Chiavennite revisited: a high-temperature in situ single-crystal X-ray diffraction study

Abstract

Chiavennite (CaMnBeSi 5 O 13 (OH) 2 · 2H 2 O, Z = 4) is a rare Be-bearing zeolite mineral with an interrupted framework of four-connected [SiO 4 ] and three-connected [BeO 4 ] tetrahedra. A sample from the type locality in the Rhetic Alps of Italy has been re-investigated at room temperature in order to clarify the space-group ambiguity discussed in the literature and to explore the system of hydrogen bonds. In addition, the thermal stability of this zeolite was tracked by in situ single-crystal X-ray diffraction between 25°C and 425°C. In spite of the pseudo-orthorhombic cell dimension, the new data indicate that at room temperature chiavennite is truly monoclinic P 2 1 / c (β ≈ 90°) and twinned as recently shown for the new mineral ferrochiavennite. The dehydration experiments under dry conditions at different temperatures showed that chiavennite continuously released water up to at least 425°C without topological modification. The loss of more than one H 2 O molecule at 250°C decreases the Ca coordination from eight- to seven-fold. After release of the first H 2 O molecule, and strong dynamic disorder of the remaining H 2 O, chiavennite evolves into orthorhombic symmetry of space group Pbcn . The continuous monoclinic–orthorhombic transition from space group P 2 1 / c to Pbcn upon dehydration is attributed to loosening of the system of hydrogen bonds with extraframework H 2 O as donor and oxygen at the cavity walls as acceptor.