• journal_title：American Mineralogist
• Contributor：G. Diego Gatta ; Pietro Vignola ; Martin Meven ; Romano Rinaldi
• Publisher：Mineralogical Society of America
• Date：2013-08-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4476
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：8-9
• firstpage：1624
• section：Articles

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) ų, 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 F_o > 4σ(F_o), the X-ray refinement led to R₁ = 0.0325 for 169 refined parameters and 2430 unique reflections with F_o > 4σ(F_o). 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.