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Alumoåkermanite, (Ca,Na)2(Al,Mg,Fe2+)(Si2O7), a new mineral from the active carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, northern Tanzania

Published online by Cambridge University Press:  05 July 2018

D. Wiedenmann
Affiliation:
Department of Geoscience, University of Fribourg, Perolles, 1700 Fribourg, Switzerland EMPA, Swiss Federal Laboratories for Material Testing and Research, Hydrogen and Energy, Ueberlandstrasse 129, Duebendorf 8600, Switzerland
A. N. Zaitsev
Affiliation:
Department of Mineralogy, Faculty of Geology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia
S. N. Britvin
Affiliation:
Department of Crystallography, Faculty of Geology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia
S. V. Krivovichev
Affiliation:
Department of Crystallography, Faculty of Geology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia
J. Keller
Affiliation:
Institut für Geowissenschaften, Mineralogie-Geochemie, Universität Freiburg, Albertstrasse 23b, 79104 Freiburg, Germany
Corresponding

Abstract

Alumoåkermanite, (Ca,Na)2(Al,Mg,Fe2+)(Si2O7), is a new mineral member of the melilite group from the active carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, Tanzania. The mineral occurs as tabular phenocrysts and microphenocrysts in melilite-nephelinitic ashes and lapilli-tuffs. Alumoåkermanite is light brown in colour; it is transparent, with a vitreous lustre and the streak is white. Cleavages or partings are not observed. The mineral is brittle with an uneven fracture. The measured density is 2.96(2) g/cm3. The Mohs hardness is ~4.5–6. Alumoåkermanite is uniaxial (–) with ω = 1.635(1) and ε = 1.624–1.626(1). In a 30 mm thin section (+N), the mineral has a yellow to orange interference colour, straight extinction and positive elongation, and is nonpleochroic. The average chemical formula of the mineral derived from electron microprobe analyses is: (Ca1.48Na0.50Sr0.02 K0.01)(Si1.99Al0.01O7). Alumoåkermanite is tetragonal, space group P421m with a = 7.7661(4) Å, c = 5.0297(4) Å, V = 303.4(1) Å3 and Z = 2. The five strongest powder-diffraction lines [d in Å, (I/I o), hkl] are: 3.712, (13), (111); 3.075, (25), (201); 2.859, (100), (211); 2.456, (32), (311); 1.757, (19), (312). Single-crystal structure refinement (R 1 = 0.018) revealed structure topology typical of the melilite-group minerals, i.e. tetrahedral [(Al,Mg)(Si2O7)] sheets interleaved with layers of (CaNa) cations. The name reflects the chemical composition of the mineral.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2009

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Alumoåkermanite, (Ca,Na)2(Al,Mg,Fe2+)(Si2O7), a new mineral from the active carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, northern Tanzania
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Alumoåkermanite, (Ca,Na)2(Al,Mg,Fe2+)(Si2O7), a new mineral from the active carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, northern Tanzania
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