Stability relations of Mn–Fe–Al piemontite

Abstract

Comparison of the stability relations of Fe-piemontite, Pm,,Ps,,Cz, (Ca2A12Mnl).S Fe,,,sSi30 I 2( OH)), with similar experimentally determined results on P~,,CZ,,~ suggests that, at moderate fo,, 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 at Pffuid = 2 kbar in cold-seal apparatus using solid oxygen buffers was completed. The data indicated breakdown of Pm17Ps17Czhtr at 645" t 10" C along the Cu20-CuO buffer, 477" 2 10" C along the Cu-Cu20 buffer, and 365" 5 lo" 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 fo, (< Cu-Cu20). At very high fo,, however, {he presence of Mn+3 in a distorted octahedral site may permit I persistence of piemontite to higher temperatures than Mn-poor epidote minerals.The compositional range of natural FeMn-AI piemontites supports these results. Cation partitioning from piemontite-bearing parageneses also indicates crystallization at high fo2, 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 fo2-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, fo, is + anorthite + fluid the major control on Fe : Mn : Al in epidote minerals.

Hydrothermal syntheses of epidote and piemontites on the join Ca2Al2Fe3+Si3O12(OH)-Ca2Al2Mn3+Si3O12(OH) at relatively low pressures of 200-400 MPa

“…Because epidote minerals occur in shallow geothermal fields at depth of only a few hundred meters, such epidotes may have crystallized at pressures of less than 100 MPa and temperatures below 200 °C (Bird and Spieler, 2004). As noted above, several experimental studies of the stability of epidote minerals (Holdaway, 1966(Holdaway, , 1972Liou, 1973;Keskinen and Liou, 1987) have confirmed that epidote minerals are stable at low pressures of 100 200 MPa, which is consistent with the natural occurrences of epidote minerals. However, the experimental results of epidote synthesis are not necessarily compatible with those on epidote stability, because Cz Ps series epidotes have never been crystallized as single phases by syntheses at low pressures less than 400 MPa.…”

“…The role of Fe 3+ in epidote group minerals has been recognized from two aspects: (1) incorporation of Fe 3+ in epidote group minerals expands their stability range toward both high and low temperatures (Holdaway, 1966(Holdaway, , 1972Liou, 1973;Keskinen and Liou, 1987); (2) f O 2 values in excess of those defined by the QFM buffer curve causes considerable expansion of the stability range of epidotes (Holdaway, 1972;Liou, 1973). Fe 3+ in epidote group minerals affects on not only the stability temperatures but also stability pressures, because the stability fields of epidote, clinozoisite and zoisite are located in their order with increasing pressure (Holdaway, 1972) and, thus, epidote is stable at lower pressures than Fe 3+ free clinozoisite and zoisite.…”

“…Although Holdaway (1972) found that epidote is stable up to 694 °C at 300 MPa in oxidizing conditions and the lower temperature limit of epidote falls with decreased f O 2 , Cz Ps series epidote has been synthesized as a single phase only at higher pressures of 500 800 MPa (Schiffman and Liou, 1983), and 460 530 MPa (Giuli et al, 1999). Reversal experiments to investigate the stability of Fe piemontite on the Cz Ps Pm join were carried out by Keskinen and Liou (1987) …”

Blueschist-facies metamorphism of manganiferous cherts: A review of the alpine occurrences

“…The former are well-known to represent higher oxidation conditions than the latter (Smith and Albee, 1967;Kawachi et al, 1983). The experimental data of Keskinen and Liou (1979) and Anastasiou and Langer (1977) imply f(0 2 ) higher than 10~2 0 -10" 10 for piemontite with 33 mol% Pm. Thus, an even higher f(0 2 ) must be assumed for the stabilization of Pm-rich piemontites such as those occurring at St. Marcel.…”

“…The aims of the present investigations are therefore the following: a) to review the existing data on sedimentary and metamorphic cherts related to ophiolites in the Alpine belt, in order to establish their peculiar geochemical characteristics, the extent of the chemical transformation undergone during the subduction metamorphism, and to identify the elements suitable to act as geochemical tracers for other outcrops where no evidence of oceanic vs. continental derivation survived the blueschist-facies metamorphism; b) to review the mineralogy and phase petrology so as to help clarify the application of experimental studies in the synthetic Mn-Si-Ca-C-O-H system currently underway (Abs-Wurmbach and Abs-Wurmbach, 1980;Maresch and Mottana, 1976;Keskinen and Liou, 1979;Peters et al, 1973;. This will eventually widen the use of rocks easily recognizable in the field from the mere condition of structural markers to that of indicators of equilibration within the high-pressurefacies sequence.…”