A novel hydrated copper(II), calcium, rare earth sulfate from the U.K. with a new structure topology

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Marco E. Ciriotti
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A novel hydrated copper(II), calcium, rare earth sulfate from the U.K. with a new structure topology

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 11:12

Referenza:
▪ Rumsey, M., Hawthorne, F., , Spratt, J. (2018): A novel hydrated copper(II), calcium, rare earth sulfate from the United Kingdom with a new
structure topology. IMA2018 Abstract book, The ever-growing mineral diversity in the Earth and the Solar System: new minerals,
including nano- and biominerals, and related nomenclature/classification issues
, 361.

Abstract:
In 1983, this material which has now gone to the IMA-CMNMC for approval as a new mineral, was discovered as a post-mining product in tailings at the Tynebottom Mine in the United Kingdom. Initially, characterisation was hampered by the difficulty of REE determination on such a small and hydrous sample, and more recently the lack of suitable crystals for structural determination. Being 35 years in development, this material is a case study on the importance of patience in new-mineral studies.
The material occurs in a complex assemblage of fairly common secondary Cu, Zn and Pb carbonates and sulphates as tufts of acicular crystals with a deep sky-blue colour. The crystals are around 200 μ m long, but have cross sections only 10 x 2 μ m. Due to the small crystal size and acicular nature, many of the other physical properties of the material cannot be adequately determined. Over 100 individual chemical analyses were done on five different samples and only the best were selected for final elemental determination; the results of 16 point analyses indicate an empirical formula of Ca0.86[REE2.02Al0.06]Cu6.00[(SO4)4.00(SiO4)0.06(PO4)0.02](OH)11.64(H2O)6, where Ce > La and Nd, and is written ideally as Ca(Ce,La,Nd)2Cu6(SO4)4(OH)12(H2O)6. Single crystal x-ray diffraction indicates monoclinic symmetry, space group C2/m, with unit-cell dimensions: a = 24.801(5), b = 6.3520(13), c = 11.245(2)Å, α = 90, β = 114.51(3), γ = 90o, V = 1611(6) Å3 and Z = 4. The structure contains three Cu sites: Cu(1) is [5]-coordinated with the anions arranged in a square pyramid, and Cu(2) and Cu(3) are both [6]-coordinated with the anions in an octahedral arrangement. The Cu(2) and Cu(3) octahedra form chains of edge-sharing octahedra extending parallel to the b axis, The Cu(1) square pyramid shares two edges with the Cu(2) octahedron and two corners with the Cu(3) octahedron, linking the chains of octahedra into a sheet. There are two S sites each of which is tetrahedrally coordinated by four O2- anions which decorate the sheet, each sharing one corner with a Cu octahedron. The sheets are held together by an interstitial REE site occupied dominantly by Ce3+ and coordinated by nine anions, one half-occupied interstitial Ca site coordinated by six anions, and by interstitial hydrogen bonding. The new material is not closely related to any of the other Cu-sulfate minerals despite the similarity of the formula to minerals such as devilline and serpierite, and is particularly interesting chemically in being one of only five sulfate minerals with essential REE and the only one with sulfate as the dominant anionic component. The characterisation of this interesting material, shows the importance of preserving known unknowns in well-curated museum collections where, in the fullness of time, appropriate collaborations or technological advances can be made that allow for full characterisation.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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