New minerals in type A inclusions from Allende and clues to processes in the early solar system: Paqueite, Ca₃TiSi₂(Al,Ti,Si)₃O₁₄, and burnettite, CaVAlSiO₆

Abstract

Paqueite (Ca₃TiSi₂[Al,Ti,Si]₃O₁₄; IMA 2013-053) and burnettite (CaVAlSiO₆; IMA 2013-054) are new refractory minerals, occurring as euhedral to subhedral crystals within aluminous melilite in A-WP1, a type A Ca-Al-rich inclusion, and CGft-12, a compact type A (CTA) from the Allende CV3 carbonaceous chondrite. Type paqueite from A-WP1 has an empirical formula of (Ca_(2.91)Na_(0.11))Ti⁴⁺Si₂(Al_(1.64)Ti⁴⁺_(0.90)Si_(0.24)V³⁺_(0.12)Sc_(0.07)Mg_(0.03))O₁₄, with a trigonal structure in space group P321 and cell parameters a = 7.943 Å, c = 4.930 Å, V = 269.37 Å3, and Z = 1. Paqueite's general formula is Ca₃TiSi₂(Al,Ti,Si)₃O₁₄ and the endmember formula is Ca₃TiSi₂(Al₂Ti)O₁₄. Type burnettite from CGft-12 has an empirical formula of Ca_(1.01)(V³⁺_(0.56)Al_(0.25)Mg_(0.18))(Si_(1.19)Al_(0.81))O₆. It assumes a diopside-type C2/c structure with a = 9.80 Å, b = 8.85 Å, c = 5.36 Å, β = 105.6°, V = 447.7 ų, and Z = 4. Burnettite's general formula is Ca(V,Al,Mg)AlSiO₆ and the endmember formula is CaVAlSiO₆. Paqueite and burnettite likely originated as condensates, but the observed grains may have crystallized from local V-rich melts produced during a later thermal event. For CGft-12, the compositions of paqueite, clinopyroxene, and perovskite suggest that type As drew from two distinct populations of grains. Hibonite grains drew from multiple populations, but these were well mixed and not equilibrated prior to incorporation into type A host melilite.

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