January 1, 2013 Journal article Open Access

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

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{
  "DOI": "10.2138/am.2013.4476", 
  "abstract": "<p>The chemical composition and the crystal structure of a gem-quality brazilianite from the Tel&iacute;rio pegmatite, near Lin&oacute;polis, Minas Gerais, Brazil, [NaAl3(PO4)2(OH)4, a = 11.2448(5) &Aring;, b = 10.1539(6) &Aring;, c = 7.1031(3) &Aring;, &beta; = 97.351(4)&deg;, V = 804.36(7) &Aring;3, 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 Tel&iacute;rio 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; 4&sigma;(Fo), the X-ray refinement led to R1 = 0.0325 for 169 refined parameters and 2430 unique reflections with Fo &gt; 4&sigma;(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 &quot;riding motion&quot; range between ~0.992 and ~1.010 &Aring;, the O&middot;&middot;&middot;O distances between ~2.67 and ~2.93 &Aring;, and the O-H&middot;&middot;&middot;O angles between ~151&deg; and ~174&deg;. The H(2a) and H(2b) are only ~1.37 &Aring; 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&middot;2H2O] are discussed. This work fulfills the need for accurate crystal-chemical data for this gem mineral.</p>", 
  "author": [
    {
      "family": "Gatta, G. Diego"
    }, 
    {
      "family": "Vignola, Pietro"
    }, 
    {
      "family": "Meven, Martin"
    }, 
    {
      "family": "Rinaldi, Romano"
    }
  ], 
  "id": "21900", 
  "issued": {
    "date-parts": [
      [
        2013, 
        1, 
        1
      ]
    ]
  }, 
  "language": "eng", 
  "note": "Funded by the European Union. Part of the European Commission FP7 Open Access pilot.", 
  "title": "Neutron diffraction in gemology: Single-crystal diffraction study of brazilianite, NaAl3(PO4)(2)(OH)(4)", 
  "type": "article-journal"
}