Letter | Published:

Three-coordinated magnesium in dehydrated offretite

Abstract

OFFRETITE is a rare natural zeolite whose synthetic equivalent, zeolite O, is commercially important as a molecular sieve1. Its aluminosilicate framework is topologically related to that of erionite2, an abundant natural zeolite3. The catalytic properties of molecular sieve zeolites have been ascribed to framework-bound protons, strong electrostatic fields, and oxygen vacancies in the framework4. As befits the small radius of Mg2+ ions (0.049 nm for four-coordination, 0.072 for six-coordination and 0.089 for eight-coordination)5, Mg normally occurs in six-coordination with oxygen and hydroxyl, though four, five and eight-coordinations occur infrequently (for example, four-akermanite: Ca2MgSi2O7, Mg–O = 0.1876 nm; five-grandidierite: (Mg,Fe)Al3O.BO4.SO4, 0.2176, 0.2054, 0.2040, 2×0.1970; farringtonite: Mg3PO4, 0.196, 0.197, 0.201, 0.206, 0.214; eight-pyrope: Mg3Al2Si3O12, 4×0.2196, 4×0.2342)6. We report here on the occurrence of three-coordinated Mg in dehydrated offretite, which should produce a strong electrostatic effect on adjacent sorbed molecules.

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