| Title |
|
|
|
The evolution of twin patterns in perthitic K-feldspar from garnitic pegmatites
| |
| Author |
|
|
|
| |
| Abstract |
|
|
|
| Grains of K-feldspar are commonly seen as heterogeneous mixtures 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-feldspars 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 31P nuclear magnetic resonance (NMR). We have found that the essential feature of this crystalline medium is the astounding capability to recrystallize in self-organized twin patterns. The mechanism involves coupling between short-range atomic motion, 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 monoclinictriclinic 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. | | |
| Language |
|
|
|
English
| |
| Source (journal) |
|
|
|
Canadian mineralogist. - Toronto | |
| Publication |
|
|
|
Toronto : 2012
| |
| ISSN |
|
|
|
0008-4476
| |
| Volume/pages |
|
|
|
50:4(2012), p. 989-1024
| |
| ISI |
|
|
|
000314174400015
| |
| Full text (Publisher's DOI) |
|
|
| | |
| Full text (publisher's version - intranet only) |
|
|
| | |
|