Lawsonite, Ca|6|Al2|6|Si2|4|O7(OH)2·H2O, is found in low-temperature, high-pressure metamorphic rocks. It is a hydrous counterpart of anorthite. A specimen from the type locality, Tiburon Peninsula, Marin County, California, was used for an X-ray study: a = 8.795(3), b = 5.847(1), с = 13.142(6)Å, V = 675.8Å, Z = 4, Dcalc = 3.088, space group Ccmm. Refinement of 865 Fobs gave an R = 0.0256. The structure is based on a three dimensional framework generated by cross-linking ribbons composed of edge sharing single chains of Al coordination octahedra and lateral bridging silicate groups. The openings of the framework accommodate the Ca atoms and the water molecules. The observed mean distances are Si|4| – О = 1.633, Al|6| – О = 1.913 and Ca|6| – О = 2.421Å. The hydrogen atoms participate in bent and bifurcated hydrogen bonds. Individual cation-anion distances conform well to the extended electrostatic valence rule.

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