Chemical Diversity of Talcs in Relation to their Origin | Szegedi Tudományegyetem, Ásványtani, Geokémiai és Kőzettani Tanszék

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Chemical Diversity of Talcs in Relation to their Origin

Acta Mineralogica-Petrographica, Szeged 2004, Vol. 45/2, pp. 35-39

Wilamowski, A.
Wiewióra, A.

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 Fe3+ suggesting simultaneous substitution of Fe3+ both in tetrahedra and in octahedra according to the mechanism MgSi Fe3+Fe3+. This was 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 were achieved after recalculation of Fe oxidation state according to the above mentioned scheme. The highest Fe3+ contents demonstrate talcs from lowtmperature crystallization environments. The likely explanation is interaction with pore fluids.

Chemical compositions of octahedral sheet, plotted in the ternary diagram Mg-Al-Fe+Mn+Ni+Zn, demonstrate 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. Samples 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. The positive correlation of tetrahedral Al and Na (R2=0.71) suggests the substitution mechanism involving the tetrahedral sheet and interlayer space: NaAl 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.




acta2004, chemical composition, crystal-chemistry, talc




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