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

¹ CIEMAT, Av. Complutense 22, 28040 Madrid, Spain
² Museo Nacional de Ciencias Naturales (CSIC), C/ Jose Gutierrez Abascal 2, 28006 Madrid, Spain
³ ISTO (CNRS), Rue de la Ferrolerie 1A, 45071 Orléans, France
⁴ Instituto Ciencia de Materiales de Madrid (CSIC), Campus Cantoblanco, 28049 Madrid, Spain
⁵ Joaquim Neves Ferreira, 238, Barrio Vila Bretas, CEP 35030-391 Governador Valdares, MG, Brazil

^* Corresponding author, e-mail: luis.sanchez{at}ciemat.es

The orthoclase-microcline transformation has been studied in perthitic K-feldspars from a granite pegmatite (Lavra do Fermín, Linópolis, Minas Gerais, Brazil) showing pristine characteristics formed in the subsolidus stage. Transitional microstructures 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 C 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 orthogonal 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 patterns 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.

Key-words: microcline, orthoclase, granite pegmatites, self-organization, twinning, Si/Al order, elastic strain accommodation, avalanches.