G.D. Gatta

M. Meven

P. Vignola

Vignola, P.

Gatta, G. D.

Meven, Martin

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A Single-Crystal Neutrons Diffraction Study of Brazilianite, NaAl3(PO4)2(OH)4

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 146  A SINGLE-CRYSTAL NEUTRONS DIFFRACTION STUDY OF BRAZILIANITE, NAAL3(PO4)2(OH)4 P. Vignola1, G. Gatta1, M. Meven2,3 1 Dipartimento di Scienze della Terra, Universita’ di Milano, Via Botticelli 23, I-20133 Milano, Italy 2 Institut für Kristallographie, RWTH Aachen, Jägerstrasse 17-19, 52056 Aachen, Germany 3 Jülich Centre for Neutron Science – Outstation at FRM II, Lichtenbergstrasse 1, 85747 Garching, Germany Brazilianite, ideally NaAl3(PO4)2(OH)4, typically forms equant to elongated monoclinic crystals with  yellow to yellowish-green color. In Brazil many gem quality locations for brazilianite are known along the Rio Doce valley mainly in Minas Gerais.  Brazilianite occurs as single-crystals or groups up to 20 cm growing in large pockets in the platy albite (cleavelandite) unit of zoned and complex granitic pegmatites. In granitic pegmatites brazilianite is considered a common product of alteration of montebrasite-amblygonite under Na-metasomatic conditions (Simmons et al. 2003). Thus far, only a few studies have been devoted to the crystal chemistry of brazilianite (Frondel and Lindberg 1948). Its crystal structure was solved by Gatehouse and Miskin (1974), on the basis of single-crystal X-ray diffraction data, in the space group P21/n, with a~11.23, b~10.14, c~7.10, and β~97.4° (Z = 4). Its structure consists of chains of edge-sharing Al-octahedra linked by P-tetrahedra forming a three-dimensional network, with Na atoms located in cavities parallel to [100]. In the structure model of Gatehouse and Miskin (1974), there are two different configurations of the Al-octahedra: trans-AlO4(OH)2 and trans-AlO3(OH)3. The crystal structure of brazilianite has been reinvestigated by means of single-crystal X-ray and neutron diffraction. A prismatic, elongated, single-crystal of pale yellow, gem-quality brazilianite, up to18 mm in length and 5 mm in diameter, coming from the Telírio pegmatite, Linópolis, Divino das Laranjeiras district (Minas Gerais, Brazil), was used for diffraction experiments and chemical analysis. The brazilianite crystal was taken from a specimen constituted by 4 gem-quality crystals of brazilianite up to 20 mm perched on platy albite (sample of 3x3x2 cm of cleavelandite) and closely associated to zanazziite crystals up to 3 mm. The WDS electron-microprobe analysis shows that our sample of brazilianite from the Telírio claim is homogeneous and approaches an almost ideal composition with the following empirical formula: (Na0.955Mg0.003Ca0.002Sr0.002K0.001)∑0.963(Al3.007Fe3+0.002)∑3.009(PO4)2(OH)4. The single-crystal X-ray and neutron structure refinement of this study confirm the general structure model of brazilianite described by Gatehouse and Miskin (1974). The building-block units of the brazilianite structure consist of chains of edge sharing trans-AlO4(OH)2 [i.e. around Al(2)] and trans-AlO3(OH)3 [i.e. about Al(1) and Al(3)] octahedra. The two chains are connected, via corner-sharing, by P-tetrahedra to form a three-dimensional framework, with Na atoms located in distorted cavities running along [100]. The Na-polyhedron, here described with a coordination number of 9 (with Na-Omax ~ 3.11 Å), is strongly distorted. Gatehouse and Miskin (1974) suggested that the distortion of the Na-polyhedron might be due to the H sites in the [100]-cavity: the effect of mutual repulsion forces the Na site to one side of the cavity, leading to a stronger Na-O interaction with oxygen sites on one side of the cavity than on the other. This can now be confirmed by our neutron structure