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- Bosi, Ferdinando, et al.
(författare)
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Lowering R3m Symmetry in Mg-Fe-Tourmalines: The Crystal Structures of Triclinic Schorl and Oxy-Dravite, and the Mineral luinaite-(OH) Discredited
- 2022
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Ingår i: Minerals. - : MDPI AG. - 2075-163X. ; 12:4, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Discreditation of the monoclinic tourmaline mineral species luinaite-(OH), ideally (Na,▯)(Fe2+,Mg)3Al6(BO3)3Si6O18(OH)4 was approved by the IMA-CNMNC (proposal 21-L) and is described. We analyzed two luinaite-(OH) samples: one from the type locality Cleveland tin mine, Luina, Waratah, Tasmania, Australia, and the other from Blue Mountain Saddle (Bald Hornet Claim), North Bend, King County, Washington, DC, USA. Biaxial (−) crystals representative of the studied samples were spectroscopically (Mössbauer, polarized Fourier transform infrared, optical absorption spectroscopy), chemically (nuclear microprobe analysis and electron microprobe analysis), and structurally characterized (single-crystal X-ray diffraction). Results show the occurrence of a triclinic structure for the studied luinaite-(OH) samples, which differs only in terms of a slight structural distortion from typical trigonal tourmaline structure (the topology of the structure is retained). As a result, following the IMA-CNMNC and tourmaline nomenclature rules, the triclinic luinaite-(OH) from the type locality (Australia) can be considered as the triclinic dimorph of schorl, as its chemical composition corresponds to schorl, and thus it should be referred as schorl-1A. Similarly, the triclinic sample from the USA can be considered as the triclinic dimorph of oxy-dravite, as its chemical composition corresponds to oxy-dravite, and then is referred to as oxy-dravite-1A.
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| 2. |
- Cámara, Fernando, et al.
(författare)
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As-bearing new mineral species from Valletta mine, Maira Valley, Piedmont, Italy: IV. Lombardoite, Ba2Mn3+(AsO4)2(OH) and aldomarinoite, Sr2Mn3+(AsO4)2(OH), description and crystal structure
- 2022
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 86:3, s. 447-458
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Tidskriftsartikel (refereegranskat)abstract
- Lombardoite, ideally Ba2Mn3+(AsO4)2(OH), and aldomarinoite, ideally Sr2Mn3+(AsO4)2(OH), are two new minerals of the arsenbrackebuschite group in the brackebuschite supergroup, discovered in Fe–Mn ore in metaquartzites of the abandoned mine of Valletta, Canosio, Val Maira, Cuneo Province, Piedmont, Italy. They occur as red–brown and orange brown, respectively, as subhedral crystals (< 0.5 mm) in thin masses, associated with quartz, aegirine, baryte, calcite, hematite, muscovite and Mn minerals such as cryptomelane, braunite and manganberzeliite. Both minerals are translucent, have yellow–orange streak and vitreous lustre. Both are brittle. Estimated Mohs hardness is 6–6½ for lombardoite (by analogy to canosioite), and 4½–5 for aldomarinoite (by analogy to tokyoite). Calculated densities are 5.124 g/cm3 for lombardoite and 4.679 g/cm3 for aldomarinoite. Both minerals are biaxial (+). Lombardoite shows 2Vz(meas.) = 78(4)° and is pleochroic with X = yellowish brown, Y = brown and Z = reddish brown (Z > Y > X). Aldomarinoite has 2Vz(meas.) = 67.1(1)°, and is pleochroic with X = brown, Y = brownish orange and Z = yellowish brown (Z > Y > X). Point analyses by electron microprobe using wavelength dispersive spectroscopy resulted in the empirical formula (based on 9 O anions): (Ba1.96Sr0.17Pb0.04Na0.02Ca0.02)Σ2.21(Mn3+0.62Fe3+0.13Al0.06Mg0.11)Σ0.92[(As0.87V0.12P0.01)Σ1.00O4]2(OH) for lombardoite, and (Sr1.93Ca0.21Ba0.04Pb0.01)Σ2.19(Mn3+0.48Al0.35Fe3+0.21Mg0.01)Σ1.05[(As0.92V0.03)Σ0.95O4]2(OH) for aldomarinoite. The absence of H2O was confirmed by Raman spectroscopy and infrared spectroscopy. Both minerals are monoclinic, P21/m, with unit-cell parameters a = 7.8636(1) Å, b = 6.13418(1) Å, c = 9.1197(1) Å, β = 112.660(2)° and V = 405.94(1) Å3, for lombardoite and a = 7.5577(4) Å, b = 5.9978(3) Å, c = 8.7387(4) Å, β = 111.938(6)° and V = 367.43(3) Å3, for aldomarinoite. The eight strongest powder X-ray diffraction lines are [d, Å (Irel) (hkl)]: 6.985 (39) (10$\bar{1}$), 3.727 (33) (111), 3.314 (100) (21$\bar{1}$), 3.073 (24) (020), 3.036 (33) (21$\bar{2}$, 10$\bar{3}$), 2.810 (87) (12$\bar{1}$, 112), 2.125 (20) (301, 11$\bar{4}$) and 1.748 (24) (321) for lombardoite and 3.191 (89) (21$\bar{1}$), 2.997 (45) (020), 2.914 (47) (21$\bar{2}$, 10$\bar{3}$), 2.715 (100) (112), 2.087 (39) (12$\bar{3}$, 1.833 (32) (31$\bar{4}$), 1.689 (36) (321), 1.664 (21) (132) for aldomarinoite. The minerals are isostructural with brackebuschite: infinite chains of edge sharing octahedra running parallel to the b axis and decorated with AsO4 groups are connected along the a and c axes through Ba and Sr atoms in lombardoite and aldomarinoite, respectively. The minerals are named after Bruno Lombardo (1944–2014), geologist and petrologist at C.N.R. (National Research Council of Italy), and Aldo Marino (b. 1942) the mineral collector and founding member of the AMI – Italian Micromineralogical Association.
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