Revision of the crystal structure and chemical formula of weeksite, K2(UO2)2(Si5O13).4H2O

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Authors

FEJFAROVA Karla PLÁŠIL Jakub YANG Hexiong CEJKA Jiri DUŠEK Michal DOWNS Robert T BARKLEY Madison C ŠKODA Radek

Year of publication 2012
Type Article in Periodical
Magazine / Source American Mineralogist
MU Faculty or unit

Faculty of Science

Citation
Doi http://dx.doi.org/10.2138/am.2012.4025
Field Solid matter physics and magnetism
Keywords Weeksite; uranyl silicate; crystal structure; X-ray diffraction; open framework
Description The previously published structure determination of weeksite from the Anderson mine, Arizona, U.S.A., suggested that it is orthorhombic, Cmmb, with a = 14.209(2), b = 14.248(2), c = 35.869(4) Angstr., and V = 7262(2) A^3, and an ideal chemical formula (K,Ba)1-2(UO2)2(Si5O13).H2O. Using single-crystal X-ray diffraction, electron microprobe analysis, and thermal analysis, we reexamined weeksite from the same locality. Our results demonstrate that weeksite is monoclinic, with the space group C2/m and unit-cell parameters a= 14.1957(4), b = 14.2291(5), c = 9.6305(3) A, beta = 111.578(3)degrees, V = 1808.96(10) A^3, and an ideal formula K2(UO2)2(Si5O13).4H2O. The previously reported orthorhombic unit cell is shown to result from twinning of the monoclinic cell. The structure refinement yielded R1 = 2.84% for 1632 observed reflections [Iobs greater than 3sigma(I)] and 5.42% for all 2379 reflections. The total H2O content derived from the structure refinement agrees well with that from the thermal analysis. Although the general topology of our structure resembles that reported previously, all Si sites in our structure are fully occupied, in contrast to the previous structure determination, which includes four partially occupied SiO4 tetrahedra. From our structure data on weeksite, it appears evident that the orthorhombic cell of the newly discovered weeksite-type mineral coutinhoite, ThxB1-2x(UO2)2Si5O13.3H2O, needs to be reevaluated.
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