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Middlebackite, a new Cu oxalate mineral from Iron Monarch, South Australia; description and crystal structure

Peter Elliott
Middlebackite, a new Cu oxalate mineral from Iron Monarch, South Australia; description and crystal structure
Mineralogical Magazine (June 2019) 83 (3): 427-433

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 approximately 2. The calculated density is 3.64 g cm (super -3) . Crystals are biaxial (+) with alpha = 1.663(4), beta = 1.748(4) and ? = 1.861(4) (measured in white light). The calculated 2V is 86.7 degrees . 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 Cu (sub 2.00) (C (sub 2) O (sub 4) )Cl (sub 0.02) (OH) (sub 1.98) . Middlebackite is monoclinic, space group P2 (sub 1) c with a = 7.2597(15), b = 5.7145(11), c = 5.6624(11) Aa, beta = 104.20(3) degrees , V = 227.73(8) Aa (super 3) and Z = 2. The five strongest lines in the powder X-ray diffraction pattern are [d(Aa), (I), (hkl)]: 7.070 (16) (100), 3.739 (100) (111), 2.860 (18) (020), 2.481 (12) (121) and 2.350 (9) (300). The crystal structure was refined from synchrotron single-crystal X-ray diffraction data to R (sub 1) = 0.0341 for 596 observed reflections with F (sub 0) > 4sigma (F (sub 0) ). 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.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 83
Serial Issue: 3
Title: Middlebackite, a new Cu oxalate mineral from Iron Monarch, South Australia; description and crystal structure
Author(s): Elliott, Peter
Affiliation: University of Adelaide, Department of Earth Sciences, Adelaide, South Aust., Australia
Pages: 427-433
Published: 20190601
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 21
Accession Number: 2019-067061
Categories: Mineralogy of non-silicatesEconomic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
S32°45'00" - S32°43'00", E138°07'60" - E138°10'00"
Secondary Affiliation: South Australian Museum, AUS, Australia
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201935
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