Acid pore waters generated within organic-rich muds destabilize carbonates and aluminosilicates and take metals, including aluminum, into solution. Sediment compaction forces some of these modified pore waters into sandstones. Once isolated from the principal source of acidity (kerogen maturation), the pH of these solutions must rise as further mineral dissolution occurs. The inevitable consequence is precipitation of kaolinite, even though solutions may still be acid. This explains why kaolinite is commonly the first precipitate to reduce secondary pore space in sandstones and can form even as other minerals are being dissolved away.--Journal abstract.

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