The upper stability limit of the assemblage paragonite + quartz and its natural occurrences |
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Authors: | Niranjan D Chatterjee |
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Institution: | (1) Institute of Mineralogy, Ruhr University, Bochum, Germany |
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Abstract: | The assemblage paragonite + quartz is encountered frequently in low- to medium-grade metamorphic rocks. With rising grade of metamorphism they react mutually to yield the condensed assemblage albite + Al2SiO5.The univariant curve pertaining to the equilibrium paragonite + quartz=albite + andalusite + H2O has been located experimentally. The reversed P
H
2
O-T data are: 1 kb: 470–490° C 2 kb: 510–530° C 3 kb: 540–560° C 4 kb: 560–580° C 5 kb: 590–600° C The univariant curve pertaining to the equilibrium paragonite + quartz=albite + kyanite + H2O runs through the following P
H
2
O-T-intervals: 5 kb: 570–625° C 6 kb: 600–630° C 7 kb: 620–640° C Thermodynamic calculations of S
298
0
, H
f,298
0
and G
f,298
0
of the phase paragonite from the experimental data presented above and those obtained from the equilibria of the reaction paragonite=albite + corundum + H2O (Chatterjee, 1970), agree within the limits of uncertainty. This prompts the idea that Zen's (1969) suggestion of a possible error of approximately 7 kcal in G
f,298
0
of the Al2SiO5 polymorphs may in fact be due to an error of similar magnitude in G
f,298
0
of corundum.A best estimate of S
298
0
, H
f,298
0
and G
f,298
0
of paragonite based on these considerations yield: S
298
0
: 67.61±3.9 cal deg–1 gfw–1
H
f,298
0
: –1411.4±2.7 kcal gfw–1
G
f,298
0
: –1320.9±4.0 kcal gfw–1 These numbers will be subject to change when better thermochemical data on corundum and albite are available.In medium-grade metamorphic rocks the assemblage paragonite + quartz is commonly found in stable coexistence with such other phases as muscovite, staurolite, andalusite, kyanite, but not with cordierite or sillimanite. However, the assemblage paragonite-sillimanite has been reported to be stable in the absence of quartz. All these petrologic observations can be explained on the basis of the stability data of the phases and phase assemblages concerned. |
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Keywords: | |
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