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

Wiedenmann, Daniel; Zaitsev, A. N.; Britvin, S. N.; Krivovichev, S. V.; Keller, J.

doi:10.1180/minmag.2009.073.3.373

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Journal article

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

Wiedenmann, Daniel Department of Geoscience, University of Fribourg, Switzerland - EMPA, Swiss Federal Laboratories for Material Testing and Research, Hydrogen and Energy, Duebendorf, Switzerland
Zaitsev, A. N. Department of Mineralogy, Faculty of Geology, St. Petersburg State University, Russia
Britvin, S. N. Department of Crystallography, Faculty of Geology, St. Petersburg State University, Russia
Krivovichev, S. V. Department of Crystallography, Faculty of Geology, St. Petersburg State University, Russia
Keller, J. Institut für Geowissenschaften, Mineralogie-Geochemie, Universität Freiburg, Germany

2009

Published in:

Mineralogical Magazine. - 2009, vol. 73, no. 3, p. 373-384

English Alumoåkermanite, (CaNa)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 μm 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: (Ca_1.48Na_0.50Sr_0.02K_0.01)_Σ2.01(Al_0.44Mg_0.30Fe²⁺_0.17Fe³⁺_0.07Mn_0.01)_Σ0.99(Si_1.99Al_0.01O₇). 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 (R₁ = 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.

Faculty

Faculté des sciences et de médecine

Department

Département de Géosciences

Language