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$A\xrightarrow { Calcination } CaO+MgO+2{ CO }_{ 2 }$
Find out A:
(a) ${ CaCO }_{ 3 }$
(b) ${ MgCO }_{ 3 }$
(c) ${ CaCO }_{ 3 }.{ MgCO }_{ 3 }$
(d) ${ CaSO }_{ 4 }.{ MgCO }_{ 3 }$

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Hint: The word calcination is derived from the Latin word calcinare which means to burn lime. The most common application of calcination is its use in the formation of cement by decomposing calcium carbonate into calcium oxide and carbon dioxide.

Complete answer:
Calcination is a process in which the concentrated ore is thermally treated to bring about its decomposition in the absence/limited supply of oxygen. The calcination products are generally called calcine. This process is done in order to remove moisture and/or carbon dioxide from the ore. The process is performed in reactors/furnaces often called as kilns and calciners. The temperatures of these furnaces could be between $550-1150^{ o }{ C }$.
Some of the examples of the calcination process include:
-To remove the carbon dioxide from the carbonate ores, they are calculated.
-To remove the water of crystallization from bauxite and gypsum, they are calcined.
-The volatile matter present in raw petroleum coke is decomposed using calcination.
-To bring about phase transformations from anatase to rutile calcination is done.
The calcination process is done at or above the thermal decomposition temperature such that the standard Gibbs free energy change for the reaction will be zero.

When limestone is calcined the following reaction takes place:

$CaC{ O }_{ 3 }(s)\xrightarrow { Calcination } CaO(s)+{ CO }_{ 2 }(g)$

This reaction takes place at 1121 K such that the standard Gibbs free energy is zero.
Now let us look into the reaction given in the question:

$A\xrightarrow { Calcination } CaO+MgO+2{ CO }_{ 2 }$

Here, A is calcined which gives rise to the calcium oxide, magnesium oxide and carbon dioxide. From the calcination reaction of limestone (calcium carbonate) given above, it is clear that when the metal carbonates are decomposed they produce their respective oxides along with carbon dioxide gas. Since both magnesium oxide and calcium oxide are being produced it implies that the ore is composed of both the carbonate of the magnesium as well as the carbonate of the calcium.
Therefore the reaction will be:

$CaC{ O }_{ 3 }.{ MgCO }_{ 3 }(s)\xrightarrow { Calcination } CaO(s)+MgO(s)+2{ CO }_{ 2 }(g)$

So, the correct answer is “Option C”.

Note: According to IUPAC, calcination refers to heating a substance at high temperatures in the presence of air or oxygen. When certain metals are calcined it sometimes leads to their oxidation. Example: tin and lead metals