Publication details
QUARTZ REPLACEMENT BY KEROLITE IN GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS FROM THE VĚŽNÁ I PEGMATITE, CZECH REPUBLIC: A COMPLEX DESILICIFICATION PROCESS RELATED TO EPISYENITIZATION
| Authors | |
|---|---|
| Year of publication | 2012 |
| Type | Article in Periodical |
| Magazine / Source | Canadian Mineralogist |
| MU Faculty or unit | |
| Citation | |
| Doi | http://dx.doi.org/10.3749/canmin.50.6.1609 |
| Field | Geology and mineralogy |
| Keywords | quartz kerolite replacement desilicification episyenitization contamination pegmatite serpentinite Věžná |
| Description | Quartz replacement by kerolite (variety of talc) was examined in coarse- to medium-grained graphic intergrowths of quartz + feldspars (oligoclase An13-15Or3/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 (Mg2.77-2.79Al0.09-010 Fe0.04 Si 4.05-4.08O10(OH)2.1.98H2O) forms aggregates, up to 2 cm in size, varying from massive fine-grained (1-10 m) to radial (needles up to ~1 mm long). Diffraction patterns exhibit a very broad peak at ~ 10 A (001) and a sharp peak at ~ 1.52 A (060). Kerolite does not swell at ethylenglycol atmosphere; cation exchange capacity (CEC) was 31+2.3 meq/100 g. Kerolite is a product of interaction between quartz and Mg-rich fluids originating from the host rock by the simplified reaction: 4SiO2 + 3MgCl2 + 6H2O = Mg3Si4O10(OH)2.2H2O + 6HCl. The process of kerolitization was evidently associated with albitization of oligoclase producing minor to trace amounts of Ba-rich K-feldspar, pectolite, analcime, natrolite, celadonite, monazite-(Ce) and niobian titanite, and possibly with vermiculitization of phlogopite. The simplified reaction including oligoclase, SiO2 released during kerolitization, and assuming immobility of Al was calculated as: 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 ~ 100-300 C and P lt; ~ 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. Similar processes such as kerolitization of quartz at Věžná occur not only in granitic pegmatites but may operate in episyenites, where a direct dissolution of quartz and consequential formation of open vugs typically occurs. |