Compressibility and high pressure structure refinement of tremolite, pargasite and glaucophane
Comodi, Paola; Mellini, Marcello; Ungaretti, Luciano; Zanazzi, Pier Francesco
European Journal of Mineralogy Volume 3 Number 3 (1991), p. 485 - 500
published: Jun 14, 1991
manuscript accepted: Jan 9, 1991
manuscript received: Jul 23, 1990
ArtNo. ESP147050303002, Price: 29.00 €
Abstract Compressibility measurements and high pressure structure refinements were performed on tremolite, glaucophane and pargasite at pressures up to 41 kbar. The diffraction data were collected with a four-circle diffractometer equipped with a Merrill-Bassett diamond anvil cell. The cell volume reduction is almost constant for the three amphiboles, with bulk moduli increasing slightly from tremolite (K = 850 kbar) to glaucophane (K = 960 kbar) and pargasite (K = 970 kbar). All three specimens show anisotropic compressibility, with ßa =5.9 in tremolite, 5.3 in glaucophane, 4.6 in pargasite (kbar-1 x 104) ; ßb = 2.7, 2.4 and 2.8, and ßc = 2.6, 2.3 and 2.4, respectively, for the three specimens. The most evident structural effect is the reduction of bowing of the tetrahedral double-chain, which assumes a flatter configuration. This effect is greatest for glaucophane, followed by tremolite and pargasite. Opposite movements have been reported for tremolite on heating. Minor variations in the dimensions of the I-beam occur along , that is the extension direction of the tetrahedral chain, and along  across the M4 sites. The tetrahedral double-chain shows an increase in ditrigonal character with increasing pressure. The A-polyhedron is the softest, followed by M4 and by M1-M2-M3. The M-polyhedra show the most prominent compositional effects ; the M2-octahedron is the hardest polyhedron in glaucophane, where it is occupied by trivalent cations, but it is the softest in tremolite and pargasite. The values of polyhedral bulk moduli are in the order: KM2>KM1>KM3>KM4 in glaucophane and KM3>KM1>KM4>KM2 in tremolite and pargasite. The geometrical results, obtained through compressibility measurements and high pressure crystal structure refinements on tremolite, glaucophane and pargasite, basically show similar pressure behaviours in the different calcalkaline amphiboles, with only minor compositional effects. The compressibility and elastic anisotropy of amphibole is intermediate between that of mica and pyroxene, as expected by considering amphibole as intermediate between the close-packed pyroxene and layer-silicate structures.