Radiometric isotype dating methods
On the upper left branch of this reaction series, olivine, the first mineral to form, Ml] react with the remaining melt to become pyroxene.
This reaction will continue until the last mineral in the series, biotite mica, is formed.
Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger.
This calls the whole radiometric dating scheme into serious question.
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.
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.
The following quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp.
55-57, (1987), gives us an idea of the tremendous complexity of the processes that occur when magma solidifies.