Segregation of Elements During the Crystallization of a Magma

The manner in which ions segregate during the long period of cooling and crystallization of a magma in the earth's crust, whereby rock bodies varying widely in composition are formed, is discussed from the standpoint of composition of liquid, temperature, and the properties of the ions. The initial magma composition taken is that of a basalt in which the quantitatively important cations are Si 4+ , AP, Ca 2+ , Mg Z+ , Fe z+ , Fe 3+ , Na + , and K + , and in which almost all of the remaining elements of the periodic table occur in minor amounts. The distribution of representative cations among early, intermediate, or late crystals formed from the liquid is considered from the standpoint of ( e ) ionic field strengths, ( b ) the noble‐gas or non‐noble‐gas character of the ions, and (c) the types of silicate or aluminosilicate frameworks which form as the temperature and compo1sition of the liquid change. An ion of low concentration in the initial magma may appear largely in early crystal structures (Sc 3+ ), throughout the series of structures (Ge 4+ , Sr 2+ ), or only in the late structures (Li + , U 4+ ).