ZAVALÍA MIGUEL ÁNGEL Galliski  GUEST EDITORS ANDREW P. BOUDREAUX  KIMBERLY T. CLARK MYLES M. FELCH KAREN L. MARCHAL LEAH R. GRASSI JON GUIDRY SUSANNA T. KREINIK C. MARK JOHNSON  COVER DESIGN BY RAYMOND A. SPRAGUE PRINTED BY RUBELLITE PRESS, NEW ORLEANS, LA PEG 2013: The 6th International Symposium on Granitic Pegmatites Abstracts Page 148  HYDROXYL GROUPS AND H2O MOLECULES IN PHOSPHATES: A NEUTRON DIFFRACTION STUDY OF EOSPHORITE, MNALPO4(OH)2·H2O P. Vignola1, G. Gatta1, G. Nénert2 1 Dipartimento di Scienze della Terra, Universita’ degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy 2 Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France Minerals belonging to the eosphorite [MnAl(PO4)(OH)2•H2O] - childrenite [FeAl(PO4) (OH)2•H2O] series are widespread secondary phosphates occurring in medium to strongly evolved rare-element  granitic pegmatites, ranging from the beryl to the petalite subtype in the classification of Černý and Ercit (2005). Eosphorite is a low-temperature metasomatic mineral in Lithium-Cesium-Tantalum (LCT) granitic pegmatites  that frequently crystallizes in open cavities occurring in platy albite (cleavelandite) units of zoned pegmatitic dikes (Simmons et al. 2003). Eosphorite constitutes one of the low temperature products of alteration of primary phosphates, mainly lithiophilite or triplite. The unit-cell constants and the possible space groups of childrenite (Bbam or Bba2) were first reported by Barnes (1949). The crystal structure of childrenite was later solved by Giuseppetti and Tadini (1984) by means of X-ray single-crystal diffraction. The authors refined the crystal structure in the acentric space group Bba2, with a = 10.395(1), b = 13.394(1), and c = 6.918(1) Å (Z = 8). The crystal-structure of eosphorite was first solved by Hanson (1960) in the centric Bbam space group, with a = 10.52, b = 13.60, and c = 6.97 Å (Z = 8) using a sample from Newry, Maine and later reinvestigated by Hoyos et al. (1993) using a sample from Taquaral, Brazil, by means of X-ray single-crystal diffraction, in the Cmca space group, with a  = 6.928(1), b = 10.445(1), and c = 13.501(2) Å [eosphorite composition: (Mn0.76Fe0.24)Al (PO4)(OH)2•H2O]. The crystal structure of both eosphorite and childrenite consists of chains parallel to the a-axis (in the Cmca space group) constituted by (Mn,Fe)-distorted octahedra, sharing opposite O-O edges, and chains of Al-octahedra. The two types of chains are connected, via corner-sharing, to form a set of (100) sheets held together by [PO4] tetrahedra (and hydrogen bonds) to form a three-dimensional network. The crystal structure of eosphorite has been reinvestigated by means of single-crystal neutron diffraction. A gemmy single-crystal of eosphorite up to 14 mm in length and 5 mm in diameter from a granitic pegmatite outcropping in the area of Chamachhu (Changmachhu), Skardu district, Baltistan, Pakistan, was used for the diffraction experiment and for the chemical analysis. The specimens from Chamachhu show eosphorite crystals perched on white platy albite (cleavelandite) or directly on pollucite or topaz crystals up to 20-30 cm contained in large miarolitic cavities. The mineralogical association of these pegmatites is fairly evolved and consists of muscovite, lepidolite, spessartine, gem elbaite, gem topaz, beryl (gem aquamarine and morganite), pollucite, tantalite-(Mn), fluorapatite, eosphorite, and väyrynenite. The WDS electron-microprobe analysis shows that our sample of eosphorite is homogeneous and approaches an almost ideal composition with the following empirical formula: (Mn2+0.95Fe2+0.02Al0.10)Σ1.07 AlPO4(OH1.93F0.07)2·H2O. Our single-crystal neutron structure refinement, based on diffraction data collected at 20 K, provides a general structural model in agreement with the one reported by Hoyos et al. (1993). The building block units of the eosphorite structure consists of chains of (Mn,Fe)-octahedra (sharing opposite O-O edges) running along the a-axis, and chains of Al-octahedra. The two types of chains are connected, via corner-sharing, to form a set of (100) sheets held together by P-tetrahedra (and hydrogen bonds) to form a three-dimensional framework. Distorted channels, confined by 6-membered ring of polyhedra, run along [100]. The (Mn,Fe)-octahedron is significantly distorted, with Δ(O-O)max ~ 0.81 Å, as the O-O distance of the shared edge is significantly shorter than the non-shared ones. We cannot exclude that this effect is due to the cation-cation repulsion, as M-M ~ 3.46 Å. The Al-octahedron and the P-tetrahedron appear to be less distorted, with Δ(O-O)max ~ 0.15 Å and Δ(O-O)max ~ 0.03 Å. The low-T structure refinement does not show any evidence of (Mn, Fe)-octahedral