Alteration of spodumene to cookeite and its pressure and temperature stability conditions in Li-bearing aplite-pegmatites from Northern Portugal

• Type: Article in International Scientific Journal
• Date: 2007

Authors

Iuliu Bobos (Author) FCUP; Philippe Vieillard (Author) Other; Bernard Charoy (Author) Other; Fernando Noronha (Author) FCUP

Journal

Title: Clays and Clay Minerals

Vol. 55

Pages: 295-310

ISSN: 0009-8604

Publisher: Springer Nature

Indexing

ISI Web of Knowledge - 23 Citations

Scopus - 25 Citations

Scientific classification

FOS: Agrarian Sciences > Agriculture, Forestry, and Fisheries

Other information

Authenticus ID: P-004-9JY

DOI: 10.1346/ccmn.2007.0550306

Abstract (EN): 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 +/- 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-layer polytypes. The thickness of one-layer cookeite packets averaged similar to 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: Al-2.0(Si3.23Al0.766)O-10(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 SUPCRT (R) 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 similar to 2.4 kbar and 240 degrees C. The cookeite + kaolinite +/- mica assemblage is still in equilibrium with quartz at similar to 2.2 kbar and 220 degrees C. The lower limit for the stability of cookeite was found at 205 degrees C and 2 kbar.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 16