The compressional behavior of P4/n vesuvianite

High-pressure in situ powder-diffraction data were collected on a sample of P4/n vesuvianite from Val d'Ala, Piemonte, Italy, between room pressure an d 13.2 GPa. No deviation from tetragonal symmetry was found at any pressure . The refined unit-cell parameters show that the highest compressibility is along the c axis [beta (a) = 2.00(4). 10(-3)GPa(-1) and beta (c) = 2.42(7) . 10(-3) GPa(-1)]. Such behavior, which is inverse to that observed with in creasing temperature, is related to weaker bonding along the 4-fold axis ow ing to the presence of hydrogen bonding and vacancies. A third-order Birch- Murnaghan equation of state was fitted to the retrieved volumes, with V-0 = 2859.47(7). K-TO = 125.5(1.3) GPa and K'(TO) = 4.8(3). The compressibility of P4/n vesuvianite probably applies to all members of the vesuvianite fam ily.