Original paper

Experimental study of the thermal stability of pyrophyllite, paragonite, and clays in a thermal gradient

Vidal, Olivier

European Journal of Mineralogy Volume 9 Number 1 (1996), p. 123 - 140

35 references

published: Dec 30, 1997
manuscript accepted: Sep 17, 1996
manuscript received: Jan 11, 1996

DOI: 10.1127/ejm/9/1/0123

BibTeX file

ArtNo. ESP147050901012, Price: 29.00 €

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Abstract

Abstract Tube-in-tube experiments involving a strong thermal gradient have been conducted in the system Na2O- Al203-Si02-H20, using a starting mixture made of quartz + paragonite + pyrophyllite. Whatever the starting proportions pyrophyllite:paragonite:quartz or the location of the starting assemblage in the thermal gradient, new clay minerals crystallized along the thermal profile. The newly formed phases were beidellite ± quartz at the cold extremity and, depending on the paragonite to pyrophyllite to quartz starting proportions, mica or pyrophyllite ± diaspore crystallized at the hot extremity of the tube. Rectorite was observed to crystallize at an intermediate position (intermediate temperature). SEM observations and microdiffraction data indicate that Na-beidellite broke down into rectorite at about 360°C, and rectorite broke down into pyrophyllite + paragonite at about 420°C. These results are consistent with synthesis experiments, using a gel of beidellite composition as starting material. On the other hand, attempts to reverse these reactions by isothermal, isobaric experiments failed or gave contradictory results. Although dealing with larger experimental uncertainties, tube-in-tube experiments seem to be well-adapted to investigate the phase relations and the temperature-composition stability relations of minerals which are difficult to obtain by reversing experiments. The experimental results are consistent with thermodynamic calculations and indicate that local solid-solution heterogeneous equilibria control the composition of solids that crystallized as well as their distribution along the thermal profile

Keywords

beidelliterectoritethermal gradienttube-in-tube experimentssystem NASHexperimental petrologyclay minerals