Hexagonal CaAl_2Si_2O_8 in a high temperature solution; metastable crystallization and transformation to anorthite

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Author(s)

    • ABE Toshiya
    • Department of Meneralogical Sciences and Geology, Faculty of Science, Yamaguchi University

Abstract

Nucleation, stability and change of hexagonal CaAl<SUB>2</SUB>Si<SUB>2</SUB>O<SUB>8</SUB> crystals in the solution of anorthite<SUB>70</SUB>forsterite<SUB>10</SUB>silica<SUB>20</SUB> wt% were investigated by high temperature in-situ observations, ex-situ runs in a furnace, and examinations of the quenched products.<BR>Hexagonal CaAl<SUB>2</SUB>Si<SUB>2</SUB>O<SUB>8</SUB> was observed to crystallize dominantly in the supercooled liquid at temperatures from 1100 to 1000°C. The incubation time becomes shorter as the temperature decreases. At 1000°C, it nucleates within a few hours. At these temperatures anorthite rarely nucleates in spite of large ΔT (300–400°C), but does at 1250–1150°C (ΔT=150–250°C). Its nucleation, however, needs time more than several hours. The nucleation sequence is explainable with supercooling degree and interfacial free energy difference for each polymorph. It was also demonstrated that the hexagonal polymorph has a metastable liquidus at 1240°C, 160°C lower than that of anorthite. This difference is consistent with that in the An<SUB>100</SUB> composition. The crystals of hexagonal CaAl<SUB>2</SUB>Si<SUB>2</SUB>O<SUB>8</SUB> grow steadily below this metastable liquidus. This can exist permanently if anorthite is not present in the system. However, hexagonal CaAl<SUB>2</SUB>Si<SUB>2</SUB>O<SUB>8</SUB> changes easily to anorthite without dissolution once the latter starts crystallization and gets in contact with the former. The contact of the two phases triggers the change from hexagonal to triclinic CaAl<SUB>2</SUB>Si<SUB>2</SUB>O<SUB>8</SUB>.

Journal

  • Mineralogical Journal

    Mineralogical Journal 17(6), 257-281, 1995-04-01

    Japan Association of Mineralogical Sciences

References:  24

Codes

  • NII Article ID (NAID)
    10008771169
  • NII NACSIS-CAT ID (NCID)
    AA00739590
  • Text Lang
    ENG
  • Article Type
    ART
  • ISSN
    05442540
  • Data Source
    CJP  J-STAGE 
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