Single-crystal neutron diffraction study of partially dehydrated laumontite at 15 K

Abstract

The structure of laumontite [(K 0.10Na 0.30Ca 3.60)Si 16.40Al 7.60O 48.n H 2O; Z = 1] was refined from low-temperature neutron diffraction data collected at the BNL HFBR. Two crystals were used. One (A) was cooled to 15 K from its equilibrium condition at room temperature and humidity; the other (B) had been soaked in water for a few hours and then sealed in a quartz capillary before cooling, in an attempt to obtain the fully hydrated variety laumontite. The structure refinements, however, proved both crystals to be partially hydrated with 13.4 and 14.2 water molecules per unit cell, respectively. The refinements (A: a = 14.690(4), b = 13.061(3), c = 7.574(2) A ̊ , β = 112.01(2)°; B: a = 14.686(10), b = 13.133(8), c = 7.533 (5) A ̊ , β = 111.20(4)°; C 2/ m) converged to final agreement factors of R( F 2) = 0.084 (A, 2136 reflections) and R( F 2) = 0.138 (B, 1784 reflections). An essentially ordered Si,Al distribution on framework tetrahedral sites was inferred from the tetrahedral distances with a minor amount of Si in the Al site in agreement with the chemical analysis. One well-defined Ca site in the cavity was found, in spite of a remarkably high degree of disorder of the water molecules. Eight positions for water oxygens were located in A and 11 in B. Most water molecules are bonded to the Ca atom and are directed toward the center of the channel in a circular arrangement of partially occupied sites. Some of the water molecules are not bonded to the extraframework cations but lie in the center of the major structural channel. They bridge opposite Ca coordination spheres across the cavity and framework oxygens through a complex system of weak hydrogen bonds. The ease of dehydration in laumontite is related to the existence of water molecules not coordinated to the extraframework cations and to the tolerance of the Ca atom to a varying number of coordinated water molecules.