Nucleation on perthite-perthite boundaries and exsolution mechanisms in alkali feldspars

A transmission electron microscope study of intracrystalline boundaries between two perthites of markedly different composition in composite crystals, one a tenary mesoperthite (Or₂₆Ab₅₂An₂₂, initially a homogeneous potassian monalbite) the other a more potassic cryptoperthite (Or₆₁Ab₃₃An₆, initially a homogeneous sodian sanidine), shows that the two perthites are in nearly parallel intergrowth. Most boundaries examined were of (hkO) type; (010) boundaries are straight, whereas other (hkO) boundaries are curved or stepped. Exsolution occurred first in the potassian monalbite (mesoperthite) and was unaffected by the boundary. Subsequent exsolution in the sodian sanidine (cryptoperthite) was affected by the boundary, but for up to only a few micrometers. Exsolution occurred by heterogeneous nucleation and growth of oligoclase on and from the intracrystalline boundary. At almost the same time the rest of the volume of sanidine exsolved by spinodal decomposition. 1–2 μm from the boundary in the intervening K-rich matrix of the sodian sanidine, further exsolution occurred by homogeneous nucleation. Time — temperature — transition curves for continuous cooling have been devised to account for the unusual complexity of the exsolution texture. Except in such exceptional circumstances as the example studied, the initial exsolution in high-temperature alkali feldspars of intermediate composition, unlike other minerals, probably does not occur by nucleation, but only by spinodal decomposition.