IMA 2016-009 = hydroxylgugiaite

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Marco E. Ciriotti
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IMA 2016-009 = hydroxylgugiaite

Messaggio da Marco E. Ciriotti » mer 12 lug, 2017 9:17

Referenza:
▪ Grice, J.D., Kristiansen, R., Friis, H., Rowe, R., Cooper, M.A., Poirier, G.G., Yang, P., Weller, M.T. (2017): Hydroxylgugiaite: A New Beryllium Silicate Mineral From the Larvik Plutonic Complex, Southern Norway and the Ilímaussaq Alkaline Complex, South Greenland; The First Member of the Melilite Group To Incorporate A Hydrogen Atom. Canadian Mineralogist, 55, 219-232.

Abstract:
Hydroxylgugiaite, ideally (Ca3□1)Σ4(Si3.5Be2.5)Σ6O11(OH)3, is a new mineral species from two localities in the Larvik plutonic complex in Porsgrunn, Telemark, Norway, and one locality in Ilímaussaq, Greenland. Hydroxylgugiaite crystals occur as squat dipyramids {111} (30 × 50 μm) or as elongate tetragonal prisms. The crystals are translucent, white to pale grey in color, with a white streak and vitreous luster. It is brittle, with no apparent cleavage. Hydroxylgugiaite is uniaxial positive with ω = 1.622 ± 0.002 and ϵ = 1.632 ± 0.002. There is no pleochroism and birefringence is low.
The average of eight analyses of a single grain of type material (oxide wt.%) gave Na2O 2.04, CaO 32.90, FeO 0.22, MnO 0.74, BeO 13.47 (LA-ICP-MS), Al2O3 0.74, SiO2 44.06, F 1.74, H2O (assuming 3 OH + F) 4.93, Total (–0.73 O = F) 100.10. Potassium, strontium, and magnesium were measured but not detected. The calculated density is 2.79 g cm–3. The empirical formula on the basis of 14 anions including 3 OH– + F– is:
(Ca2.76Na0.31Mn0.05Fe0.01)Σ3.13(Si3.45Be2.53Al0.07)Σ6.05O11[(OH)2.57F0.43]Σ3. The formula from crystal-structure analysis of the Saga specimen is: (Ca3.02□0.98)Σ4(Si1.79Be0.21)Σ2(Be2.29Si1.71)Σ4O11(OH)3. Combined structural and chemical data gives the following formula for the Nakkaalaaq specimen: (Ca2.88□0.98Na0.12Mn0.02)Σ4(Si1.80Be0.17Al0.03)Σ2(Be2.32Si1.68)Σ4O11[(OH)2.70F0.30]Σ3; with simplified formula (Ca,□)4(Si,Be)2(Be,Si)4O11(OH)3.
The crystal structure of hydroxylgugiaite is tetragonal in acentric space group P-421/m, with a 7.4151(2), b 7.4151, c 4.9652(1) Å, V 272.9(1) Å3, and Z = 1. It has been refined to an R index of 0.028 on the basis of 342 observed reflections and a correction for the {110} twin law. It is an H-bearing member of the melilite group. The structure has two distinct layers. The one crystallographically distinct Ca site with eight-fold coordination is a square antiprism polyhedron. The Ca polyhedra are in a layer with the H atoms. A second layer consists of corner-sharing Si/Be atoms in tetrahedral coordination with O. One H atom is bonded to an apical O atom that is not shared by two tetrahedra. This H atom is present only when there is a Ca-site vacancy. The other H atom is loosely bonded to the same O atom but at a different site. The IR spectrum supports this H-bonding scheme.
Additional hydroxylgugiaite data is given for the other localities.
Marco E. Ciriotti

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