Uvarovite: Stability of uvarovite-andradite solid solutions at low pressure |
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Authors: | H. G. Huckenholz D. Knittel |
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Affiliation: | (1) Mineralogisch-Petrographisches Institut der Universität München, Theresienstr. 41, D-8000 München 2, Federal Republic of Germany |
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Abstract: | Uvarovite (Ca3Cr2Si3O12) forms a complete solid solution series with andradite (Ca3Fe2+3Si3O12) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO3)+eskolaite (Cr2O3) at 1,385 ± 10 ° C. The incorporation of Ca3Fe2+3Si3O12 component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnetss (Ca3(Cr,Fe+32)Si3O12)+pseudowollastonite (CaSiO3)+hemeskolaitess(Cr,Fe+3O3). Pure andradite decomposes to pseudowollastonite (CaSiO3)+hematite (Fe2O3) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca3Cr2Si3O12 component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO3-Cr2O3-Fe2O3 ternary system. This minimum is located at about 64.5 wt.-% CaSiO3, 0.5 wt.-% Cr2O3, and 35.0 wt.-% Fe2O3. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr+3 by Fe+3 increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2). |
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