The Evolution of twin patterns in perthitic K-Feldspar from granitic pegmatites

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Sánchez Muñoz, Luis
García Guinea, Javier
Zagorsky, Victor Ye.
Juwono, Tjipto
Modreski, Peter J.
Van Tendeloo, Gustaaf
Moura, Odulio J. M. de
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Mineralogical assoc Canada
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K-feldspar in rocks is commonly seen as a heterogeneous mixture of mineral species and varieties with random microstructures. Most consider that observable features arise from incomplete re-equilibrations owing to slow kinetic and localized effects of aqueous fluids (catalyst), with geological environment and chemical impurities playing only a secondary role. Here, an alternative approach is explored by studying well-preserved regularities in the twin patterns of K-feldspar formed in the subsolidus stage from a historical perspective. Selected samples from granitic pegmatites were studied by polarized light optical microscopy (PLOM), electron-probe micro-analysis (EPMA), scanning (SEM) and transmission electron microscopy (TEM), cathodoluminescence imaging (CL), micro-Raman spectroscopy (MRS) and P-31 nuclear magnetic resonance (NMR). We have found that the essential feature of this crystalline medium is the astounding ability to recrystallize into self-organized twin patterns. The mechanism involves coupling between short-range atomic motions and long-range displacive correlations propagated as ideal and non-ideal Albite and Pericline orientations. We suggest a general evolutionary process to explain the development of macroscopic twin patterns in microcline, based on three twin generations as microtwins, macrotwins and cryptotwins. Evolutionary variants also were identified; they depend on both internal crystallochemical features and an external geological stimulus. We suggest a continuous monoclinic-triclinic transformation for impure K-feldspar, whereas a discontinuous inversion occurs where the starting composition is close to the ideal chemical formula. Twin patterns can evolve by twin coarsening to single-orientation microcline if the system "releases" energy, or by twin fragmentation to finely twinned microcline if the system "stores" energy. Hence, K-feldspar is seen here as a very sensitive medium in which precious geological information is recorded in the form of twin patterns, and thus useful for general geological challenges.
© Geoscienceworld This work has been supported in part by MAT2010–17753 and MAT2010–21088–C03–01 projects. Isabel Sobrados and Jesús Sanz, from Instituto de Ciencia de Materiales de Madrid (ICMM, CSIC) are thanked for the NMR spectra. We thank two anonymous reviewers for the suggestions and comments, and guest editor David London and Robert F. Martin for their help in improving this work. LSM thanks Petr Černý for his efforts in communicating his ideas with such clarity for so many years.
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