IMA 2015-115 = middlebackite

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Marco E. Ciriotti
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IMA 2015-115 = middlebackite

Messaggio da Marco E. Ciriotti » mer 04 lug, 2018 10:11

Prossima pubblicazione.

▪ Elliott, P. (2018): Middlebackite, a new Cu oxalate mineral from Iron Monarch, South Australia: Description and crystal structure. Mineralogical Magazine, 82, (in press).

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 to 0.3 mm across comprising individual crystals to 0.05 mm in length associated with atacamite and mottramtite. Crystals are translucent with a vitreous lustre and has 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), γ = 1.861(4) (measured in white light). The calculated 2V is 86.7°. Pleochroism is X = colourless, Y = very pale blue, 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, P21/c with a = 7.2597(15), b = 5.7145(11), c = 5.6624(11) Å,  = 104.20(3) º, V = 227.73(8) Å3 and Z = 2. The five strongest lines in the X-ray powder diffraction pattern are [d(Å), (I), (hkl)]: 7.070 (16) (100), 3.739 (100) (11-1), 2.860 (18) (020), 2.481 (12) (12-1), 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.
Marco E. Ciriotti

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