If a body of magma is subjected to fractional crystallization, the first rock that is expected to crystallize from the magma will be

The first rock expected to crystallize is Olivine. Olivine is a silicate mineral that is green in colour. Fractional crystallization is the process by which the crystals formed in a cooling magma are segregated from the remaining liquid.
The Bowen's reaction series provides a model to show that as magma temperature decreases olivine and other minerals crystallize in a particular order. As magma cools, minerals crystallize at different temperatures and settle out of the magma.

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