Substitutional and thermal expansion in MAlSi2O6 aluminosilicates with keatite structure
Sternitzke, Martin; Müller, Gerd
European Journal of Mineralogy Volume 3 Number 5 (1991), p. 769 - 776
published: Oct 2, 1991
manuscript accepted: Apr 17, 1991
manuscript received: Dec 18, 1990
ArtNo. ESP147050305013, Price: 29.00 €
Abstract A series of isostructural keatite-type MAlSi2O6 aluminosilicates with M+ ions ranging from H to K can be synthesized by low-temperature ion echange. Because of its particular framework topology the keatite structure can accommodate large cations by c-axis expansion with concomitant slight α-axis contraction. This flexibility results from changes in T-O-T angles and finds its limit at the ionic size of K, Rb ions not even being incorporated in small quantities. From lattice constant variations the substitutional expansion coefficients are derived. Structural modelling by Distance-Least-Squares (DLS) calculations shows that unacceptable framework deformation would result from substitution by ions larger than K. For the same reason, the thermal expansion of the MOx polyhedra is very restricted in KAlSi2O6, leading to anomalous lattice thermal expansion. DLS calculations further reveal a close correlation between substitutional and thermal expansion in the keatite series. Anomalous bond lengths and anomalous thermal expansion occur in the MOx polyhedra of the ionexchanged, unstable members of the series.