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GEOREF RECORD

The crystal structure of vladimirite, with a revised chemical formula, Ca (sub 4) (AsO (sub 4) ) (sub 2) (AsO (sub 3) OH).4H (sub 2) O

Hexiong Yang, Stanley H. Evans, Robert T. Downs and Robert A. Jenkins
The crystal structure of vladimirite, with a revised chemical formula, Ca (sub 4) (AsO (sub 4) ) (sub 2) (AsO (sub 3) OH).4H (sub 2) O
The Canadian Mineralogist (August 2011) 49 (4): 1055-1064

Abstract

Two samples of vladimirite, one from Bou Azzer, Morocco, and the other from a new occurrence in Copiapo, Chile (designated as R100075 and R080001, respectively), were examined with an electron microprobe, single-crystal X-ray diffraction, and Raman spectroscopy. Our results show that vladimirite is monoclinic with space group P2 (sub 1) /c and unit-cell parameters a 5.8279(2), b 10.1802(4), c 22.8944(10) Aa, beta 96.943(2) degrees , and V 1348.35(9) Aa (super 3) for R100075 and a 5.8220(1), b 10.1750(2), c 22.8816(6) Aa, beta 96.902(1) degrees , and V 1345.66(5) Aa (super 3) for R080001. The structure determinations, with R (sub 1) =0.022 and 0.023 for R100075 and R080001, respectively, yielded an ideal chemical formula Ca (sub 4) (AsO (sub 4) ) (sub 2) (AsO (sub 3) OH).4H (sub 2) O (Z=4) for this mineral, in contrast to Ca (sub 5) (AsO (sub 4) ) (sub 2) (AsO (sub 3) OH) (sub 2) .5H (sub 2) O (Z=3) documented in the literature. The chemical compositions for R100075 and R080001 are Ca (sub 4.03) (AsO (sub 4) ) (sub 2) (As (sub 0.99) O (sub 3) OH).4H (sub 2) O and Ca (sub 3.97) (AsO (sub 4) ) (sub 2) (As (sub 1.01) O (sub 3) OH).4H (sub 2) O, with trace Zn and Mn, respectively. The structure is characterized by undulating layers formed by the four nonequivalent, rather irregular Ca polyhedra [Ca1O (sub 6) (H (sub 2) O), Ca2O (sub 6) (H (sub 2) O), Ca3O (sub 4) (H (sub 2) O) (sub 3) , and Ca4O (sub 5) (H (sub 2) O) (sub 3) ] linked through the sharing of edges and vertices. These undulating layers are parallel to (010) and are interconnected by AsO (sub 4) and AsO (sub 3) OH tetrahedra, as well as hydrogen bonds, along the b axis. Vladimirite is remarkable inasmuch as one of the hydrogen-bonded O-H...O distances (O12H-H...O3) is only 2.465(2) Aa, which is the shortest donor-acceptor distance of all known Ca-bearing arsenate minerals, similar to the short donor-acceptor distances observed in several synthetic compounds containing AsO (sub 3) OH groups.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 49
Serial Issue: 4
Title: The crystal structure of vladimirite, with a revised chemical formula, Ca (sub 4) (AsO (sub 4) ) (sub 2) (AsO (sub 3) OH).4H (sub 2) O
Affiliation: University of Arizona, Department of Geosciences, Tucson, AZ, United States
Pages: 1055-1064
Published: 201108
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 36
Accession Number: 2011-103543
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
N30°34'60" - N30°34'60", W06°45'00" - W06°45'00"
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201152
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