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Micro-structural phenomena in agate/chalcedony; spiral growth

Jens Goetze, Harry Berek and Klaus Schaefer
Micro-structural phenomena in agate/chalcedony; spiral growth
Mineralogical Magazine (April 2019) 83 (2): 281-291

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 mu m 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 alpha -quartz, whereas the surrounding matrix is composed of strongly disordered or amorphous SiO (sub 2) 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.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 83
Serial Issue: 2
Title: Micro-structural phenomena in agate/chalcedony; spiral growth
Affiliation: Freiberg University of Mining and Technology, Institute of Mineralogy, Freiberg, Germany
Pages: 281-291
Published: 201904
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 30
Accession Number: 2019-065536
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch map
N49°07'00" - N49°37'60", E06°19'60" - E07°20'60"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201934
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