Crystallization of dmisteinbergite (hexagonal <scp> CaAl ₂ Si ₂ O ₈ </scp> ) from type B <scp>CAI</scp> analog melt

Abstract Dmisteinbergite is a metastable polymorph of CaAl 2 Si 2 O 8 , which has been found in calcium−aluminum‐rich inclusions (CAIs) in chondrites, although anorthite is the predominant CaAl 2 Si 2 O 8 phase in natural type B CAIs. The occurrence/absence of igneous dmisteinbergite would put tighter constraints on the thermal history of CAIs. However, little attention has been paid for crystallization of dmisteinbergite in CAI melts even though its kinetically controlled crystallization (i.e., crystallization from a supercooled melt) has been proposed. In this study, we conducted dynamic crystallization experiments on CAI analogs in the presence of low‐pressure hydrogen gas (1–10 Pa) to simulate the formation of igneous CAIs and to understand crystallization behavior of CaAl 2 Si 2 O 8 phases. Dmisteinbergite was found in samples cooled at rates of 1–20 °C h −1 , while anorthite was found in samples cooled at 5 °C h −1 , suggesting that the slow cooling (likely slower than on the order of 1 °C h −1 ) might promote the formation of anorthite. Because the slow cooling rates during the final stage of crystallization of CAIs likely reflect conditions approaching the background temperature of CAI‐forming regions, CaAl 2 Si 2 O 8 phases in type B CAIs could constrain the thermal history and formation environment of those regions.