ordering, which should lead to a lowering of the symmetry. In addition, the principal root-mean-square components of the atomic displacement parameters do not show any pronounced displacement about the equilibrium position, compared with those of the other atomic sites, suggesting the absence of a local split. The Fe-content deduced on the basis of the structure refinement is 0.165 a.p.f.u. where the chemical PEG 2013: The 6th International Symposium on Granitic Pegmatites Abstracts Page 149  analysis shows a Fe-content of about 0.025 a.p.f.u. This difference can be ascribed to a multi-element population at the M site, in which Mn, Fe and Al might coexist. In fact, the EPMA-WDS data show an excess of Al, as often reported in other eosphorite analyses (e.g. samples from Black Mountain and Buckfield, Maine; Hurlbut 1950). The neutron structure refinement of this study allowed an unambiguous location of the H-sites along with the description of the H-bonding scheme in eosphorite structure. We can now describe the structure as made by (Mn, Fe) O4(OH, H2O)2 and AlO2(OH)2 (OH, H2O)2 octahedra (Figure 1). The O3 site is the OH- group (i.e. O3-H3) oxygen, whereas O4 is the oxygen of OH- group (i.e. O4-H1) and H2O molecules (i.e. H2-O4-H1). In particular, H2O molecules have site occupancy of 50%, so that the two H2O molecules generated by the mirror plane are mutually exclusive. In other words, the two equivalent O4 sites generated by the mirror plane are respectively the oxygen of one OH group and one H2O molecule, with a local breaking of the symmetry. Among the three independent Al-O bond distances of the AlO2(OH)2(OH,H2O)2 octahedron, the Al-OH bond distance (i.e. Al-O3) is the shortest and the Al-(OH,H2O) distance (i.e. Al-O4) the longest. In the (Mn, Fe)O4(OH,H2O)2 polyhedron, the M-(OH,H2O)2 bond distance (i.e. M-O4) is the longest. The geometry of the H2O molecule and the OH group, along with the hydrogen bonding scheme in eosphorite, are now well defined. The O4-H1 and O4-H2 distances, corrected for “riding motion”, are ~1.003 and ~1.071 Å, respectively, and two strong energetically favorable hydrogen bonds occur: O4···O5 = 2.711(1) Å, H1···O5 = 1.757(1) Å and O4-H1···O5 = 161.9(1)°; O4···O4 = 2.504(1) Å, H2···O4 = 1.454(1) Å and O4-H1···O4 = 170.9(1)° (Fig. 1). In other words, symmetry-related O4 act as donor and as acceptor of the H-bond. The H1-O4-H2 angle approaches the ideal value (i.e. 107.2(1). The OH-group shows: O3-H3 distance, corrected for “riding motion”, of ~ 0.995 Å, O3···O1 = 2.836(1) Å, H3···O1 = 1.930(1) Å and O3-H3···O1 = 154.0(1)°. The O1 site (i.e. the acceptor of the H bond of the O3-H3 group) is under-bonded, as it is shared by two (Mn, Fe)- octahedra and one P-tetrahedron (Fig. 1). The anisotropic structure refinement shows that the principal root-mean-square components of the atomic displacements parameters of the H3 site (belonging to the OH group) are larger in magnitude than those of the H1 and H2 sites.                     Fig. 1: Hydrogen location, configuration of OH groups and H2O molecules, along with the H-bond scheme, in the structure of eosphorite. The occupancy factor of the H2 site is 50%. References: Barnes, W.H. (1949) The unit cell and space group of childrenite. American Mineralogist, 34, 12-18. Černý, P. and Ercit, S. (2005): The classification of granitic pegmatites revisited. Canadian Mineralogist, 43, 2005-2026. Giuseppetti, G., and Tadini, C. (1986) The crystal structure of childrenite from Tavistock (SW England), Ch89E011 term of childrenite-eosphorite series. Neues Jahrbuch für Mineralogie Monatshefte, 263-271. Hanson, A.W. (1960) The crystal ctructure of eosphorite. Acta Crystallographica, 13, 384-387. Hoyos, M.A., Calderon, T., Vergara, I., and Garcia-Solé, J. (1993) New structure and spectroscopic data for eosphorite. Mineralogical Magazine, 57, 329-336. Hurlbut, C. (1950) Childrenite-eosphorite series. American Mineralogist, 35, 793–805. Simmons, W., Webber, K.L., Falster, A.U., and Nizamoff, J.W. (2003) Pegmatology – Pegmatite Mineralogy, Petrology and Petrogenesis. Rubellite Press, New Orleans, LA, 176pp. 

HYDROXYL GROUPS AND H2O MOLECULES IN PHOSPHATES: A NEUTRON DIFFRACTION STUDY OF EOSPHORITE, MNALPO4(OH)2·H2O

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HYDROXYL GROUPS AND H2O MOLECULES IN PHOSPHATES: A NEUTRON DIFFRACTION STUDY OF EOSPHORITE, MNALPO4(OH)2·H2O

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HYDROXYL GROUPS AND H2O MOLECULES IN PHOSPHATES: A NEUTRON DIFFRACTION STUDY OF EOSPHORITE, MNALPO4(OH)2&#183;H2O

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HYDROXYL GROUPS AND H2O MOLECULES IN PHOSPHATES: A NEUTRON DIFFRACTION STUDY OF EOSPHORITE, MNALPO4(OH)2·H2O