Middlebackite, a new Cu oxalate mineral from Iron Monarch, South Australia: Description and crystal structure

Abstract

Middlebackite is a new supergene mineral formed in the upper levels of the Iron Monarch quarry, South Australia. It occurs as aggregates of blue, prismatic crystals up to 0.3 mm across comprising individual crystals up to 0.05 mm in length associated with atacamite and mottramite. Crystals are translucent with a vitreous lustre and have a pale blue streak. Middlebackite is brittle with one perfect cleavage and uneven fracture. Mohs hardness is ~2. The calculated density is 3.64 g cm–3. Crystals are biaxial (+) with α = 1.663(4), β = 1.748(4) and γ = 1.861(4) (measured in white light). The calculated 2V is 86.7°. Pleochroism is X = colourless, Y = very pale blue and Z = dark sky blue; Z > Y > X. The empirical formula unit, based on six oxygen atoms per formula unit is Cu2.00(C2O4)Cl0.02(OH)1.98. Middlebackite is monoclinic, space group P21/c with a = 7.2597(15), b = 5.7145(11), c = 5.6624(11) Å, β = 104.20(3)°, V = 227.73(8) Å3 and Z = 2. The five strongest lines in the powder X-ray diffraction pattern are [d(Å), (I), (hkl)]: 7.070 (16) (100), 3.739 (100) (11$\bar{1}$), 2.860 (18) (020), 2.481 (12) (12$\bar{1}$) and 2.350 (9) (300). The crystal structure was refined from synchrotron single-crystal X-ray diffraction data to R1 = 0.0341 for 596 observed reflections with F0 > 4σ(F0). The structure is based on sheets of edge- and corner-sharing octahedra parallel to the bc plane. Sheets link in the a direction via oxalate anions.

Synchrotron radiation diffraction study of the mineral moolooite, and synthetic copper oxalates

“…M Bussell, formerly of CSIRO Division of Mineralogy, is acknowledged for preparing the CSIRO-SYN synthetic moolooite, R Hermann of the Department of Chemistry, Curtin University for element analysis (CHN) of synthetic moolooite and D Allen of MBS Environmental for advice on the chemical synthesis. P Elliott of The University of Adelaide is acknowledged for providing a pre-proof copy of the accepted article Elliott (2018).…”

X-ray diffraction study of middlebackite [CuII2C2O4(OH)2, di-copper oxalate dihydroxide], using a mineral specimen from Mooloo Downs Station, Western Australia and chemically synthesized material

“…The Smith–Snyder FOM and unit cell dimensions for the synthetic material from LXRD data (Table I) were F 18 = 30.2 (0.017, 35) and a = 7.272(2) Å, b = 5.757(2) Å, c = 5.679(2) Å; β = 104.77(2)°; V c = 229.9 Å 3 . The middlebackite lattice parameters result from the present study are compared with those from the Elliott (2018) and Demartin et al . (2017) publications on the Iron Knob and Passo di San Lugano middlebackite materials, respectively – Table II.…”

“…The approach taken here, focusing on chemical bonding, within and between the molecular ribbons, complements that followed by Demartin et al . (2017) and Elliott (2018) both of whom mainly described the structure in terms of polyhedra distributions.…”

“…Elliott (2018) has quoted LXRD d-I diffraction data. These are compared with SRD d-I data from the current study in Table VIII, which shows sound agreement as expected from the close agreement between the lattice parameters.…”

“…Initial accounts of the mineral middlebackite, Cu II 2 C 2 O 4 (OH) 2 from the Iron Monarch Quarry, Iron Knob, South Australia and from Passo di San Lugano, near Trento, Italy have been described by Elliott (2016, 2018) and Demartin et al . (2017), respectively.…”