Though calcite and aragonite form together and in similar environments, ancient limestones generally are all calcite. Why is this?

• A. Aragonite forms first, is less stable, and transforms with time to calcite
• B. Calcite forms first and remains calcite
• C. Quartz replaces carbonate in old rocks
• D. Dolomite forms first and dominates the rock

Answer: (A)

In carbonate rocks, the mineral that ends up dominating the ancient limestone is the more stable form of calcium carbonate under Earth's surface conditions. Aragonite is a metastable form that can precipitate in modern seawater, but as soon as the sediment is buried and diagenetic fluids move through it, aragonite tends to dissolve and re-precipitate as calcite. Calcite is thermodynamically more stable at typical temperatures and pressures, so over geologic time the aragonite minerals transform into calcite. This diagenetic re-crystallization is why ancient limestones are largely composed of calcite, even though aragonite can form in similar environments today. The other options don’t reflect this common diagenetic transformation to the more stable calcite.