Back to Search
Start Over
Micro-structural phenomena in agate/chalcedony: spiral growth.
- Source :
-
Mineralogical Magazine . Apr2019, Vol. 83 Issue 2, p281-291. 11p. - Publication Year :
- 2019
-
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. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 0026461X
- Volume :
- 83
- Issue :
- 2
- Database :
- Academic Search Index
- Journal :
- Mineralogical Magazine
- Publication Type :
- Academic Journal
- Accession number :
- 136612194
- Full Text :
- https://doi.org/10.1180/mgm.2018.156