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The crystal structure and composition of pottsite, (Pb (sub 3x) Bi (sub 4-2x) )(VO (sub 4) ) (sub 4) .H (sub 2) O (0.8 < X < 1.0)

Abstract

Pottsite from the Linka mine (Nevada, USA) was studied by scanning electron microscopy, energy- and wavelength-dispersive analyses, and single-crystal X-ray diffraction. The formula, calculated on the basis of (V + As) = 4, shows that the Pb/Bi atomic ratio in pottsite varies between 0.86 and 1.48, and a plot of Pb vs. Bi (atoms per formula unit) suggests that the 3Pb (super 2+) <-->2Bi (super 3+) substitution is operative defining two compositional groups: one with low Pb/Bi atomic ratio (0.86-1.01), the other with high Pb/Bi ratio (1.22-1.48). Structure refinement shows that the crystal studied by X-ray diffraction has a low Pb/Bi ratio. Pottsite crystallizes in the tetragonal space group I4 (sub 1) /a, a = 11.0839(5), c = 12.6516(6) Aa, V = 1554.29(16) Aa (super 3) , Z = 1. The crystal structure was solved by direct methods and refined to R (sub 1) =0.040 for 1167 unique reflections with F>4sigma (sub F) . The structure contains one A site split into partially occupied A(1) and A(2) sites. The A(1) site is populated by Pb (sub 0.500) Bi (sub 0.380) Ca (sub 0.030) , whereas A(2) is occupied by Pb (sub 0.068) Ca (sub 0.032) , with a site occupation factor (s.o.f.) of 0.07. The Bi(1) site has a s.o.f. of 0.84 and forms a distorted but symmetrical square antiprismatic coordination environment, which indicates a low degree of stereochemical activity of the lone electron pair on Bi (super 3+) . The symmetrically independent V(1) site has a V (sub 0.983) As (sub 0.017) population and is coordinated by four oxygen atoms to form a distorted VO (sub 4) tetrahedron. The minor amount of chlorine in pottsite is restricted to the O (sub w) site (O (sub 0.928) Cl (sub 0.072) ) which belongs to the H (sub 2) O molecule.The atomic arrangement in the structure of pottsite can be described as the packing of isolated V (super 5+) O (sub 4) tetrahedra, linked with heavy cations via common oxygen atoms to form 2D blocks or double-layered (DL) structural units that are parallel to (001). The DL units are formed by pairs of vanadate tetrahedra arranged opposite one to each other and oriented in opposite directions. Water molecules are located inbetween vanadate tetrahedral groups to form [(VO) (sub 4) (H (sub 2) O)] (super 3-) chains which are separated from adjacent chains in the (001) plane by rows of Pb (super 2+) and Bi (super 3+) cations extending in the [120] direction. Two adjacent DL units are shifted one relative to the other by a translation of + or -1/2 unit along the b axis, with an interlayer spacing of about 1.13 Aa, measured as the shortest distance between adjacent O-O edges of the VO (sub 4) tetrahedra. The structure of pottsite is compared to that of related vanadate minerals based on isolated V (super 5+) O (sub 4) tetrahedra with formation of DL units.The observed variations in Pb and Bi contents and the single-crystal X-ray diffraction data suggest a general formula of pottsite as (Pb (sub 3) (sub x) Bi (sub 4-2) (sub x) )(VO (sub 4) ) (sub 4) .H (sub 2) O, 0.8 < X < 1.0, and an ideal mineral formula of (Pb (sub 3) Bi)Bi(VO (sub 4) ) (sub 4) .H (sub 2) O.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 28
Serial Issue: 1
Title: The crystal structure and composition of pottsite, (Pb (sub 3x) Bi (sub 4-2x) )(VO (sub 4) ) (sub 4) .H (sub 2) O (0.8 < X < 1.0)
Affiliation: Saint Petersburg State University, Department of Crystallography, St. Petersburg, Russian Federation
Pages: 137-145
Published: 20150805
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 41
Accession Number: 2015-089101
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
N39°19'60" - N39°19'60", W116°49'60" - W116°49'60"
Secondary Affiliation: Natural History Museum, GBR, United Kingdom
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201538
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