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. 




SEM image of the two new minerals, paqueite and burnettite, in the Allende meteorite