Chemical composition of pollucite + harmotome + "chabazite" nodule from Véžná (Czech Republic)

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Marco E. Ciriotti
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Chemical composition of pollucite + harmotome + "chabazite" nodule from Véžná (Czech Republic)

Messaggio da Marco E. Ciriotti » mer 24 ott, 2018 10:28

Referenza:
▪ Toman, J. & Novák, M. (2018): Textural Relations and Chemical Composition of Minerals from a Pollucite + Harmotome + Chabazite Nodule in the Véžná I Pegmatite, Czech Republic. Canadian Mineralogist, 56, 375-392.

Abstract:
Minerals and their textural relations from a pollucite + harmotome + chabazite nodule (PHC) enclosed in a volumetrically negligible albite-pollucite unit (1–2 dm3) in the Věžná I beryl-columbite pegmatite were examined by EMP and XRD. The albite-pollucite unit consists of a dominant albite-lepidolite-topaz subunit (Ab + Qz + Kf + Ms + Lpd + Tpz) as a matrix surrounding minor nodular subunits: the examined PHC and triplite-apatite nodules. Small vugs with crystals of harmotome and chabazite growing from the PHC nodule and polylithionite with sokolovaite rims, albite, fluor-elbaite, and K-feldspar extending from the host albite-lepidolite-topaz subunit are locally developed at the margin. Several textural/paragenetic types of the individual minerals in the PHC were distinguished from center to margin of the nodule and vugs. The minerals exhibit very complicated textural relations varying from evident replacement of early pollucite by analcime to evident replacement of pollucite–analcime by late zeolites (harmotome + chabazite). However, the pollucite types have almost identical compositions and underwent strong analcimization along the contact with the albite-lepidolite-topaz subunit. Harmotome evolves from K-rich harmotome I (harmotome > phillipsite-K) via Ca,K-moderate harmotome II to Ca,K-poor and Ba-rich harmotome III and chabazite evolves from Ca-rich chabazite I via slightly K-enriched chabazite II to moderately K-enriched chabazite III (chabazite-Ca > chabazite-K) from center to margin and vugs, respectively. Pollucite was replaced by the assemblage harmotome + chabazite according to the simplified equation pollucite + Ba + 2Ca + 25 H2O = harmotome + chabazite. The temperatures of the processes leading to the assemblage pollucite + analcime + harmotome + chabazite in the PHC were inferred from the experimental and empirical P-T stabilities of the minerals: primary pollucite T > ∼350 °C, analcimization T < ∼150 °C, harmotome + chabazite T < ∼100–150 °C.
Marco E. Ciriotti

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