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Subsolidus Phase Relations in the System MgO–SiO 2 –Cr–O and Thermodynamic Properties of Related Cr 2+ –Containing End–members
Author(s) -
Jianping Li,
Neill H, St, C O',
Seifert F.
Publication year - 1996
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/j.1755-6724.1996.mp9004008.x
Subject(s) - pyroxene , spinel , olivine , ternary operation , phase (matter) , solid solution , analytical chemistry (journal) , mineralogy , metal , materials science , chemistry , metallurgy , organic chemistry , chromatography , computer science , programming language
Subsolidus phase relations have been determined in the systems SiO 2 –Cr–O and MgO–SiO 2 –Cr–O in equilibrium with metallic Cr, at 1100 to 1500°C and 0 to 2.88 GPa. The results show that there are no ternary phases in the SiO 2 –Cr–O system at these conditions, i.e., only the assemblage eskolaite–Cr–metal–quartz (or tridymite) is found. In the MgO–containing system, however, extensive substitution of Cr 2+ for Mg is observed in (Mg, Cr 2+ ) 2 SiO 4 olivine, (Mg, Cr 2+ ) 2 Si 2 O 6 pyroxene, and (Mg, Cr 2+ )Cr 2 O 4 spinel. Cr 3+ levels in olivine and pyroxene are below detection limits. The pyroxene is orthohombic at X Px Cr2+ <0.2, monoclinic at higher X Px Cr2+ . The structure of the spinels becomes tetragonally distorted at X Px Cr2+ >0.2. The experimental data have been fitted to a thermodynamic model, and the authors obtained the mixing parameter (W) of Mg‐Cr 2+ in olivine, pyroxene and spinel, and the relation between temperatures and free energies of formation for the end–members: Cr 2+ –olivine (Cr 2 SiO 4 ), Cr 2+ –pyroxene (Cr 2 Si 2 O 6 ) and Cr 2+ –spinel (Cr 3 O 4 ).