January 1, 2013 Journal article Open Access

Gatta, G. Diego; Vignola, Pietro; Meven, Martin; Rinaldi, Romano

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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:creator>Gatta, G. Diego</dc:creator>
  <dc:creator>Vignola, Pietro</dc:creator>
  <dc:creator>Meven, Martin</dc:creator>
  <dc:creator>Rinaldi, Romano</dc:creator>
  <dc:date>2013-01-01</dc:date>
  <dc:description>The chemical composition and the crystal structure of a gem-quality brazilianite from the Telírio pegmatite, near Linópolis, Minas Gerais, Brazil, [NaAl3(PO4)2(OH)4, a = 11.2448(5) Å, b = 10.1539(6) Å, c = 7.1031(3) Å, β = 97.351(4)°, V = 804.36(7) Å3, space group P21/n, Z = 4], have been reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode, single-crystal X-ray and neutron diffraction. The chemical analysis shows that brazilianite from Telírio Claim approaches almost ideal composition. The neutron anisotropic structural refinement was performed with final agreement index R1 = 0.0290 for 211 refined parameters and 2844 unique reflections with Fo &gt; 4σ(Fo), the X-ray refinement led to R1 = 0.0325 for 169 refined parameters and 2430 unique reflections with Fo &gt; 4σ(Fo). The building-block units of the brazilianite structure consist of chains of edge-sharing AlO4(OH)2 and AlO3(OH)3 octahedra. 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]. Five independent H sites were located, here labeled as H(1), H(2a), H(2b), H(3), and H(4). The configuration of the OH groups, along with the complex hydrogen-bonding scheme, are now well defined. The O-H distances corrected for "riding motion" 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°. The H(2a) and H(2b) are only ~1.37 Å apart and mutually exclusive (both with site occupancy factor of 50%). The differences between the crystal structure of brazilianite and wardite [ideally NaAl3(PO4)2(OH)4·2H2O] are discussed. This work fulfills the need for accurate crystal-chemical data for this gem mineral.</dc:description>
  <dc:identifier>https://www.openaccessrepository.it/record/21900</dc:identifier>
  <dc:identifier>10.2138/am.2013.4476</dc:identifier>
  <dc:language>eng</dc:language>
  <dc:relation>url:https://www.openaccessrepository.it/communities/itmirror</dc:relation>
  <dc:rights>info:eu-repo/semantics/openAccess</dc:rights>
  <dc:subject>FP7</dc:subject>
  <dc:subject>European Commission</dc:subject>
  <dc:subject>Research Infrastructures</dc:subject>
  <dc:subject>EC</dc:subject>
  <dc:subject>SP4-Capacities</dc:subject>
  <dc:title>Neutron diffraction in gemology: Single-crystal diffraction study of brazilianite, NaAl3(PO4)(2)(OH)(4)</dc:title>
  <dc:type>info:eu-repo/semantics/article</dc:type>
  <dc:type>publication-article</dc:type>
</oai_dc:dc>