Magnesian-iron carbonates

Fig. 93. Equilibrium of magnesian-iron minerals in rocks with excess silica. A. In silicate iron-formations (aqueous fluid. Cum + Px -t- OH- Q association). B. In carbonate iron-rich rocks (carbonic acid fluid. Car -(- Px -I- 01 -t- Q association). Figures indicate maximum iron content of orthopyroxene in association with quartz and olivine.

In addition to the quartz-magnetite iron formation, Dymek and Klein (1988) described magnesian iron formation, aluminous iron formation, graphitic iron formation, and carbonate rich iron formation. They pointed out that the composition of the iron formation as a whole is very similar to that of other Archean and Proterozoic iron formations that have been metamorphosed to the amphibolite facies. 

Theoretically this sequence should be observed in the deposition of purely carbonate sediments. In the actual conditions of nature, including deposition of the BIF, a substantial part of the Fe and Mg went into the composition of primary silicates of greenalite, minnesotaite, and iron chlorite type. Taking ferrous greenalite—FejSi205(0H)4)—and magnesian saponite — Mg3Si40,o(OH)2 — as the stable end members, one can find the interrelationships between carbonates and silicates. 

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