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Coexistence of twisted and untwisted crystals: An impurity/structural order model with implications for agate patterns

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Published/Copyright: April 1, 2015
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American Mineralogist
From the journal American Mineralogist Volume 92 Issue 11-12

Abstract

Coexistence of twisted and untwisted crystals is explained via a model that accounts for the coupling of the entropic and energetic effects of impurities and a supra-lattice-scale structural order parameter. It is shown that twisted impure crystals can be in equilibrium with untwisted purer ones. The model explains how coexistence can occur in agates and other systems under hydrostatic stress. The model implies that untwisted crystals grown under one set of conditions could undergo a phase separation that, when accompanied by an imposed compositional gradient, leads to commonly observed, alternating bands of twisted and untwisted crystals and, when occurring in the absence of an external gradient, mossy patterns of crystal texture can emerge. This phenomenon is not related to anisotropic applied stress. Rather coexistence is a consequence of a compositional segregation/twist phase transition. Since twist coexistence is a compositional equilibrium, it arises from the exchange between bulk phases; hence, the detailed nature of the atomic structure within an interface between twisted and untwisted zones is not relevant. The approach places crystal-twist phenomena within the theory of order/disorder phase transitions.

Keywords: Agate; agate banding; free energy model; phase separation; quartz fibers; self-organization; twisted crystals; twisted/untwisted crystal equilibria
Received: 2006-12-28
Accepted: 2007-7-3
Published Online: 2015-4-1
Published in Print: 2007-11-1

© 2015 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am.2007.2547
Keywords for this article
Agate; agate banding; free energy model; phase separation; quartz fibers; self-organization; twisted crystals; twisted/untwisted crystal equilibria

Articles in the same Issue

  1. Evaluation of image classification routines for determining modal mineralogy of rocks from X-ray maps
  2. Atomic structure and formation mechanism of (301) rutile twins from Diamantina (Brazil)
  3. Significance of the depth-related transition montmorillonite-beidellite in the Bouillante geothermal field (Guadeloupe, Lesser Antilles)
  4. High-pressure infrared spectra of talc and lawsonite
  5. Proton dynamics in letovicite: Part I. Static 1H and 15N NMR MAS experiments and lineshape simulations
  6. Proton dynamics in letovicite: Part II. Static 2H NMR experiments and lineshape simulations
  7. Chemical and structural variations at augite (100) deformation twin boundaries
  8. Comparative compressibility and structural behavior of spinel MgAl2O4 at high pressures: The independency on the degree of cation order
  9. Multiple breakdown and chemical equilibrium of silicic clinopyroxene under extreme metamorphic conditions in the Kontum Massif, central Vietnam
  10. Direct EXAFS evidence for incorporation of As5+ in the tetrahedral site of natural andraditic garnet
  11. Deformation-controlled cation diffusion in tourmaline: A microanalytical study on trace elements and boron isotopes
  12. Scandium-45 NMR of pyrope-grossular garnets: Resolution of multiple scandium sites and comparison with X-ray diffraction and X-ray absorption spectroscopy
  13. Partitioning of trace elements among coexisting crystals, melt, and supercritical fluid during isobaric crystallization and melting
  14. Thermal diffusivity of aluminous spinels and magnetite at elevated temperature with implications for heat transport in Earth’s transition zone
  15. High-pressure structure and bonding in CaIrO3: The structure model of MgSiO3 post-perovskite investigated with time-of-flight neutron powder diffraction
  16. Application of major- and trace-element geochemistry to refine U-Pb zircon, and Sm/Nd or Lu/Hf sampling targets for geochronology of HP and UHP eclogites, Western Gneiss Region, Norway
  17. Measurement of U-Pb ages of uraninite and davidite by laser ablation-HR-ICP-MS
  18. A modeling approach to understanding the role of microstructure development on crystalsize distributions and on recovering crystal-size distributions from thin slices
  19. Neptunium substitution in synthetic uranophane and soddyite
  20. Coexistence of twisted and untwisted crystals: An impurity/structural order model with implications for agate patterns
  21. Intracrystalline boron isotope partitioning in illite-smectite: Testing the geothermometer
  22. Mineralogy of the hardpan formation processes in the interface between sulfide-rich sludge and fly ash: Applications for acid mine drainage mitigation
  23. Dissolution of radiation-damaged zircon in lateritic soils
  24. Hydrogen analysis in minerals by continuous-flow mass spectrometry
  25. Re-examination of berlinite (AlPO4) from the Cioclovina Cave, Romania
  26. Letter. Valence state partitioning of Cr between pyroxene-melt: Effects of pyroxene and melt composition and direct determination of Cr valence states by XANES. Application to Martian basalt QUE 94201 composition
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