The crystal structure of epifanovite
Panikorovskii T. L., Krivovichev S. V., Yakovenchuk V. N., Ivanyuk G. Yu.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 3. P. 39-50
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Language: Russian
Abstract
The crystal structure of epifanovite, NaCaCu5(PO4)4[AsO2(OH)2] · 7H2O, a new mineral from the oxidation zone of the Kester tin deposit, was determined by direct methods and refined to R1 = 0.087 on the basis of 2147 independent observed reflections. The mineral is monoclinic, P21/m, a = 9.6911(8), b = 9.7547(9), c = 9.9632(14) Å, β = 102.237(10)°, V = 920.46(17) Å3, Z = 2. The crystal structure of epifanovite contains four symmetrically independent Cu sites, each coordinated by five O atoms by forming short equatorial (1.905—1.964 Å) and one elongated (2.467—2.720 Å) Cu—O bonds. Four CuO5 pyramids centered by the Cu2, Cu3 and Cu4 sites are linked at the O11 atom to form [Cu4O13] tetramers. The [Cu4O13] tetramers and Cu1O5 pyramids are linked via phosphate groups into the complex, which is incrustated by disordered [AsO2(OH)2]– group to form fundamental building blocks in the structure. The blocks are linked through phosphate groups to form layers parallel to the (001) plane. The layers are joined into a three-dimensional framework by sharing of the apical atoms of the Cu1O5 pyramids and O atoms of disordered arsenate groups. Epifanovite is related to the group of minerals and inorganic compounds based upon the [Cu4O(TO4)4] layers (T = As, P). The most closely related to epifanovite are monoclinic polytypes of the andyrobertsite-calcioandyrobertsite KMeCu5(AsO4)4[As(OH)2O2] · 2H2O (Me = Cd, Ca).
Key words: epifanovite, crystal structure, disorder, copper phosphate-arsenate, Kester deposit.
