Synthesis of feldspar bicrystals by direct bonding

We have produced synthetic feldspar bicrystals using a direct bonding techn ique. A gem-quality orthoclase crystal from Itrongay, Madagascar, was used for the bonding experiments. Microprobe analysis shows only minor concentra tions of iron and sodium. Orthoclase single crystal plates oriented paralle l (0 0 1) were cut and chemomechanically polished with silica slurry. From interferometry, final roughness of the square crystal plates was about 0.34 nm. Specimens were wet-chemically cleaned using deionised water. The bondi ng procedure produced an orthoclase bicrystal with an optically straight gr ain boundary-oriented parallel (0 0 1), which was investigated by HREM. Alo ng the interface no amorphous layer was observed between lattice fringes of both crystals. We suggest that the bicrystals formed by initial hydrogen b onding and subsequent water loss and polymerisation of silanol and aluminol groups at elevated temperatures.