Abstract

Quartz replacement by “kerolite” (a variant of talc) was examined in coarse- to medium-grained graphic intergrowths of quartz + feldspars (oligoclase An13–15Or3 or orthoclase Or95–99Ab5–1) from the desilicated granitic pegmatite hosted in serpentinized apoharzburgite, at the locality Věžná I, western Moravia, Czech Republic. Pale brownish to white “kerolite” (Mg2.77–2.79Al0.09–010Fe0.04Si4.05–4.08O10(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(Na0.85Ca0.10K 0.05)Al1.15Si2.85O8 + 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.

You do not currently have access to this article.