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Micro-structural phenomena in agate/chalcedony: spiral growth

Published online by Cambridge University Press:  03 October 2018

Jens Götze ,
Harry Berek  and
Klaus Schäfer
Jens Götze*
Affiliation:
TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, 09599 Freiberg
Harry Berek
Affiliation:
TU Bergakademie Freiberg, Institute of ceramics, glass and construction materials, Agricolastraße 17, 09599 Freiberg
Klaus Schäfer
Affiliation:
Flurstraße 19, 55758 Vollmersbach
*
*Author for correspondence: Jens Götze, Email: jens.goetze@mineral.tu-freiberg.de
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Abstract

Agates with spectacular micro-structural features were found in volcanic rocks at several occurrences in the Saar-Nahe region (Germany). These agates include spirals of several tens up to several hundreds of μm in size within zones lacking the characteristic structural agate banding. A combined mineralogical study by polarising microscopy, scanning electron microscopy, cathodoluminescence microscopy and spectroscopy, and electron backscatter diffraction provided evidence that the spirals consist of well-ordered trigonal α-quartz, whereas the surrounding matrix is composed of strongly disordered or amorphous SiO2 phases. The quartz micro-crystals show a systematic rotation of the crystal orientation perpendicular to the direction of the spiral loops indicating helical growth.

It is assumed that the spiral growth is initiated by dislocations with a screw component. The lacking symmetry of the strongly disordered or amorphous matrix initiated a curved development by a screw dislocation in a system far from equilibrium. The atoms/molecules are packed into spiral layers, which is energetically favoured in comparison with the incorporation into plane crystal faces. Such self-organisation growth and polymerisation initiated by a screw dislocation can produce variable spiral morphologies sometimes resembling living forms.

Keywords

agatesilicaspiral growthpolarising microscopycathodoluminescenceelectron backscatter diffraction
Type
Article
Information
Mineralogical Magazine , Volume 83 , Issue 2 , April 2019 , pp. 281 - 291
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Martin Lee

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