Petrology of rapakivi granites in Salmi massif

Crystallization of rapakivi is accompanied by a progressive decrease in the alkali content of the high-temperature granitic magma and by the concurrent increase in its iron content, during the crystallization of potassium feldspar which is followed by the crystallization of biotite. The pressure is not high enough to permit a simultaneous crystallization of potassium feldspar, as e. g. of albite. The potassium feldspar-biotite sequence is upset by Mg, in the ovoid varieties of rapakivi in which the potassium feldspar crystallization is arrested, short of its completion, by the crystallization of Mg biotite. – IGR Staff.