QUARTZ REPLACEMENT BY “KEROLITE” IN GRAPHIC QUARTZ–FELDSPAR INTERGROWTHS FROM THE VĚŽNÁ I PEGMATITE, CZECH REPUBLIC: A COMPLEX DESILICATION PROCESS RELATED TO EPISYENITIZATION

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• journal_title：The Canadian Mineralogist
• Contributor：Marek Dosbaba ; Milan Novák
• Publisher：Mineralogical Association of Canada
• Date：2012-12-01
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.50.6.1609
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：50
• issue：6
• firstpage：1609
• section：Mineral Assemblages and Evolution

摘要

<p id="p-1">Quartz replacement by “kerolite” (a variant of talc) was examined in coarse- to medium-grained graphic intergrowths of quartz + feldspars (oligoclase An_13–15Or₃ or orthoclase Or_95–99Ab_5–1) from the desilicated granitic pegmatite hosted in serpentinized apoharzburgite, at the locality Věžná I, western Moravia, Czech Republic. Pale brownish to white “kerolite” (Mg_2.77–2.79Al_0.09–010Fe_0.04Si_4.05–4.08O₁₀(OH)₂•1.98H₂O) forms aggregates up to 2 cm in size, varying from massive fine-grained (1–10 μm) to radial (needles up to ~1 mm long). Diffraction patterns exhibit a very broad peak at ~10 Å (001) and a sharp peak at ~1.52 Å (060). “Kerolite” does not swell at an ethylene glycol atmosphere; its cation-exchange capacity (CEC) is 31±2.3 meq/100 g. “Kerolite” is a product of an interaction between quartz and Mg-rich fluids originating from the host rock according to the simplified reaction: 4SiO₂ + 3MgCl₂ + 6H₂O = Mg₃Si₄O₁₀(OH)₂•2H₂O + 6HCl. The process of “kerolite” formation was evidently associated with the conversion of oligoclase to albite, producing minor to trace amounts of Ba-rich K-feldspar, pectolite, analcime, natrolite, celadonite, monazite-(Ce) and niobian titanite, possibly with the transformation of phlogopite to vermiculite. The simplified reaction, including oligoclase and SiO₂ released during “kerolite” formation, with an assumption of Al immobility, gives 20(Na_0.85Ca_0.10K _0.05)Al_1.15Si_2.85O₈ + 14SiO₂ + 4.5H₂O + 6NaCl = 21NaAlSi₃O₈ + 1KAlSi₃O₈ + 1NaCa₂Si₃O₈(OH) + 1NaAlSi₂O₆•H₂O + 6HCl. This process likely proceeded at T in the range 100–300°C and P below ~0.5–1 kbar, under high activity of alkalis and low tectonic stress. Pseudomorphs of “kerolite”, locally with rare relics of quartz, were later almost completely removed because of surface weathering, which produced empty cavities in the graphic unit. The process of transformation of quartz to “kerolite” at Věžná occurs not only in granitic pegmatites but may operate in episyenites, where a direct dissolution of quartz leading to the formation of open vugs typically occurs. p>