saussuritization

saussuritization, process by which calcium-bearing plagioclase feldspar is altered to a characteristic assemblage of minerals called saussurite; the typical assemblage formed includes zoisite, chlorite, amphibole, and carbonates. Residual fluids present during the late stages of magmatic crystallization can react with previously formed plagioclase feldspar to form saussurite; the saussurite will be spread through the plagioclase or located near its outer margin. The plagioclase may be reconstituted into a more sodium-rich variety (albite), although the original form of the crystal is retained. Later hydrothermal alteration can produce the same result. Mafic rocks are especially susceptible to saussuritization owing to their high calcium content; the more calcium-rich portions of plagioclase in acidic rocks also are often saussuritized.