Growth and ordering processes in synthetic hydrothermal anorthite

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.