Early diagenetic siderites from marine and fresh-water depositional environments are characterized by distinctive compositional trends. Siderite from fresh-water environments is often relatively pure (i.e., greater than 90 mol% FeCO3) and commonly attains end-member composition. Siderite from marine environments, however, is always extremely impure and has extensive substitution of Mg (up to 41 mol%) and, to a lesser extent, Ca (up to 15 mol%) for Fe in the siderite lattice. In addition, marine siderite generally contains less Mn and has a higher Mg/Ca ratio than fresh-water siderite.
This compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters, inasmuch as early marine pore waters generally have a higher Mg2+/Ca2+ ratio and contain less Mn2+ and Fe2+ and more Ca2+ and Mg2+ than meteoric waters.