The reasons for the growth of the initial temperature value of the sideroplesite decomposition compared with the this temperature value for the siderite have been established. It has been shown that the sideroplesites like carbonates are the ionic crystals to a known approximation and are solid ionic solutions of the iron, magnesium, and manganese cations have been presented. With the use of the developed theory of the regular ionic solutions, the equation connecting the iron carbonate activity with its activity coefficient has been obtained. It has been shown that the calculation data makes it possible to determine the real fuel consumption for firing the siderite ore in the thermal plants and to optimize the operating conditions of its heat treatment.