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 °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 °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 (O 2– )– Y (AlAlAl)– V (OH) 3 – Z [AlAlAlAlAl(Al,Mg)]. These extended clusters remain stable to temperatures close to the observed start of decomposition (~900 °C).