Exsolution during metamorphism with particular reference to feldspar solid solutions

SUMMARY. In the aureole of the syn-orogenic Hasvik Gabbro a variety of exsolution phenomena developed during the contact metamorphism. These exsolution phenomena have similar textures to those produced uring the age-hardening of metal alloys. In feldspars the exsolution precipitates are closely related to the pattern of excesses over and deficiencies from the formula requirements, par-ticularly in terms of SiO ~ and A12Oa. The exsolution products are dependent upon both the tempera-tures of ageing and the details of the mineral chemistry. Evidence supporting an exsolution theory of origin for myrmekite is provided, and it is shown that precipitation from heterotype f ldspar solid solutions during metamorphism may in fact be a complex process involving a number of precipitate phases. The nucleation of sillimanite as a precipitate phase is discussed and it is shown how sillimanite develops at a number of distinctive sites within feldspar and garnet. From the textural relations of sillimanite precipitates and their host minerals a control exerted by the thermal stress configuration over the nucleation of precipitate phases is suggested. It is argued that the thermal stresses exerted across grain boundaries and within minerals during metamorphism may have considerable r levance to the interpretation ftextures in metamorphic rocks