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1 Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstr. 14, A-1090 Wien, Austria
2 Centre for Industrial and Applied Mineralogy, School of Science, Food and Horticulture, BCRI Parramatta Campus, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia
3 School of Natural Science, BCRI Parramatta Campus, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia
* Corresponding author, e-mail: gerald.giester{at}univie.ac.at
The crystal structures of lavendulan from the Hilarion mine, Laurion, Greece, and isotypic sampleite from the Northparkes mine, New South Wales, Australia, have been determined from single-crystal diffraction data, resulting in a revision of the symmetry. The structure of lavendulan, NaCaCu5(AsO4)4Cl · 5H2O (space group P21/n, a = 10.011(1), b = 19.478(2), = 10.056(1) Å, ß = 90.37(1)°, V = 1960.8(3) Å3, Z = 4) was solved from X-ray data (MoK , CCD area detector) and refined to R1(F) = 5.35 % for 4179 observed reflections with Fo > 4
(Fo). The structure of sampleite, NaCaCu5(PO4)4Cl · 5H2O, was determined from synchrotron data (CCD area detector) obtained from a twinned crystal (space group P21/n, a = 9.676(2), b = 19.284(4), c = 9.766(2) Å, ß = 90.07(1)°, V = 1822.3(6) Å3, Z = 4) and refined to R1(F) = 4.44 % for 2601 observed reflections with Fo > 4 (Fo). The structure type is based on Cu(O,H2O,Cl)5 pyramids forming [Cu4O12Cl] clusters, which are connected by XO4 (X = As,P) groups into heteropolyhedral sheets. These sheets ideally have p4mm symmetry and are aligned parallel to (010), i.e., the plane of the excellent cleavage, and are linked to adjacent parallel sheets via NaO6 and CaO7 polyhedra as well as by weak hydrogen bonding.
A polymorph of sampleite with the chemical formula NaCaCu5(PO4)4Cl · nH2O (n ~ 4.5) has been found at Lake Boga, Victoria, Australia. The crystal structure (space group P21/, a = 9.695(2), b = 9.673(2), c = 19.739(4) Å, ß = 102.61(3)°, V = 1806.5(6) Å3, Z = 4) was determined using single-crystal X-ray diffraction data (MoK
, CCD area detector) and refined to R1(F) = 14.67 % for 3189 observed reflections with Fo > 4 (Fo). The heteropolyhedral sheet in the polymorph is topologically identical to that of sampleite (and lavendulan), but the stacking of these sheets is slightly different. Thus, the polymorph may be considered a polytype if the apparently slightly reduced water content of the polymorph is neglected. Some physicochemical properties of the sampleite polymorph are given.
Sheets identical to those in lavendulan are encountered in zdenekite, the Pb-analogue of lavendulan. Very similar sheets are found in andyrobertsite, calcio-andyrobertsite(–1M), calcio-andyrobertsite-2O, richelsdorfite, and probably bleasdaleite, mahnertite, lemanskiite and the inadequately characterized, lavendulan-related species shubnikovite.
Key-words: lavendulan, sampleite, crystal structure, crystal chemistry, polymorphism.
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