A new titanium‐bearing calcium aluminosilicate phase: I. Meteoritic occurrences and formation in synthetic systems

Abstract

A new titanium‐bearing calcium aluminosilicate mineral has been identified in coarse‐grained calcium‐aluminum‐rich inclusions (CAIs) from carbonaceous chondrites. The formula for this phase, which we have temporarily termed “UNK,” is Ca3Ti(Al, Ti)2(Si, Al)3O14, and it is present in at least 8 of the 20 coarse‐grained CAIs from the Allende CV3 chondrite examined as part of this project. The phase occurs in Types A and B1 inclusions as small tabular crystals oriented along two mutually perpendicular planes in melilite. UNK crystallizes from melts in dynamic crystallization experiments conducted in air from four bulk compositions modeled after Types A, B1, B2 and C inclusions. Cooling rates resulting in crystallization of UNK ranged from 0.5 to 200 °C/h from maximum (initial) temperatures of 1375 to 1580 °C. Only below 1190 °C does UNK itself begin to crystallize. To first order, the presence or absence of UNK from individual experiments can be understood in terms of the compositions of residual melts and nucleation probabilities. Compositions of synthetic and meteoritic UNK are very similar in terms of major oxides, differing only in the small amounts of trivalent Ti (7–13% of total Ti) in meteoritic samples. UNK crystallized from the Type A analog is similar texturally to that found in CAIs, although glass, which is typically associated with synthetic UNK, is not observed in meteoritic occurrences. A low Ti end‐member of UNK (“Si‐UNK”) with a composition near that of Ca3Al2Si4O14 was produced in a few samples from the Type B1 analog. This phase has not been found in the meteoritic inclusions.

A new titanium‐bearing calcium aluminosilicate phase: III. Crystals from a mixer furnace slag

“…The papers by El Goresy et al (1984) and Paque et al (1986) reporting the occurrence of a Ti-bearing Al-silicate, which we refer to as UNK for the purposes of this paper, in meteoritic inclusions and slag-like synthetic compositions, raised the possibility that the furnace slag crystals were also the UNK phase. This idea has been confinued, as described below, and has led to use of crystals from the slag for the determination of the space group of the mineral described in Paque et al (1986Paque et al ( , 1994 and Barber et al (1994) by means of convergent beam electron diffraction (CBED).…”

The effect of TiO2 additions on CaO–MgO–Al2O3–SiO2 (CMAS) crystallization behavior from the melt

“…XRD confirmed the presence of paqueite (Ca 3 TiSi 2 (Al,Ti,Si) 3 O 14 ) along with melilite for the 1300/900°C (5.5 hours) profile (Figure ). Paqueite, which was so named only in 2013, is a mineral that has been discovered in meteoritic calcium‐aluminum‐rich inclusions (CAIs) of carbonaceous chondrites, as well as in synthetic CMAS‐TiO 2 samples . A cross‐section of the product material suggested that the paqueite phase formed as spherulites within the glassy matrix.…”

Crystallization of calcium‐aluminum‐rich inclusions: Experimental studies on the effects of repeated heating events

“…2g). The intergrowth consists mainly of anorthite, pyroxene, ikermanitic-rich melilite, and in one case unknown (Paque et al, 1994). The presence of the intergrowth is not indicative of low-temperature precrystallization and has been produced in single-stage experiments (Stolper and Paque, 1986).…”