Minerals siderite, FeCO

The major cations found in the carbonate minerals in coals are calcium, magnesium, and iron. The rather pure end member calcite (CaC03) is dominant in some coals, whereas siderite (FeCO ) is dominant in others. Calcite and ankerite (a mixed crystal composed of Ca, Mg, and Fe carbonates) are abundant in some coals. 

Although the Ca-Mg carbonates constitute the most frequently encountered carbonate minerals, other metal carbonates can also be important. Of these, the most abundant are siderite (FeCO ) and rhodocrosite (MnC03). The divalent metal carbonates typically occur as. solid. solutions and so contain trace to major amounts of one or more secondary metal cations (cf. Reeder 1983). Solubilities of some binary metal-carbonate solid solutions are known (cf. Glynn 1990). 

FeCOs which occurs naturally as the mineral siderite has a rhombohedral structure. The octahedron of oxygens around the iron is trigonally distorted along the c axis which is the magnetic axis below the Neel temperature. The ground state is a doublet and the high symmetry eases interpretation of the data. 

Minerals containing the carbonate ion are plentiful. The principal carbonate minerals are caldte (CaCOs), magnesite (MgCOs), dolomite [MgCa(C03)2], and siderite (FeCOs). Calcite is the principal mineral in limestone rock, large deposits of which occur in many parts of the world. It is also the main constituent of marble, chalk, pearls, coral reefs, and the shells of marine animals such as clams and oysters. Although CaCOs has low solubility in pure water, it dissolves readily in acidic solutions with evolution of CO2. 

The most important feature of mineral chemistry in carbonates of Chehelkureh deposit is the large range in the FeCOsiMgCOs ratio which can indicate large variations in chemistry of hydrothermal fluids. Compositions extend along the magnesite-siderite join, from 22 to 72 mole% FeCOs (Fig. 3). Dolomite and ankerite compositions extend between 0 to 27 mole% FeCOs. 

Carbonate cement content in the studied rocks varies from 0 to 45 vol. %, mostly forming the pore filling. The following carbonates were observed (Koztowska 2004) siderite, Fe-dolomite, ankerite and Fe-calcite. The term siderite corresponds to minerals from the isomorphic group FeCOs-MgCOs with 60-100 mol percent FeCOs. Most siderites fall into the interval siderite - sideroplesite (Fig. 3). 

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