Clays and Clay Minerals, Vol. 55, No. 3, 295–310, 2007.
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Authors
Bobos, Iuliu
Vieillard, Philippe
Charoy, Bernard
Noronha, Fernando
Issue Date
2007
Type
Article
Language
Keywords
Periodicals , Geology , Cookeite , Crystal Chemistry , Electron Microscopy , Kaolinite , Polytype , Spodumene Breakdown , Thermodynamic Stability
Alternative Title
Alteration Of Spodumene To Cookeite And Its Pressure And Temperature Stability Conditions In Li-bearing Aplite-pegmatites From Northern Portugal
Abstract
The alteration of spodumene to cookeite has been identified in a Li-bearing aplite-pegmatite from northern Portugal. Optical microscopy and scanning and transmission electron microscopy (SEM/ TEM) were used to characterize the alteration products in both cookeite + quartz and cookeite + kaolinite F mica assemblages. Mutual relationships between the minerals were assessed using back-scattered electron imaging. The first assemblage occurs along the cleavage planes of spodumene, whereas the second forms as the result of spodumene breakdown. Fine mica grains surrounded the cookeite aggregates in the second assemblage only. Precipitations of cookeite and quartz in open pore spaces and parallel packets of cookeite and kaolinite were identified by TEM. Selected area electron diffraction carried out on cookeite identified a layer-stacking sequence from highly disordered to one-layer order-disorder with a lesser participation of two-layerpoly types. The thickness of one-layercookeite packets averaged ~85-100 nm. Disordered cookeite has a mean thickness of 450 nm. The chemistry of cookeite was analyzed by both electron and ionmicroprobe techniques. The structural formula of cookeite from the first assemblage corresponds to: Al2.0(Si3.23Al0.766)O10(OH)2(Al1.80Li1.166Mg0.004Fe0.09)(OH)6. Two distinct compositional varieties were found: one corresponding to ideal cookeite, whereas the second belongs to the cookeite–donbassite join. The thermodynamic stability field for the observed minerals was calculated using the SUPCRT1 programme. Two distinct stages of cookeite crystallization are discussed. The lower pressure and temperature stability of the reaction of spodumene to cookeite + quartz were estimated at ~2.4 kbar and 240ºC. The cookeite + kaolinite F mica assemblage is still in equilibrium with quartz at ~2.2 kbar and 220ºC. The lowerlimit for the stability of cookeite was found at 205ºC and 2 kbar.
Description
Clays and Clay Minerals, Vol. 55, No. 3, 295–310, 2007. Alteration Of Spodumene To Cookeite And Its Pressure And Temperature Stability Conditions In Li-bearing Aplite-pegmatites From Northern Portugal. Iuliu Bobos; Philippe Vieillard; Bernard Charoy; Fernando Noronha. DOI: 10.1346/CCMN.2007.0550306. Copyright © 2007, The Clay Minerals Society
Citation
Clays and Clay Minerals, Vol. 55, No. 3, 295–310, 2007.
Publisher
The Clay Minerals Society
License
Copyright © 2006-2018, The Clay Minerals Society