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

Authors: Wiedenmann, D.; Zaitsev, A.N.; Britvin, S.N.; Krivovichev, S.V.; Keller, J.

Source: Mineralogical Magazine, Volume 73, Number 3, June 2009 , pp. 373-384(12)

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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 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.

Keywords: ALUMOÅKERMANITE; ÅKERMANITE; MELILITE; NEW MINERAL; VOLCANIC ROCK; CHEMICAL COMPOSITION; CRYSTAL STRUCTURE; OLDOINYO LENGAI; TANZANIA

Document Type: Research article

DOI: http://dx.doi.org/10.1180/minmag.2009.073.3.373

Publication date: 2009-06-01

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