Strätlingite: crystal structure, chemistry, and a reexamination of its polytpye vertumnite
Rinaldi, Romano; Sacerdoti, Michele; Passaglia, Eliο
European Journal of Mineralogy Volume 2 Number 6 (1990), p. 841 - 850
published: Dec 19, 1990
manuscript accepted: Sep 7, 1990
manuscript received: Jun 9, 1990
ArtNo. ESP147050206011, Price: 29.00 €
Abstract The crystal structure of strätlingite Ca2Al(AlSi)O2(OH)10 • 2.25H2O (Z=3) from Montalto di Castro (VT), Italy, was solved and refined in space group R3̅m (isotropic R=0.080). The structure contains ordered CaVII and A1VI polyhedra fully occupying "octahedral" brucite-type layers (main layers of all aluminate hydrates) hydrogen bonded to partially occupied double tetrahedral (Si/Al=1) layers formed by sharing the apical oxygens of the two component tetrahedral sheets. Cell dimensions: α=5.745(7); c=37.77(l)Å imply the stacking of three main ("octahedral") layers and three "interlayers" (double-T layers) along the  direction as compared with two main layers for the mineral vertumnite which has therefore been proved to be a polytype with a c parameter equal to two thirds that of strätlingite. New chemical data on vertumnite holotype, obtained by accurate electron probe microanalysis together with those obtained for strätlingite holotype as well as for this second occurrence, show very close analogies between the two mineral species also in terms of their crystal chemistry, with a tendency of strätlingite to have a larger proportion of vacant T sites and a higher hydration degree than the polytype vertumnite.