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Stability relations of Mn–Fe–Al piemontite
Author(s) -
KESKINEN MARY,
LIOU J.G.
Publication year - 1987
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1987.tb00398.x
Subject(s) - epidote , magnetite , anorthite , mineral redox buffer , electron microprobe , hematite , analytical chemistry (journal) , mineralogy , allanite , octahedron , thermal stability , crystallization , geology , quartz , oxygen , crystallography , materials science , chemistry , chlorite , crystal structure , metallurgy , organic chemistry , chromatography , biotite
Comparison of the stability relations of Fe‐piemontite, Pm 17 Ps 17 Cz 66 (Ca 2 Al 2 Mn 0.5 Fe 0.5 Si 3 O 12 (OH)), with similar experimentally determined results on Pm 33 Cz 67 suggests that, at moderate f o 2 , addition of Fe to piemontite raises its thermal stability and slightly decreases its sensitivity to oxygen fugacity. Reversal study of the reaction: Fe‐piemontite + quartz = garnet (Gr 33 Sp 27 And 40 ) + anorthite + fluid at P fluid = 2 kbar in cold‐seal apparatus using solid oxygen buffers was completed. The data indicated breakdown of Pm 17 Ps 17 Cz 66 at 645°± 10° C along the Cu 2 O–CuO buffer, 477°± 10° C along the Cu–Cu 2 O buffer, and 365°± 10° C for the hematite‐magnetite buffer. Mn‐free clinozoisite and epidote have been shown to be stable to much higher temperatures than piemontite at moderate f o 2 (< Cu–Cu 2 O). At very high f o 2 , however, the presence of Mn +3 in a distorted octahedral site may permit persistence of piemontite to higher temperatures than Mn‐poor epidote minerals. The compositional range of natural Fe–Mn–Al piemontites supports these results. Cation partitioning from piemontite‐bearing parageneses also indicates crystallization at high f o 2 , but application of the cited results should be made with caution. Synthetic phases were too fine‐grained for optical or microprobe analysis, and reaction was often incomplete. The assumption that all Fe + Mn was contained in piemontite and garnet, respectively, in the low‐ and high‐temperature assemblages is supported by the cell dimensions for these synthetic minerals, which are close to predicted values. The reaction is pseudo‐univariant, as solid solution in garnet and piemontite would be f o 2 ‐dependent in a more complex chemical system. Although bulk rock and fluid compositions may also have a marked effect on the stability of natural epidote–piemontite, f o 2 is the major control on Fe : Mn : Al in epidote minerals.