Ferro-magnesian calcite occurs as cement in terrigenous sandstones as old as Ordovician from North America and the West Indies. Based on microprobe analyses, ferro-magnesian calcite cements have 1 to 10 wt. % FeCO 3 and 11 to 39 wt. % MgCO 3 . Cement of this nature previously has not been analytically documented. Because of the propensity of iron to substitute for magnesium in the calcite lattice, what appears to be high magnesian calcite, based on X-ray diffraction techniques, actually may be ferro-magnesian calcite in terms of chemical composition. Subsurface occurrences indicate that with depth, a transition may take place from low-magnesian calcite through ferro-magnesian calcite to dolomite.