Division
of Geological and Planetary Sciences, California Institute of
Technology,
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 Å, c = 4.930 Å, V= 269.37 Å3,
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 Å, b = 8.85 Å, c = 5.36 Å, β = 105.6°, V= 447.7 Å3,
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.