Beckettite, Ca₂V₆Al₆O₂₀, a new mineral in a Type A refractory inclusion from Allende and clues to processes in the early solar system

Abstract

Beckettite (Ca₂V₆Al₆O₂₀; IMA 2015-001) is a newly discovered refractory mineral, occurring as micrometer-sized grains intergrown with hibonite and perovskite, and surrounded by secondary grossular, anorthite, coulsonite, hercynite, and corundum. It occurs within highly altered areas in a V-rich, Type A Ca-Al-rich inclusion (CAI), A-WP1, from the Allende CV3 carbonaceous chondrite. The type beckettite has an empirical formula of (Ca_(1.99)Na_(0.01))(V³⁺_(3.47)Al_(1.40)Ti⁴⁺_(0.57)Mg_(0.25)Sc_(0.08)Fe²⁺_(0.04))(Al_(5.72)Si_(0.28))O₂₀, with a triclinic structure in space group P1 and cell parameters a = 10.367 Å, b = 10.756 Å, c = 8.895 Å, α = 106.0°, β = 96.0°, γ = 124.7°, V = 739.7 Å3, and Z = 2, which leads to a calculated density of 3.67 g cm⁻³. Beckettite’s general formula is Ca₂(V,Al,Ti,Mg)₆Al₆O₂₀ and the endmember formula is Ca2V6Al6O20. Beckettite is slightly ¹⁶O-depleted (Δ¹⁷O = −16 ± 2‰) compared to the coexisting hibonite and spinel −24 ± 2‰. Beckettite is a primary high-temperature mineral resulting from igneous crystallization of an ¹⁶O-rich V-rich CAI melt together with V-bearing hibonite, perovskite, burnettite, spinel, and paqueite. Subsequently, beckettite experienced an incomplete isotope exchange with an ¹⁶O-poor aqueous fluid (Δ¹⁷O = −3 ± 2‰) on the Allende parent asteroid.