%0 Journal Article
%A Sánchez Muñoz, Luis
%A García Guinea, Javier
%A Zagorsky, Victor Ye.
%A Juwono, Tjipto
%A Modreski, Peter J.
%A Cremades Rodríguez, Ana Isabel
%A Van Tendeloo, Gustaaf
%A Moura, Odulio J. M. de
%T The Evolution of twin patterns in perthitic K-Feldspar from granitic pegmatites
%D 2013
%@ 0008-4476
%U https://hdl.handle.net/20.500.14352/33400
%X 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.
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