Radiaxial calcite is a common and distinctive pore fill in ancient limestones, but the origin of this fabric is highly controversial. We present data from the Late Cretaceous equatorial Pacific that suggest that radiaxial calcite forms through alteration of early magnesian calcite (≥7 mol% MgCO3) marine cement. These data suggest that radiaxial calcites are not straightforward indicators of pore-fluid chemistry. On the other hand, the presence of radiaxial calcite in limestones may provide a distinctive petrographic proxy for earlier magnesian calcite cement and thus a generally warm CaCO3-saturated marine environment.

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