Abstract: A new mineral, deltalumite, which is an analogue of the spinel-type synthetic δ-Al₂O₃, the second natural modification of alumina after corundum, α-Al₂O₃, has been found in products of two eruptions of the Ploskiy Tolbachik Volcano, Kamchatka, Russia. It occurs in the pores of basalt lava and basalt scoria altered by fumarolic gas. The mineral forms roundish segregations up to 0.2 mm across, which consist of blocky coarse prismatic individuals of up to 0.03 mm. Deltalumite is pale yellowish, pale beige, or white, translucent, with a vitreous luster. The mineral is brittle. D_calc = 3.663 g/cm³. Deltalumite is optically uniaxial, negative, ω = 1.654(2), ε = 1.653(2) (λ = 589 nm). The chemical composition (electron microprobe data) is, wt %: 99.74 Al₂O₃ and 0.04 SiO₂; the total is 99.78. The strongest reflections of the 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, with the P-4m2 space group (similar to synthetic δ-Al₂O₃). The unit-cell dimensions are a = 5.608(1), c = 23.513(7) Å, V = 739.4(4) ų, and Z = 16. Deltalumite belongs to the spinel subgroup within the oxy-spinel group; its structural formula can be written as (Al_0.67◻_0.33)Al₂O₄, where ◻ is a vacancy. The new mineral can be clearly distinguished from other modifications of alumina using its powder X-ray diffraction pattern or IR spectrum.