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A late-stage hydrothermal phosphate-bearing montmorillonite argillitization from the tourmaline-bearing pegmatite of Alto dos Quintos Mine, northeast Brazil

Published online by Cambridge University Press:  09 July 2018

H. G. Dill ,
S. Kaufhold  and
R. Dohrmann
H. G. Dill*
Affiliation:
Bundesanstalt für Geowissenschaften und Rohstoffe, P.O. Box 51 01 53, D-30631 Hannover, Germany
S. Kaufhold
Affiliation:
Bundesanstalt für Geowissenschaften und Rohstoffe, P.O. Box 51 01 53, D-30631 Hannover, Germany
R. Dohrmann
Affiliation:
Bundesanstalt für Geowissenschaften und Rohstoffe, P.O. Box 51 01 53, D-30631 Hannover, Germany Landesamt für Bergbau, Energie und Geologie, P.O. Box 51 01 53, D-30631 Hannover, Germany
*
*E-mail:dill@bgr.de
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Abstract

The Alto dos Quintos Mine, Rio Grande do Norte, northeast region of Brazil, is one of the few mines where a LCT (Li-Cs-Ta-enriched) pegmatite is exploited for the deep blue gemstone called “Paraiba Tourmaline (PT)”. Blue cuprian elbaite, a fault-bound pink montmorillonite bearing some relict apatite and newly-formed Ca-Al-phosphate of the Al-P-sulphate group (APS minerals) also occur in the underground mine. The montmorillonite was studied using extended clay mineralogical tools (XRD, IR spectroscopy, XRF, SEM-EDX, cation exchange capacity (CEC), DTA). The structural formula method for calculation of the smectite formula based on EDX data yielded an extremely Fe-poor montmorillonite: (Ca0.22+)(Al1.7Fe0.0Mg0.4)(Si3.8Al0.2). The charge on the tetrahedral sheets accounts for approximately 30% of the total permanent negative charge. However, based on the more precise Hofmann & Klemen test, tetrahedral charge values of 11% to 13% were calculated. This is indicative of the dominance of montmorillonite rather than beidellite among the smectite minerals, which is independently proved by a pronounced IR band at 630 cm-1. The formation of pink montmorillonite is not directly related to the emplacement of the Li-bearing PT pegmatite. The sheet silicate developed after a considerable hiatus (Mesozoic?), when the Neoproterozoic pegmatites had already undergone different stages of hypogene (cookeite) and supergene (illite, kaolinite) alteration, during which Li was flushed out to a degree that formation conditions of expandable Li-bearing phyllosilicates were no longer favoured. The nature of the pinkish tint is not yet clarified. The montmorillonite formed in a temperature regime close to 250°C, when apatite was transformed into Ca-bearing APS minerals under acidic conditions. Cainozoic volcanic activity and the reactivation of deep-seated fault zones are considered to be responsible for the formation of this pink P-bearing montmorillonitic clay.

Keywords

montmorilloniteIR spectroscopyphosphateParaiba tourmaline pegmatiteCainozoichydrothermalBrazil
Type
Research Article
Information
Clay Minerals , Volume 46 , Issue 3 , September 2011 , pp. 473 - 485
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2011

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