refinement, as Na – H(4) distance is ~3.06 Å,  Na – H(3) is ~3.16 Å and Na – H(2) is ~3.21 Å, and H(4), H(3) and H(2) lie on the same side of the cavity. Both the X-ray and neutron structure refinements show that the Al-octahedra appear to be significantly distorted, with Δ[Al(1)-O]max ~ 0.32 Å, Δ[Al(2)-O]max ~ 0.31 Å, and Δ[Al(3)-O]max ~ 0.17 Å (i.e. the difference between the longest and the shortest Al-O distances, based on the X-ray structure refinement). The longest Al-O bond distances are those with the bridging oxygen shared between two Al-octahedra and one P-tetrahedron. The shortest Al-O bond distances are those with oxygen of OH-groups. P-tetrahedra appear to be less distorted, as Δ[P(1)-O]max ~ 0.044 Å and Δ[P(2)-O]max ~  0.036 Å (i.e. the difference between the longest and the shortest P-O distances, based on the X-ray structure refinement). The neutron structure refinement of this study provides an unambiguous location of the H-sites, allowing the description of the H-bonding scheme in brazilianite structure. Five independent H sites have been located, here labelled as H(1), H(2a), H(2b), H(3) and H(4). The configuration of the OH groups (i.e. O(1)–H(1), O(2)–H(2a), O(2)–H(2b), O(3)–H(3), O(4)–H(4)), along with the hydrogen bonding scheme, are now well defined (Fig. 1). O(1), O(2), O(3) and O(4) act as donors, whereas O(2), O(4), O(9) and O(12) as acceptors. PEG 2013: The 6th International Symposium on Granitic Pegmatites Abstracts Page 147  Symmetry-related O(2) act both as donor and as acceptor of H-bond. The O-H distances corrected for “riding motion” (Busing and Levy 1964), range between ~0.992 and ~1.010 Å, the O···O distances between ~2.67 and ~2.93 Å, and the O–H···O angles between ~151 and ~174° (Table 5). The H(2a) and H(2b) are only ~1.37 Å apart. The neutron structure refinement was conducted without any restrain on the site occupancy factor (s.o.f.) of H(2a) and H(2b), leading to s.o.f. (H2a) = 0.546(6) and s.o.f. (H2b) = 0.446(6), respectively. We can thus consider a general s.o.f. of 50% of both H(2a) and H(2b), and so the two H-sites are mutually exclusive. Additional test refinements have been performed in order to check if this H-site split reflects a lower symmetry than P21/n, but unsuccessfully. The key to understanding the split in two mutually exclusive H(2a) and H(2b) sites is in the H-bonding scheme of the structure. In fact, if only one “virtual” H(2) site occurs, located between H(2a) and H(2b), it should have an H-bond configuration energetically not favourable with O(2)-H(2)·O(9) ~ 113° and O(2)-H(2)···O(2) ~ 127°, whereas O(2)-H(2a)···O(9) ~ 151° and O(2)-H(2b)···O(2) ~171°. Fig. 1: Hydrogen site locations and, H-bonding scheme in the structure of brazilianite based on the neutron structure refinement of this study. The sites H2a and H2b are mutually exclusive. Thermal ellipsoid probability factor: 60%. References: Busing W.R., Levy H.A. (1964): The effect of thermal motion on the estimation of bond lengths from diffraction measurements. Acta Crystallographica, 17, 142-146. Frondel, C., Lindberg, M.L. (1948) Second occurrence of brazilianite. American Mineralogist, 33, 135–141. Gatehouse, B.M., Miskin, B.K. (1974): The crystal structure of brazilianite, NaAl3(PO4)2(OH)4. Acta Crystallographica, 30, 1311–1317. Simmons, W.B., Webber, K.L., Falster, A.U. & Nizamoff J.W. (2003) Pegmatology: pegmatite mineralogy, petrology & petrogenesis. Rubellite Press, New Orleans.     

A SINGLE-CRYSTAL NEUTRONS DIFFRACTION STUDY OF BRAZILIANITE, NAAL3(PO4)2(OH)4

https://air.unimi.it/retrieve/handle/2434/222953/282962/2013_Vignola%20et%20al_%20brazilianite%20PEG%202013%20abstract.pdf

A SINGLE-CRYSTAL NEUTRONS DIFFRACTION STUDY OF BRAZILIANITE, NAAL3(PO4)2(OH)4

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