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Sanidine is the monoclinic high-T modification of K-rich alkalifeldspars. Annealing at T > 9000C usually causes disordering of Al/Si distribution at the two non-equivalent tetrahedral sites, but it is supposedly possible to disorder samples of sanidine from Volkesfeld/Eifel at notably lower temperatures and shorter times [1]. To investigate this behavior and compare various approaches to obtain the Al/Si distribution, samples from different Eifel locations and Madagascar have been studied. Al/Si order was determined by direct and indirect methods, including X-ray and neutron diffraction of powder and single crystal samples. Neutron powder diffraction experiments were executed at the Fine Resolution Neutron Powder Diffractometer E9, single crystal neutron diffraction at the 4-circle Diffractometer E5 and diffuse neutron scattering experiments at the Flat-Cone Diffractometer E2, all located at the Berlin Research Reactor BERII. The Al/Si distribution was determined directly, refining site occupancies by applying Rietveld analysis to powder diffraction data and XTAL for single crystal data. This approach is inapplicable when using X-ray data, due to similar atomic form factors of Al3+ and Si4+, thus indirect methods [2,3,4] were applied. X-ray powder diffraction was performed at Helmholtz Centre Berlin, single crystal X-ray diffraction was done at Ruhr-University Bochum. The obtained data was processed using Rietveld refinement and ShelXL software, respectively. It was possible to verify a dependency of decreasing Al/Si order on increasing annealing times and temperatures. Interestingly we observed different results from direct and indirect methods, regardless whether samples were untreated or annealed. Applying the direct determination method, a stronger change of Al/Si distribution during annealing was revealed. Moreover, diffuse scattering of untreated and annealed samples was detected, which may arise from hydron incorporated in the crystal lattice.
