At 25°C, synthetic schwertmannite, a common iron compound in acid mine drainage, completely converted in water to a sulphate-containing goethite within ~100 days. The rate of transformation doubled as the pH increased from 4.0 to 7.2. In deviation from its normal acicular habit, the goethite appeared as spherical aggregates.
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Schwertmann et al. (2005) studied this question.
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