3RD-GENERATION SYNCHROTRON X-RAY-DIFFRACTION OF 6-MU-M CRYSTAL OF RAITE, ATE-TO-NA3MN3TI0.25SI8O20(OH)2-CENTER-DOT-10H(2)O, OPENS UP NEW CHEMISTRY AND PHYSICS OF LOW-TEMPERATURE MINERALS

The crystal structure of raite was solved and refined from data collec ted at Beamline Insertion Device 13 at the European Synchrotron Radiat ion Facility, using a 3 x 3 x 65 mu m single crystal, The refined latt ice constants of the monoclinic unit cell are a = 15.1(1) Angstrom; b = 17.6(1)Angstrom; c = 5.290(4) Angstrom; beta = 100.5(2)degrees; spac e group C2/m, The structure, including all reflections, refined to a f inal R = 0.07, Raite occurs in hyperalkaline rocks from the Kola penin sula, Russia, The structure consists of alternating layers of a hexago nal chicken-wire pattern of 6-membered SiO4 rings, Tetrahedral apices of a chain of Si six-rings, parallel to the c-axis, alternate in point ing up and down. Two six-ring Si layers are connected by edge-sharing octahedral bands of Na+ and Mn3+ also parallel to c, The band consists of the alternation of finite Mn-Mn and Na-Mn-Na chains, As a conseque nce of the misfit between octahedral and tetrahedral elements, regions of the Si-O layers are arched and form one-dimensional channels bound ed by 12 Si tetrahedra and 2 Na octahedra. The channels along the shor t c-axis in raite are filled by isolated Na(OH,H2O)(6) octahedra, The distorted octahedrally coordinated Ti4+ also resides in the channel an d provides the weak linkage of these isolated Na octahedra and the mix ed octahedral tetrahedral framework, Raite is structurally related to intersilite, palygorskite, sepiolite, and amphibole.