IMA 2016-027 = deltalumite

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Marco E. Ciriotti
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IMA 2016-027 = deltalumite

Messaggio da Marco E. Ciriotti » gio 12 set, 2019 14:34

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▪ Pekov I.V., Anikin, L.P., Chukanov, N.V., Belakovskiy, D.I., Yapaskurt, V.O., Sidorov, E.G., Britvin, S.N., Zubkova, N.V. (2019): Deltalumite, a new natural modification of alumina with spinel-type structure. Zapiski RMO, 148(4), (in press).

The new mineral deltalumite, an analogue of the spinel-type synthetic δ-Al2O3, the second, after corundum α-Al2O3, natural modification of alumina, was found in products of two eruptions of the Ploskiy Tolbachik volcano (Kamchatka, Russia). It occurs in pores of basalt lava and basalt scoria altered by fumarolic gas. The mineral forms roundish aggregations up to 0.2 mm across which consist of blocky, coarse prismatic individuals up to 0.03 mm in size. Deltalumite is pale yellowish, pale beige or white, translucent, with vitreous lustre. The mineral is brittle. Dcalc = 3.663 g cm–3. Deltalumite is optically uniaxial (–), ω = 1.654(2), ε = 1.653(2) (λ = 589 nm). Chemical composition (electron microprobe) is: Al2O3 99.74, SiO2 0.04, total 99.78 wt.%. The strongest reflections of powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 2.728(61)(202), 2.424(51)(212), 2.408(49)(213), 2.281(42)(206), 1.993(81)(1.0.11, 220, 221), 1.954(48)(0.0.12) and 1.396(100)(327, 400, 2.1.14). The mineral is tetragonal, space group P-4m2 (by analogy with synthetic δ-Al2O3), unit-cell dimensions are: a = 5.608(1), c = 23.513(7) Å, V = 739.4(4) Å3 and Z = 16. Deltalumite belongs to the spinel subgroup within the oxyspinel group, its structural formula can be written as (Al0.67☐0.33)Al2O4 in which ☐ means vacancy. The new mineral can be clearly distinguished from other modifications of alumina using powder X-ray diffraction pattern or IR spectrum
Marco E. Ciriotti

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