TECHNICAL ARTICLE Synchrotron radiation diffraction study of the mineral moolooite, and synthetic copper oxalates B. H. O’Connor, 1,a) R. M. Clarke, 2 and J. A. Kimpton 3 1 Department of Physics and Astronomy, Curtin University, Kent St, Bentley, Perth, WA 6102, Australia 2 ChemCentre, PO Box 1250, Bentley, WA 6983, Australia 3 Australian Synchrotron, 800 Blackburn Road, Clayton, Vic 3168, Australia (Received 4 September 2018; accepted 14 January 2019) The orthorhombic mineral moolooite, CuC 2 O 4 . nH 2 O, described by Clarke and Williams (1986) using Debye-Scherrer photographic data, has a fully-disordered stacking fault (FDSF) structure. Related monoclinic models have been reported for various synthesised samples based on Schmittler (1968). In the present study, synchrotron radiation diffraction data for moolooite and syn- thesised specimens have been examined with particular reference to crystallographic disorder. The moolooite data correspond to space group Pnnm, with a = 5.3064(2), b = 5.6804(2), c = 2.5630(1) Å; V c = 77.26(1) Å 3 ; and Z = 1; and the FDSF structure along the b-direction has been confirmed. The synthetic specimen data from the study indicate partial ordering, with space group P2 1 /n; and the cell parameters for one specimen being a = 5.957(7), b = 5.611(5), c = 5.133(7) Å; β = 115.16 (2)°; V c = 155.27 Å 3 and Z = 2. The level of zeolitic water in the materials has been considered using the approach of Schmittler based on thermogravimetry and pycnometry. The new data for nat- ural topotype material correspond to CuC 2 O 4 .1.0H 2 O. It is postulated that the level of water for nat- ural and synthetic specimens may be attributed to the conditions under which the material forms. © 2019 International Centre for Diffraction Data. [doi:10.1017/S0885715619000101] Key words: natural moolooite from Mooloo Downs, synthetic moolooite, synchrotron radiation diffraction data, order-disorder structural character, zeolitic water content I. INTRODUCTION Papers addressing the structural crystallography for hydrated copper (II) oxalate, CuC 2 O 4 . nH 2 O, extend back over 50 years. Schmittler (1968), in studying a suite of synthetic samples using Guinier X-ray diffraction (XRD), observed sample-to-sample differences involving XRD line shifts, line broadening differences, and the presence of addi- tional lines in some samples. The differences were interpreted in terms of order-disorder (OD) theory (Fichtner-Schmittler, 1979), with the sample-to-sample differences being explained in terms of (i) an orthorhombic, fully-disordered stacking fault (FDSF) structural model (designated the “superposition struc- ture’) with space group Pnnm; or (ii) an ordered monoclinic structure with space group P2 1 /n which was described by Schmittler as “pseudo-orthorhombic’. The space groups, unit-cell parameters and atom coordinates for both models were first proposed by Schmittler (1968). Subsequently, Kondrashev et al.(1985) designated the orthorhombic and monoclinic structures as the α- and β-forms, respectively, with the unit cells of the α- and β-forms being related through an orthorhombic-monoclinic transformation. Other papers which have contributed to understanding the structure forms of copper oxalates include Michalowicz et al.(1979); Gleizes et al. (1980); Fichtner-Schmittler (1984) and Christensen et al.(2014). Also relevant is the report of the crystal structure of fully-ordered orthorhombic monohydrate [diaqua-μ-oxalato-copper(II) monohydrate] (Wu and Zhai, 2007).” A study of natural hydrated copper oxalate material, des- ignated moolooite by Clarke and Williams (1986), which had been extracted from mineral specimens collected 12 km from Mooloo Downs Station homestead in Western Australia, described sound fits to Debye-Scherrer XRD data using an orthorhombic cell. Clarke and Williams reported unit-cell dimensions, a = 5.35, b = 5.63, c = 2.56 Å (V c = 77.1 Å 3 ), after which Chisholm et al.(1987) gave very similar values, a = 5.348–5.381, b = 5.625–5.639, c = 2.548–2.559 Å (V c = 77.17 Å 3 ) for moolooite material extracted from Scandinavian lichens. While the work of Schmittler and others has provided atom coordinates for synthetic material, to date there has been no report of atom coordinates for natural moolooite. Christensen et al.(2014) described the use of laboratory XRD, synchrotron diffraction data (SRD), and neutron diffrac- tion data for synthetic copper oxalate powders to develop a disordered monoclinic P2 1 /n structural model for synthetic Cu oxalate specimens, and also provided evidence for there being no crystal water in the structure of the material exam- ined. The structural model has two randomly-occupied Cu and oxalate sites, which are attributed to the existence of anisotropic nano-sized crystallites. The issue of water content in copper (II) oxalates has not been fully explained. Schmittler (1968) used thermogravimet- ric analysis and pycnometry to estimate the water content of chemically synthesised oxalate powders and provided plots of unit-cell parameters vs. water content for the a and b a) Author to whom correspondence should be addressed. Electronic mail: brian_oconnor@iprimus.com.au 21 Powder Diffraction 34 (1), March 2019 0885-7156/2019/34(1)/21/14/$18.00 © 2019 JCPDS-ICDD 21