The origin of myrmekitic intergrowths and a comparison with rod-eutectics in metals

Abstract

SummaryA review is given of the metallurgical literature relating to the formation of metal rodeutectics, which are texturally similar to myrmekite. Since no satisfactory explanation for rodeutectics has been advanced, a comparison is made with myrmekite. It is proposed that this intergrowth develops as a result of the constriction of quartz during its recrystallization and inclusion in an expanding blastic growth of plagioclase, and the forces involved lead to the prolongation along the growth directions of quartz in the form of rods with a relatively small surface area (i.e. with circular cross-sections). Possible forces that could produce the same geometry in metal intergrowths are those that result from the relative contractions of the two components during freezing; in all cases for which quantitative data are known, the metallic rods have a greater contraction on freezing than the host substance. Examples of myrmekite and myrmekite-like intergrowths in the Constant Gneiss are described in relation to their particular origin. Two factors, the lack of proportionality of qtartz to feldspar and the intimate association of myrmekite with myrmekite-like intergrowths, support the proposed mechanism of constriction of pre-existing quartz.