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TEM study of mineral transformations in fired carbonated clays: relevance to brick making

Published online by Cambridge University Press:  09 July 2018

B. Bauluz ,
M. J. Mayayo ,
A. Yuste ,
C. Fernandez-Nieto  and
J. M. Gonzalez Lopez
B. Bauluz*
Affiliation:
Cristalografía y Mineralogía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50.009 Zaragoza, Spain
M. J. Mayayo
Affiliation:
Cristalografía y Mineralogía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50.009 Zaragoza, Spain
A. Yuste
Affiliation:
Cristalografía y Mineralogía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50.009 Zaragoza, Spain
C. Fernandez-Nieto
Affiliation:
Cristalografía y Mineralogía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50.009 Zaragoza, Spain
J. M. Gonzalez Lopez
Affiliation:
Cristalografía y Mineralogía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50.009 Zaragoza, Spain
*
*E-mail: bauluz@unizar.es
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Abstract

This study uses transmission electron microscopy (TEM) and analytical electron microscopy (AEM) supported by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the mineralogical and textural changes produced in carbonated clays by firing. Sample bars were prepared using raw clays composed of quartz, illite and carbonates with minor amounts of smectite, chlorite, feldspars and Fe oxides. The raw samples were then fired at temperatures of between 800 and 1050°C. The XRD data show that increases in firing temperature result in dehydroxylation of clay minerals, carbonate decomposition and the formation of Ca-bearing silicates (e.g. gehlenite, wollastonite, pyroxenes and anorthite). The sizes of the Ca-silicate crystals make the use of the SEM inappropriate since they lie below the resolution threshold. However, TEM/AEM do provide the required textural and compositional characterization, revealing that there is a broad range of pyroxene compositions, some of which resemble fassaite, and that Ca/Mg ratios increase with temperature. The TEM also shows significant dehydroxylation and vitrification of the clay-rich matrix at T of ~800°C. Observed mineralogical and textural changes probably occurred in a system with a local disequilibrium much like small-scale, high-temperature metamorphic reactions (i.e. pyrometamorphism). The importance of these results is that they enable the selection of more appropriate raw clay composition and firing dynamics (temperature, firing duration and cooling rate) for both the brickmaking industry and brick conservation in the field of cultural heritage.

Keywords

AEMceramicsclaysmineral transformationpyroxenesTEMbrick makingcultural heritage
Type
Research Article
Information
Clay Minerals , Volume 39 , Issue 3 , September 2004 , pp. 333 - 344
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

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