The crystal structure of tugtupite – a new mineral, Na8Al2Be2Si8O24(Cl,S)2

The structure of the new mineral tugtupite NasA12BezSi8Oz4(Cl,S)2 has been determined by singlecrystal X-ray methods.The crystals are tetragonal with space group 12[.They are strongly piezoelectric but show no pyroelectric effect.Optically the mineral is orthorhombic with values of 2 V~ up to 10 ° and n~, -n• = 1 "496 _+ 0"001, n~, = 1 "502 + 0"002.The specific gravity is 2.30 +_ 0.02.The unit-cell constants are a= 8.583 + 0.004, e= 8"817_+ 0-004 A,.Atomic and thermal vibrational coordinates were refined to R = 8.9 ~o by full-matrix least-squares methods and with three-dimensional data.The following bond lengths were found:Be-O --1-608 + 0.012 .A, A1-O = 1-762_+ 0.014 Si -O --1-601 _+ 0.008The last value is an average of four independent determinations.