Alumoakermanite, (Ca,Na)_2(Al,Mg,Fe~(2+))(Si_2O_7), a new mineral from the active carbonatite-nephelinite-phonoiite volcano Oldoinyo Lengai, northern Tanzania

摘要

Alumoakermanite, (Ca,Na)_2(Al,Mg,Fe~(2+))(Si_2O_7), 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. Alumoakermanite 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~3. The Mohs hardness is approx 4.5-6. Alumoakermanite is uniaxial (-) with omega = 1.635(1) and epsilon = 1.624-1.626(1). In a 30 mu m thin section (+N), the mineral has a yellow to orange interference colour, straight extinction and gositive elongation, and is nonpleochroic. The average chemical formula of the mineral derived from electron microprobe analyses is: (Ca_(1.48)Na_(0.50)Sr_(0.02) K_(0.01))_(8888882.01)(Al_(0.44)Mg_(0.30)Fe_(0.17)~(2+)Fe_(0.07)~(3+)Mn_(0.01))_(888880.99)(Si_(1.99)Al_(0.01)O_7). Alumoakermanite is tetragonal, space group P42_1m with a = 7.7661(4) A, c = 5.0297(4) A, V = 303.4(1) A~3 and 2=2. The five strongest powder-diffraction lines [d in A, (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)(Si_2O_7)] sheets interleaved with layers of (CaNa) cations. The name reflects the chemical composition of the mineral.