TY - JOUR
T1 - Revision of the crystal structure and chemical formula of weeksite, K2(UO2)2(Si5O13)̇4H2O
AU - Fejfarová, Karla
AU - Plášil, Jakub
AU - Yang, Hexiong
AU - Cejka, Jirí
AU - Dušek, Michal
AU - Downs, Robert T.
AU - Barkley, Madison C.
AU - Škoda, Radek
PY - 2012/4
Y1 - 2012/4
N2 - 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) Å, and V = 7262(2) Å3, and an ideal chemical formula (K,Ba)1-2(UO2)2(Si 5O13)̇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) Å, b = 111.578(3)°, V = 1808.96(10) Å3, and an ideal formula K2(UO2)2(Si 5O13)̇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 > 3s(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)2Si5O 13̇3H2O, needs to be reevaluated
AB - 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) Å, and V = 7262(2) Å3, and an ideal chemical formula (K,Ba)1-2(UO2)2(Si 5O13)̇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) Å, b = 111.578(3)°, V = 1808.96(10) Å3, and an ideal formula K2(UO2)2(Si 5O13)̇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 > 3s(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)2Si5O 13̇3H2O, needs to be reevaluated
KW - Crystal structure
KW - Open framework
KW - Uranyl silicate
KW - Weeksite
KW - X-ray diffraction
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U2 - 10.2138/am.2012.4025
DO - 10.2138/am.2012.4025
M3 - Article
AN - SCOPUS:84860326629
SN - 0003-004X
VL - 97
SP - 750
EP - 754
JO - American Mineralogist
JF - American Mineralogist
IS - 4
ER -