2. Given that most biochemical (other than coal) rocks react with hydrochloric acid, what does that tell you about organisms?

Reaction with HCL of biochemical rocks (carbonate rocks) tells us about their biologic origin thanks to their compositions.

Explanation:

While some carbonate rocks form as simple chemical precipitates most carbonate rocks are the product of marine organisms such as molluscs and corals. They precipitate calcite (calcium carbonate, CaCO3) or other similar carbonate minerals directly from the dissolved chemicals in the water to create their shells. Limestone is the product. At some later time (e.g.,. after burial) calcite may be transformed into dolomite, CaMg(CO3)2. Calcite will react vigorously with dilute hydrochloric acid (HCl). Powdered dolomite will react sluggishly with HCl.

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