Thermal stability of extended clusters in dravite: a combined EMP, SREF and FTIR study

A dravite from Yemen of near end-member composition was treated in air and hydrogen atmospheres at 600-900 A degrees C to reveal changes in Mg and Al order over the octahedrally coordinated Y and Z sites, and to explore related changes in the characteristic vibrational bands in the principal (OH)-stretching frequency. Relevant information was obtained using electron microprobe analysis (EMPA), structural refinement (SREF) and polarized infrared (IR) single-crystal spectroscopy. Overall, the EMPA, SREF and IR data show that only minor changes occur during thermal treatment up to at least 800 A degrees C, including variations in structural parameters, Mg-Al order-disorder and (OH)-stretching bands, indicating limited hydrogen loss. Untreated and treated dravite samples have very similar long-range and short-range atomic structures, which may be related to the occurrence of stable Al-Mg extended clusters around the O1 (=W) and O3 (=V) sites: (W)(F)-(Y)(MgMgMg)-(V)(OH)(3)-(Z)[AlAlAlAlAl(Al,Mg)]; (W)(OH)-(Y)(MgMgAl)-(V)(OH)(3)-(Z)[AlAlAlAlAl(Al,Mg)]; (W)(O2-)-(Y)(AlAlAl)-(V)(OH)(3)-(Z)[AlAlAlAlAl(Al,Mg)]. These extended clusters remain stable to temperatures close to the observed start of decomposition (similar to 900 A degrees C).