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

Moxon, T. and Petrone, C. M. and Reed, S. J. B. (2013) Characterization and genesis of horizontal banding in Brazilian agate: an X-ray diffraction, thermogravimetric and electron microprobe study. Mineralogical Magazine, 77 (3). pp. 227-248. ISSN 0026-461X, Online ISSN: 1471-8022 DOI 10.1180/minmag.2013.077.3.02

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Official URL: http://minmag.geoscienceworld.org/content/77/3/227

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.

Item Type: Article
Uncontrolled Keywords: 2013AREP; IA65;
Subjects: 99 - Other
03 - Mineral Sciences
Divisions: 03 - Mineral Sciences
Journal or Publication Title: Mineralogical Magazine
Volume: 77
Page Range: pp. 227-248
Identification Number: 10.1180/minmag.2013.077.3.02
Depositing User: Sarah Humbert
Date Deposited: 08 Jun 2013 12:39
Last Modified: 23 Jul 2013 10:06
URI: http://eprints.esc.cam.ac.uk/id/eprint/2802

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