Mineral self-organization during the orthoclase-microcline transformation in a granite pegmatite

The orthoclase-microcline transformation has been studied in perthitic K-feldspars from a granite pegmatite (Lavra do Fermin, Linopolis, Minas Gerais, Brazil) showing pristine characteristics formed in the subsolidus stage. Transitional mi- crostructures were analysed by optical microscopy (OM), electron microprobe analyses (EMPA) and Raman microprobe (RMP) spectroscopy. The solid-state transformation initially occurs by many discrete inputs in form of avalanche-like events following four orientations, starting from the interfaces with albite veins, compatible with a C2/m to C1 transition. It results in discrete microcline twins, within orthoclase, those which are tabular on (010) displaying power law statistics in length, as well as in or- thogonal intersections forming chess-board patterns. The mineral change progresses by self-assemblage of microcline variants with the same orientation because of long distance correlations, reinforcing the re-equilibration process and producing twins with sharp wall boundaries. Raman microprobe can resolve low, intermediate and high microcline schemes of local Si/Al order in each individual transformation event. It is suggested that microcline with regular, large and pseudo-periodic twins and chess-board pat- terns could be the result of a crystal evolution process by self-organization in which the mineral transformation is controlled not only by "short-range" Si/Al ordering but also by "long-range" elastic strain accommodation.