Failure of the exsolution silica-pump model for the orgin of myrmekite: examination of K-feldspar crystals in the Sharpners Pond tonalite, Massachusetts, USA

Evidence from studies of K-feldspar and myrmekite in the same terrane as that examined by Castle and Lindsley (1993) shows that when these investigators formulated their "exsolution silica-pump model" for the origin of myrmekite, an inadequate analysis was made of the feldspars in transitions to adjacent non-myrmekite-bearing rocks. In their proposed model the myrmekite originates "in response to kinetic effects associated with the exsolution of calcic alkalic feldspar into discrete potassium feldspar and plagioclase phases." This hypothesis was an "outgrowth of the speculations of Tuttle (1952) and Tuttle and Bowen (1958) intended to explain many 'subsolvus' granites." Castle and Lindsley suggested that the diffusion rates of tetrahedral Al and Si through an exsolving ternary feldspar in the presence of excess silica result in quartz concentrating as vermicules in myrmekite where Ca and Al are coupled in plagioclase. Therefore, the "exsolution silica-pump model" depends upon the prior existence of a K-rich, high-temperature, alkali feldspar whose composition is "close to the crest of the solvus, where the potential for subsolvus exsolution is high." Such a composition would produce nearly equal percentages of K-feldspar and plagioclase.