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American Mineralogist Volume 72 pages491 506 1987 Determining At Si distribution and strain in alkali feldsparsusing lattice parametersand diffraction peak positions A review Hnnnpnr Knor r Institut fiir Mineralogie WestftilischeWilhelms Universitlit Corrensstrasse24 D 4400 Miinster West Germany Paur H Rrnnr Department of Geological Sciences Viryinia Polytechnic Institute and State University Blacksburg Virginia 24061 U S A Ansrnc cr Al contents tr of the T tetrahedral sites have been estimated from T 4 bond lengthsof 38 K rich alkali feldspars Thesedata were usedtogetherwith lattice parameters and selecteddiffraction peak positions in degrees20 CuKa radiation for thesefeldspars plus corresponding referencevalues for low albite low microcline analbite and high sanidine to obtain new regressionequationsfor estimating structural states to 0 02 For monoclinic alkali feldspars 2tt 1 b 24 8095 74 9054C 3 326r 19 5012C z 20 060 12 1814 t 04093120 204 l 0 6112 o or 5 92t20 204 l For triclinic alkali feldspars 2t t o t m LIr b 2r 5398 53 8405C 2 r567 15 8583C 20 060 8 3063 0 96459120 204 1 The b c equations produce nearly linear plots for alkali exchangeseries a substantial improvement over the relationships involving b and c cell parameters The familiar a 7 plot gives At t o t m for triclinic alkali feldspars the regressionequation is Atr 4 44 778 0 50246a 6 6 4 6 0 0 5 0 6 1 4 Equations in terms of 2d I 30 20 l 30 and 20 201 are given for At in K and Na rich feldspars In addition other determinative methods for 2t and At involving the translations trfl l0 and trfl10 are presented New quantitative strain indexes S I in o o are defined for phasesthat occur in coherent or semicoherentperthitic intergrowths for K rich feldspars S I 3391 84 67 960a 42 516 b c and for Na rich feldspars S I 1028 00 193 8834 28 472 b c Similar equations were derived using dro and do o d2oo is mole fraction anorthite content T O is the mean of four Al SilO bond lengths for the T tetrahedron and In



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