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Growth and ordering processes in synthetic hydrothermal anorthite

X-ray diffraction and transmission electron microscopy

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Abstract

Anorthite crystals were synthetized from gel under hydrothermal conditions (P=1000 bar, T=700° C) with run durations from 2 h to 110 days. The products were observed at room temperature by optical microscopy, X-ray powder diffraction and transmission electron microscopy (TEM). It has been shown that crystallization progresses as a function of time as follows: from t=2 h to t=7 days, the proportion of amorphous material decreases from 80 to 0 percent; the average crystal size grows from 1 to 20 μm; the number of crystals per unit volume decreases from 2.1011 to 5.108cm−3, indicating substantial recrystallization. The average A1, Si state of order increases with time and reaches a steady state after seven days. The crystals exhibit a core few micrometers in diameter and a rim whose width increases with time from 1 to 10 μm. The core, highly disordered in Al, Si, displays weak b reflexions and small antiphase domains (APDs), the rim, more ordered in Al, Si, differs from the core by sharp b reflexions and larger APDs extending radially. Both macroscopic results and microscopic observations show that crystals grow by two successive processes: at first, nucleation and growth from gel giving rise to the core, then recrystallization giving rise to the rim.

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Copreaux, J., Gandais, M. & Dujon, S.-. Growth and ordering processes in synthetic hydrothermal anorthite. Phys Chem Minerals 16, 545–550 (1989). https://doi.org/10.1007/BF00202209

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Keywords

  • Crystallization
  • Transmission Electron Microscopy
  • Geochemistry
  • Recrystallization
  • Optical Microscopy