Micro-structural phenomena in agate/chalcedony: spiral growth

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Marco E. Ciriotti
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Micro-structural phenomena in agate/chalcedony: spiral growth

Messaggio da Marco E. Ciriotti » mer 03 ott, 2018 11:34

Prossima pubblicazione.

▪ Götze, J., Berek, H., Schäfer, K. (2018): Micro-structural phenomena in agate/chalcedony: spiral growth. Mineralogical Magazine, 82, (in press).

Agates with spectacular micro-structural features were found in volcanic rocks of 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 polarizing microscopy, scanning electron microscopy (SEM), cathodoluminescence (CL) microscopy and spectroscopy as well as electron backscatter diffraction (EBSD) provided evidence that the spirals consist of well-ordered trigonal α-quartz, whereas the surrounding matrix is composed of strongly disordered 18 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 kind of self-organization growth and polymerization initiated by a screw dislocation can produce variable spiral morphologies sometimes resembling living forms.
Marco E. Ciriotti

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