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

Published online by Cambridge University Press:  02 July 2018

Peter Elliott
Peter Elliott*
Affiliation:
Department of Earth Sciences, School of Physical Sciences, The University of Adelaide, Adelaide, South Australia 5005, Australia South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia
*
*Author for correspondence: Peter Elliott, Email: peter.elliott@adelaide.edu.au
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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.

Keywords

middlebackitenew mineral speciescopper oxalatecrystal structureIron MonarchSouth AustraliaAustralia
Type
Article
Information
Mineralogical Magazine , Volume 83 , Issue 3 , June 2019 , pp. 427 - 433
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: František Laufek

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