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Crystallography and Mineralogy |
1 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
2 Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow 117071, Russia
E-mail address: umuvarov{at}cc.umanitoba.ca
The crystal structure of "nickelalumite", ideally NiAl4(SO4)(OH)12(H2O)3, from the Kara–Tangi uranium deposit, Batken region, Kyrgyzstan, monoclinic, a 10.2567(5), b 8.8815(4), c 17.0989(8) Å, ß 95.548(1)°, V 1550.3(1) Å3, space group P21/n, Z 
4, Dcalc 2.231 g/cm3, has been solved by the Patterson method and refined to an R1 index of 5.7% for 1554 unique (Fo > 4
F) reflections collected on a Bruker single-crystal P4 diffractometer with a 4K CCD detector and MoK
X-radiation. "Nickelalumite" occurs with quartz, calcite, alumohydrocalcite, allophane, crandallite and kyrgyzstanite. An electron-microprobe analysis gave Al2O3 39.94, SiO2 0.17, SO2 12.16, V2O3 0.29, FeO 0.15, NiO 8.00, ZnO 6.21, (H2O)calc 31.87, sum 98.79 wt.%; the amount of H2O was determined by crystal-structure analysis. There is one tetrahedrally coordinated S site, with <S–O> 1.468 Å. There are five octahedrally coordinated sites. At the M site, Ni is dominant over Zn, with <M–O> = 2.079 Å. Four sites are occupied solely by Al, with a grand <Al–O> distance of 1.900 Å. There are 19 anion sites: four sites are occupied by O atoms, 12 sites by (OH) groups, and three sites by (H2O) groups. The main building unit of the "nickelalumite" structure is a (001) sheet of Al and M octahedra. The Al and M octahedra share common edges to form an interrupted sheet of the form [MAl4(OH)12]2+. Intercalated between the M–Al–OH sheets are layers of (SO4) tetrahedra and (H2O) groups. All H atoms of (OH) and (H2O) groups were located, and details of hydrogen bonding are discussed. The structure of "nickelalumite" is similar to those of alvanite, ZnAl4(VO3)2(OH)12(H2O)2, and ankinovichite, NiAl4(VO3)2(OH)12(H2O)2, and stoichiometry and cell dimensions suggest that mbobomkulite and hydrombobomkulite also are related to "nickelalumite". All structures are based on a positively charged sheets of octahedra of the form [
M2+ M3+ 4 (OH)12]2+ that are intercalated with interstitial complexes of oxyanions and (H2O) groups. The Lewis acidity of the structural unit is 0.167 vu, and the character of the interstitial complexes is constrained by the valencesum rule of bond-valence theory. Possible interstitial oxyanions are (SO4)2–, (NO3)1– 2 and [VO3]1– 2, whereas (SiO4)4–, (BO3)3– 2, (PO4)3– and (BO4)5– have Lewis basicities that are too high and hence cannot form "nickelalumite"-type structures.
Keywords: "nickelalumite", crystal structure, electron-microprobe analysis, hydrogen bonding.
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