MODULATED STRUCTURES IN KAISI(3)O(8) - A STUDY BY HIGH-RESOLUTION ELECTRON-MICROSCOPY AND SI-29 MAS-NMR SPECTROSCOPY

Modulated structures in KAlSi3O8:(orthoclase) have been studied by ele ctron microscopy and electron diffraction and by Si-29 MAS-NMR spectro scopy. Electron diffraction as well as NMR data are consistent with th e local triclinic symmetry of microcline. TEM images show the presence of nanometer scale 'tweed' contrast. As the gamma angle deviates fro m 90 degrees, the heterogeneous perturbed lattice without discrete int ernal boundaries is first transformed into a microstruture with discre te nanodomains joined along diffuse or ill-defined boundaries and afte rwards into a domain-structure with discrete twin planes. Si-29 spectr a are compatible with a Si/Al distribution in which the Al atoms, pref erentially in T-1 sites, are homogeneously dispersed throughout the fr amework. Na atoms probably stabilize as pinning centres the (010) boun daries following the albite-twin orientation.