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This left branch is called a discontinuous reaction series because each mineral has a different crystalline structure.Recall that olivine is composed of a single tetrahedra and that the other minerals in this sequence are composed of single chains, double chains, and sheet structures, respectively.Oftentimes the rate of cooling occurs rapidly enough to prohibit the complete transformation of calcium-rich feldspar into sodium-rich feldspar.In these instances, the feldspar crystals will have calcium-rich interiors surrounded by zones that are progressively richer in sodium.As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.

However, geologists have found that various eruptive stages of the same volcano often extrude lavas exhibiting somewhat different mineral compositions, particularly if an extensive period of time separated the eruptions.Evidence of this type led them to look into the possibility that a single magma might produce rocks of varying mineral content. Bowen discovered that as magma cools in the laboratory, certain minerals crystallize first.A pioneering investigation into the crystallization of magma was carried out by N. At successively lower temperature, other minerals begin to crystallize as shown in Figure 3.6.But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium.Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization.

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