These are crystals of aragonite, one of the polymorphs of CaCO3. The chemistry of the ocean varies and is somewhat dependent on the amounts of hot new rock being created on the seafloor. During periods with a large amount of seafloor spreading the ratio of manganese to calcium is lower, and this promotes the precipitation of calcite. In periods with less seafloor spreading the magnesium/calcite ratio is higher and aragonite is favoured. We currently have aragonite oceans, but the oceans favoured calcite during the cretaceous. The Earth has switched between calcite and aragonite oceans many times in its history.
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