Vein filling low magnesium calcite in septarian veins exhibits fibrous, bladed and equant fabrics, all of which occur also in limestone cements. These fabrics are associated with particular crystal habits (fibrous-combined prism and rhombohedron; bladed-steep rhombohedra; equant-flat rhombohedra) and magnesium content (fibrous and bladed approximately 2.0 mole percent MgCO 3 ; equant generally less than 0.4 mole % MgCO 3 ). These relationships indicate that the amount of magnesium in aqueous solution controls the crystal habit of crystallizing calcite which in turn affects the final fabric. The sequence of fibrous or bladed calcite followed by equant calcite suggests crystallization in waters where magnesium content decreased with time (changing diagenetic environment in marine lutites with burial). The early formed fibrous calcite may have crystallized as aragonite or as high magnesium calcite (micron-sized or fibrous cement), but was later modified through neomorphism. Concurrent increase in ferrous iron content (from early to late formed calcite) indicates parallel changes in those chemical factors (iron content, Eh?) that control the incorporation of iron into the calcite lattice.