• journal_title：Mineralogical Magazine
• Contributor：D. Wiedenmann ; A. N. Zaitsev ; S. N. Britvin ; S. V. Krivovichev ; J. Keller
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2009.073.3.373
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：73
• issue：3
• firstpage：373
• section：Articles

Alumoåkermanite, (CaNa)<sub>2sub>(Al,Mg,Fe<sup>2+sup>)(Si<sub>2sub>O<sub>7sub>), 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/cm<sup>3sup>. 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<sub>1.48sub>Na<sub>0.50sub>Sr<sub>0.02sub>K<sub>0.01sub>)<sub>Σ2.01sub>(Al<sub>0.44sub>Mg<sub>0.30sub><span class="inline-formula" id="inline-formula-1">ss="math tex" alt="Formula" src="373/embed/tex-math-1.gif" />span><span class="inline-formula" id="inline-formula-2">ss="math tex" alt="Formula" src="373/embed/tex-math-2.gif" />span>Mn<sub>0.01sub>)<sub>Σ0.99sub>(Si<sub>1.99sub>Al<sub>0.01sub>O<sub>7sub>). Alumoåkermanite is tetragonal, space group P<span class="inline-formula" id="inline-formula-3">ss="math tex" alt="Formula" src="373/embed/tex-math-3.gif" />span>2<sub>1sub>m with a = 7.7661(4) Å, c = 5.0297(4) Å, V = 303.4(1) Å<sup>3sup> and Z = 2. The five strongest powder-diffraction lines [d in Å, (I/I<sub>osub>), 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<sub>1sub> = 0.018) revealed structure topology typical of the melilite-group minerals, i.e. tetrahedral [(Al,Mg)(Si<sub>2sub>O<sub>7sub>)] sheets interleaved with layers of (CaNa) cations. The name reflects the chemical composition of the mineral.