Rhabdophane group and alunite supergroup minerals from Valence Mountains, Hungary

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Marco E. Ciriotti
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Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy

Rhabdophane group and alunite supergroup minerals from Valence Mountains, Hungary

Messaggio da Marco E. Ciriotti » mer 04 lug, 2018 9:59

Prossima pubblicazione.

▪ Ondrejka, M., Bačík, P., Sobocký, T., Uher, P., Škoda, R., Mikuš, T., Luptáková, J., Konečný, P. (2018): Minerals of the rhabdophane group and the alunite supergroup in microgranite: products of low-temperature alteration in a highly acidic environment from the Velence Mountains, Hungary. Mineralogical Magazine, 82, (in press).

An assemblage of alunite-supergroup minerals (ASM), rhabdophane-group minerals (RGM), goethite and associated clay minerals occur in Permian A-type porphyritic microgranite in the eastern part of the Velence Hills, Hungary. The secondary sulfates/phosphates include jarosite, plumbian jarosite and alunite, corkite, hinsdalite, rhabdophane-(Ce), -(La) and -(Nd) members. Detailed EPMA and Raman spectroscopy reveal a wide miscibility among RGM end-members representing an uncommon intermediate rhabdophane–tristramite–brockite solid solution of most extensive compositional variation recorded. Moreover, ASM show heterogeneous composition and complex substitution mechanisms among alunite, beudantite and plumbogummite groups. The formation of this rare mineral assemblage reveals extensive remobilization of REE, Th, U, P, S, Fe, and Pb under supergene conditions. Compositional variations and substitution trends of investigated RGM indicate that Th, U, Ca, and Pb are incorporated rhabdophane structure by (Ca, Pb)2+ + (Th, U)4+ ↔ 2REE3+ substitution mechanism. Consequently, we suggest following end-member formulae of divalent and tetravalent RGM members: (Ca0.5Th0.5)PO4·H2O for brockite, (Pb0.5Th0.5)PO4·H2O for grayite, and (Ca0.5U0.5)PO4·2H2O for tristramite. The ASM and RGM originated from total leaching of primary magmatic REE, Th, U and P minerals of the microgranite [most probably allanite-(Ce), fluorapatite, possibly also xenotime-(Y)], together with input of Pb and S in low-temperature, acid sulfate solutions, connected with adjacent Paleogene andesite-diorite intrusion and accompanying hydrothermal sulphide mineralisation.
Marco E. Ciriotti

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