Neutron Diffraction Refinement of an Ordered Orthoclase Structure
American Mineralogist, Volume 58,pages 500-507, 1973 NeutronDiffraction Refinement of an orderedorthoclase Structure Erweno PnrNcB Inst,ttute for Materials Research, National Bureau o.f Sta:ndards, Washington, D. C. 20234 Gnenrprrp DoNNnylNo R. F. MenrrN Department of Geological Sciences, McGiII Uniuersity, Montreal, Canada Abstract The crystal structure of a pegmatitic monoclinic potassium feldspar, (IG rnnonn) "Nao (Si,,*Al'*) t0"*(OH)o.1, from the Himalaya mine in the Mesa Grande pegmatitedistrict, California, has been refined with 3-dimensionalneutron-diffraction data to an unweighted R value of 0.031 for 721 symmetry-independentobserved reflections. Atomic coordinatesdiffer by no more than 3 estimated standard deviations from those of Spencer B adularia, yet the specimen does not have the adularia morphology, and no diffuse reflections with (h + k) odd have been observed.Direct refinement of the tetrahedral cation distribution shows that the Al content of the T(2) sites is not significantly different from zero (actually -0.016 with an e.s.d. of 0.029); in other words the Al-Si ordering in the tetrahedral sites is essentially complete. The mean SiO distance in the T(2) sites is 1.616A, appreciably greater than the values predicted by various regression lines relating bond distance to aluminum content. This indicates that the observed mean T,(m)-O, ?,(O)-O, and Tn(m)-O bond lengths re- ported for low albite and maximum microcline are consistentwith full Si occupancy. This ordered orthoclase occurs in gem pockets in a microcline-bearingpegmatite. The association suggestsstable growth of ordered orthoclase above the field of stability of microcline and metastablepersistence to lower temperatures.Perhaps because of more rapid crystal growth, the bulk of the pegmatitic K-feldspar ordered to common orthoclase, then transformed to maximum microcline.
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