Chemical diversity of talcs in relation to their origin

Apart from main constituents, chemical compositions of talc show various elements, such as Fe, Al, Cr, Ni, Mn , alkalies and fluorine. Several analyses of talcs from Egypt (El-Sharkawy 2000) show the rectilinear negative correlation of Si and Fe 3+ suggesting simultaneous substitution of Fe 3 '...

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Bibliographic Details
Main Authors: Wilamowski Andrzej
Wiewióra Andrzej
Format: Article
Published: Department of Mineralogy, Geochemistry and Petrology, University of Szeged Szeged 2004
Series:Acta mineralogica-petrographica 45 No. 2
Kulcsszavak:Földtan, Ásványtan, Kőzettan
Subjects:
Online Access:http://acta.bibl.u-szeged.hu/25131
Description
Summary:Apart from main constituents, chemical compositions of talc show various elements, such as Fe, Al, Cr, Ni, Mn , alkalies and fluorine. Several analyses of talcs from Egypt (El-Sharkawy 2000) show the rectilinear negative correlation of Si and Fe 3+ suggesting simultaneous substitution of Fe 3 ' both in tetrahedra and in octahedra according to the mechanism MgSi<-»Fe 3 +Fe 3 This wa s veryfied in talks deficient in Si and rich in Fe. Essentially better calculated formulas and agreement of the layer charge with the theoretical value 22 wer e achieved after recalculation of Fe oxidation state according to the above mentioned scheme. The highest Fe 3 contents demonstrate talcs from lowtmperature crystallization environments. The likely explanation is interaction with por e fluids. Chemical compositions of octahedral sheet, plotted in the ternary diagram Mg-Al-Fe+Mn+Ni+Zn , demonstrat e a quite regular grouping of projection points into 3 fields: I. near-stoichiometric talcs around 100 % Mg, II. talcs with elevated Fe+Mn+Ni+Zn contents and III. Mg-Al talcs. Sample s representing all genetic types gather in the first field, in the second dominate talcs from high-pressure and ultramafic environments, and in the third - dolomite-related talcs. T he positive correlation of tetrahedral Al and Na (R2 =0.71) suggests the substitution mechanism involving the tetrahedral sheet and interlayer space: NaAI<->Si. Much weaker correlation shows octahedral Al (R2 =0.50). Sodium content shows positive correlation both with temperature and pressure of talc crystallization. It is likely that high PT-conditions favour incorporating Na into the talc structure.
Physical Description:35-39
ISSN:0365-8066