Micro-structural phenomena in agate/chalcedony: spiral growth. (3rd October 2018)

• Record Type: Journal Article
• Authors: Götze, Jens; Berek, Harry; Schäfer, Klaus
• Is Part Of: Mineralogical magazine. Volume 83:Number 2(2019)
• Page Start: 281
• Page End: 291
• Publication Date: 2018-10-03
• Subjects: agate, -- silica, -- spiral growth, -- polarising microscopy, -- cathodoluminescence, -- electron backscatter diffraction; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2018.156
• Languages: English
• ISSNs: 0026-461X

Abstract

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.