Additional crystallographic data are given for the recently reported mineral middlebackite, which has been described for discoveries at Iron Knob in South Australia and Passo di San Lugano near Trento, Italy. The material examined in the present study was from a third finding of the mineral, viz. from a quartz outcrop at Mooloo Downs Station in Western Australia within which it was co-located with the chemically- and structurally-related mineral moolooite, CuIIC2O4·nH2O, reported by Clarke and Williams (1986). In this study, the crystal structure was elucidated independently of the other studies using a combination of the a priori charge flipping and simulated annealing methods with synchrotron radiation diffraction (SRD) powder data. The principal crystal data for the Mooloo Downs material are: space group P21/c with lattice parameters a = 7.2659(18) Å, b = 5.7460(11) Å, c = 5.6806(11) Å, β = 104.588(3)°; Vc = 229.46(18) Å3; empirical formula CuII2C2O4(OH)2 with 2 formula units per unit cell; and calculated density = 3.605 g cm−3. The lattice parameters agree approximately with values given for the other studies, but not within the reported error estimates. The atom coordinates, interatomic distances, and angles for the Mooloo Downs material are compared with those from the other studies using single crystal data, with the values from all three studies agreeing approximately, but again not within the reported uncertainties. The crystal chemistry found for middlebackite received strong confirmation through the synthesis for the first time of di-copper oxalate di-hydroxide. Laboratory X-ray diffraction powder data for the synthetic form of the mineral from this study agree closely with the SRD data for the natural mineral.

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.