Thomsonite: a detailed refinement with cross checking by crystal energy calculations

The thomsonite [12399-54-1], from Death Valley, California, is orthorhombic, space group Pncn, with a = 13.089, b = 13.047, and c = 13.218 Å. The structure was refined to a final R of 0.035. The ordering scheme for the Si-Al distribution doubles the value of the c parameter, typical of all other fibrous zeolites. The possible Si-Al distribution is discussed for samples with an Si/Al ratio >1. Two cation sites were found. One site, occupied by Ca and Na, is at the center of a regular square antiprism, which is linked to 2 equiv. polyhedrons forming a chain parallel to c. The other site, occupied by Ca and Sr, statistically distributed over 2 equiv. positions near a center of symmetry, and is 6-fold coordinated. Four H2O mols. were found at all the H atoms located. The cation distribution and the H positions were verified by electrostatic energy calcns.