THE CRYSTAL-STRUCTURE OF PREISWERKITE

The structure of preiswerkite, ideally Na(Mg2Al)(Si2Al2)O10(OH)2, from the type locality at Geisspfad in Switzerland, has been determined by single-crystal X-ray diffraction and refinement. The polytype is 1M with space group C2/m. The unit-cell parameters are a = 5.228(7), b = 9.049(10), c = 9.819(12) angstrom, beta = 100.42(13)degrees. As expected as a consequence of the tschermakitic substitution [6]Mg + [4]Si half arrow right over half arrow left [6]Al + [4]Al, coupled with the K half arrow right over half arrow left Na substitution, the ditrigonal distortion of the tetrahedral sheet is considerable (alpha = 20-degrees), as in ephesite, clintonite, and margarite, because of a concomitant increase in tetrahedral dimensions, decrease in octahedral dimensions, and decrease in the [W-O] bond distance. The coordination of the interlayer cation becomes sixfold because of the very large value observed for the outer W-O distances, as in paragonite. The charge imbalance in the basal 0 atoms due to the high [4]Al content is compensated partly by a significant shortening of the T-O(basal) distances relative to the T-O(apical) distances and partly by some H bonding interactions that inhibit the replacement of OH by F. The O4-H bond is nearly parallel to c*, as in phlogopite. Partial ordering can occur between the octahedral sites; long-range ordering in the tetrahedral sites has not been found. Preiswerkite thus shares specific structural properties with a number of micas with very different compositions; apart from tetrahedral ordering, the closest structural analogue seems to be ephesite, ideally Na(LiAl2)(Si2Al2)O20)(OH)2, which differs chemically only by the octahedral substitution Mg + Mg half arrow right over half arrow left Li + Al.