Between aragonite and calcite, which is more stable at Earth's surface due to diagenetic transformation?

• A. Aragonite
• B. Calcite
• C. Dolomite
• D. Quartz

Answer: (B)

Calcite is the more stable form of calcium carbonate at Earth’s surface. Aragonite is metastable under near-surface conditions and tends to transform into calcite during diagenesis, the process by which sediments recrystallize and settle into rock at low temperatures and pressures. This transformation is driven by thermodynamics: calcite represents a lower-energy, more stable arrangement for CaCO3 under typical surface waters, so aragonite dissolves and reprecipitates as calcite, making calcite the prevalent and more stable mineral in diagenetically altered rocks. Dolomite and quartz aren’t the primary species involved in this specific near-surface aragonite-to-calcite stability path.