Low-Magnesian and High-Magnesian Calcite

Calcite (CaCO₃) can accommodate varying amounts of magnesium in its crystal structure because magnesium ions and calcium ions are similar in size and charge, allowing Mg²⁺ to substitute for Ca²⁺ in the rhombohedral calcite lattice. ^[1] This substitution creates two distinct varieties of calcite that are distinguished by how much MgCO₃ they contain.

The Two Varieties

Low-magnesian calcite (also called simply “calcite”) contains less than about 4 percent MgCO₃. ^[1]

High-magnesian calcite contains more than 4 percent MgCO₃. ^[1] Despite its elevated magnesium content, high-magnesian calcite still retains the crystal structure of calcite, with Mg ions randomly substituting for Ca ions in the crystal lattice rather than occupying ordered positions. ^[1] This random substitution is the critical distinction between high-magnesian calcite and true dolomite: in dolomite, Mg ions occupy exactly half the cation sites and are organized in well-ordered alternating planes rather than scattered randomly through the lattice.

Why Mg Does Not Substitute in Aragonite

Mg²⁺ ions commonly do not substitute for Ca²⁺ ions in the larger spaces available in the more open orthorhombic lattice of aragonite. ^[1] The orthorhombic aragonite structure has larger Ca sites that are too big for the smaller Mg ion to fit into comfortably — the ionic size match that makes Mg substitution work in the rhombohedral calcite structure does not apply in aragonite.

Biological Producers

Different groups of marine organisms preferentially secrete different forms of calcium carbonate. Echinoids, crinoids, bottom-dwelling (benthonic) forams, and coralline red algae are composed mainly of high-magnesian calcite, while planktonic (floating) forams, coccoliths, and brachiopods have low-magnesian calcite shells or tests. ^[1] Many molluscs (pelecypods, gastropods, pteropods, chitons, and cephalopods), calcareous green algae, stromatoporoids, scleractinian corals, and annelids build skeletons of aragonite rather than calcite. ^[1] The biological control on carbonate mineralogy means that a carbonate rock’s Mg content and mineral form are partly a record of which organisms lived in the depositional environment.

Diagenetic Fate

High-magnesian calcite is metastable under near-surface conditions and tends to lose its Mg during diagenesis, transforming toward low-magnesian calcite. Combined with the conversion of aragonite to calcite, this means that most ancient carbonate rocks ultimately converge on low-magnesian calcite as their dominant mineral, regardless of what mix of calcite and aragonite varieties was originally deposited.

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