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Neutron diffraction in gemology; single-crystal diffraction study of brazilianite, NaAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 4)

G. Diego Gatta, Pietro Vignola, Martin Meven and Romano Rinaldi
Neutron diffraction in gemology; single-crystal diffraction study of brazilianite, NaAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 4)
American Mineralogist (August 2013) 98 (8-9): 1624-1630

Abstract

The chemical composition and the crystal structure of a gem-quality brazilianite from the Telirio pegmatite, near Linopolis, Minas Gerais, Brazil, [NaAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 4) , a = 11.2448(5) Aa, b = 10.1539(6) Aa, c = 7.1031(3) Aa, beta = 97.351(4) degrees , V = 804.36(7) Aa (super 3) , space group P2 (sub 1) /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 Telirio Claim approaches almost ideal composition. The neutron anisotropic structural refinement was performed with final agreement index R (sub 1) = 0.0290 for 211 refined parameters and 2844 unique reflections with F (sub o) > 4sigma (F (sub o) ), the X-ray refinement led to R (sub 1) = 0.0325 for 169 refined parameters and 2430 unique reflections with F (sub o) > 4sigma (F (sub o) ). The building-block units of the brazilianite structure consist of chains of edge-sharing AlO (sub 4) (OH) (sub 2) and AlO (sub 3) (OH) (sub 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 approximately 0.992 and approximately 1.010 Aa, the O...O distances between approximately 2.67 and approximately 2.93 Aa, and the O-H...O angles between approximately 151 degrees and approximately 174 degrees . The H(2a) and H(2b) are only approximately 1.37 Aa apart and mutually exclusive (both with site occupancy factor of 50%). The differences between the crystal structure of brazilianite and wardite [ideally NaAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 4) .2H (sub 2) O] are discussed. This work fulfills the need for accurate crystal-chemical data for this gem mineral.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 98
Serial Issue: 8-9
Title: Neutron diffraction in gemology; single-crystal diffraction study of brazilianite, NaAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 4)
Affiliation: Universita degli Studi di Milano, Dipartimento di Scienze della Terra, Milan, Italy
Pages: 1624-1630
Published: 201308
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 21
Accession Number: 2013-073847
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables
S23°30'00" - S14°00'00", W51°00'00" - W39°45'00"
Secondary Affiliation: Rheinisch-Westfaelische Technische Hochschule Aachen, DEU, GermanyForschungszentrum Juelich, DEU, GermanyUniversita degli Studi di Perugia, ITA, Italy
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201344
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