Physics Department, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
A new titanium-bearing calcium aluminosilicate phase: I. Meteoritic occurrences and formation in synthetic systems
Article first published online: 15 JUN 2012
1994 The Meteoritical Society
Volume 29, Issue 5, pages 673–682, September 1994
How to Cite
Paque, J. M., Beckett, J. R., Barber, D. J. and Stolper, E. M. (1994), A new titanium-bearing calcium aluminosilicate phase: I. Meteoritic occurrences and formation in synthetic systems. Meteoritics, 29: 673–682. doi: 10.1111/j.1945-5100.1994.tb00783.x
- Issue published online: 15 JUN 2012
- Article first published online: 15 JUN 2012
- Received 1993 September 28; accepted in revised form 1994 May 3
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.