New minerals in fluffy type A inclusions from Allende and clues to processes in the early solar system.

Part I: Paqueite, Ca3TiSi2(Al,Ti,Si)3O14, and burnettite, CaVAlSiO6

Chi Ma, John R.  Beckett, Francois Tissot, George R. Rossman

 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA 91125-2500


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 fluffy Type A (FTA) Ca-Al-rich inclusions (CAIs), A-WP1 and CG-12, from the Allende CV3 carbonaceous chondrite. The type paqueite from A-WP1 has an empirical formula of (Ca2.91Na0.11)Ti4+Si2(Al1.64Ti4+0.90Si0.24V3+0.12Sc0.07Mg0.03)O14, with a trigonal structure in space group P321 and cell parameters a =7.943 , c = 4.930 , V= 269.37 3, and Z = 1, which leads to a calculated density of 3.39 g/cm3. Paqueite’s general formula is Ca3TiSi2(Al,Ti,Si)3O14 and the end-member formula is Ca3TiSi2(Al2Ti)O14.   Based on Sc-Zr, there are two populations of paqueite in FTAs with most paqueites from CG-12 in the low Sc-Zr group and all of the paqueites in A-WP1 in the high-Sc population.  Vanadium displays a similar pattern with all paqueites from A-WP1 in a low-V population and most paqueites from CG-12 in a high-V population.

Type burnettite from CG-12 has an empirical formula of Ca1.01(V3+0.56Al0.25Mg0.18)(Si1.19Al0.81)O6. It assumes a diopside-type C2/c structure with a = 9.80 , b = 8.85 , c = 5.36 , β = 105.6, V= 447.7 3, and Z = 4, leading to a calculated density of 3.44 g/cm3. Burnettite’s general formula is Ca(V,Al,Mg)AlSiO6 and the end-member formula is CaVAlSiO6.  Compositions of clinopyroxenes included in FTA melilite generally form arrays of roughly constant V/Sc but they can have highly variable Ti.  Spinel and perovskite have highly restricted V concentrations (0.14 – 0.73 wt.%) but V concentrations in hibonite are variable.  A common theme among V-bearing phases in A-WP1 and CG-12 is that V concentrations are restricted to one or two concentrations, suggesting that the phase included in FTA melilite were drawn from a highly restricted set of environments.

Burnettite is a refractory mineral, likely condensates or crystallized from a melt, and among the first solid materials formed in the solar nebula. Burnettite is a V-rich clinopyroxene that can be Sc-, Ti-rich (coexisting melilite is Sc-, V-poor, < 0.03 wt% of the oxides), joining other Sc-rich refractory minerals from carbonaceous chondrites. The high Sc in some burnettites is consistent with derivation from an ultrarefractory parent and the high Ti3+/Ti4+ (0.8–1.3), which is similar to values for davisite and grossmanite from the same inclusion (0.6–0.9), suggests reducing conditions. Paqueite coexisting with clinopyroxene has lower Ti3+/Ti4+ (0.0–0.1 in bulk) but both phases imply reducing conditions. The subequal amounts of V, Sc, and Ti in some burnettites, davisites, and grossmanites suggests that there is a complete solid solution for these end-member clinopyroxenes. Some paqueite, a Ti-rich silicate, likely condensed in the nebula but other grains may have formed through exsolution or been involved in partial melting.