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Characterization and genesis of horizontal banding in Brazilian agate: an X-ray diffraction, thermogravimetric and electron microprobe study

Published online by Cambridge University Press:  05 July 2018

T. Moxon ,
C. M. Petrone  and
S. J. B. Reed
T. Moxon*
Affiliation:
55 Common Lane, Auckley, Doncaster DN9 3HX, UK
C. M. Petrone
Affiliation:
Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
S. J. B. Reed
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
*
* E-mail: [email protected]
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Abstract

Characterization of Brazilian agates containing a lower horizontally banded section and an upper chamber with bands parallel to the walls shows that these agates formed much later than the 135 Ma Paraná basalt host rock. Age differentiation between the two types of banding shows that the horizontal bands formed between 43 to 63 Ma ago with a final infill of wall-lining bands between 7 and 27 Ma later. The horizontal bands have a higher Al3+ concentration and a greater crystallite size than the wall-lining layers; they have a lower mogánite content and defect-site water content. The formation of these agates appears to be the result of a three-stage process. After the separate formation of horizontally banded and wall-lining agate, a silica infill seals the gap between the agate and the cavity wall. The detection of cristobalite in some specimens indicates that genesis of both the horizontally banded and wall-lining deposits in the Brazilian samples proceeds along an amorphous silica → opal-CT → opal-C → chalcedony pathway.

Keywords

agatechalcedonymogánitecristobalitehorizontal bandsgenesis
Type
Research Article
Information
Mineralogical Magazine , Volume 77 , Issue 3 , April 2013 , pp. 227 - 248
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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