Crystal structures and compressibilities of synthetic 2M1 and 3T phengite micas

Abstract

Abstract:The crystal structures of co-existing monoclinic 2M 1 and trigonal 3T polytypes of phengitic micas synthesized at 11 GPa and 900 ºC have been refined at ambient conditions. The compositions of both . The molar volumes of the two polytypes are identical within error (approximately one part in 4000). The structures show closely similar distortions consistent with the nearly pure silicate tetrahedral layer. The tetrahedral rotation angles, α, are both about 2.4º and thus the smallest yet reported for dioctahedral micas. There is no indication of tetrahedral ordering of Al and Si. The 3T polytype contains two distinct octahedral sites that appear to be distinctly different in size indicating possible ordering of Mg and Al. The unit cell parameters of the 2M 1 sample have been measured at several pressures up to 7.5 GPa and those of the 3T sample to 4.0 GPa. Fitting compression data to a third-order Birch-Murnaghan equation of state gives a K 0 of 57 ± 3 GPa with K' of 9.2 ± 1.7 for the 2M 1 and a K 0 of 62 ± 2 GPa with a fixed K' of 9 for the 3T. These are statistically identical and represent the largest bulk modulus yet measured for any mica. As with other micas, compressional anisotropy is large with compression normal to the layers being about seven times that within the layers.

Structural evolution of a 3T phengite mica up to 10 GPa: an in-situ single-crystal X-ray diffraction study

“…Previous experiments were confined to the investigation of the unitcell parameters with P, and were aimed to describe the isothermal EoS and the phase-stability of 2M 1 -and 3T-phengites (Pavese et al, 1999;Smyth et al, 2000, Curetti et al, 2006.…”

“…To date, only a few studies have been devoted to the high-pressure (HP) behaviour of dioctahedral micas (Vaughan and Guggenheim, 1986;Sekine, Rubin, Ahrens, 1991;Faust and Knittle, 1994;Catti, Ferraris, Hull, Pavese, 1994;Comodi and Zanazzi, 1995;1997) and in particular of phengites (Pavese, Ferraris, Pischedda, Mezouar, 1999;Smyth, Jacobsen, Swope, Angel, Arlt, Domanik, Holloway, 2000;Curetti, Levy, Pavese, Ivaldi, 2006). Pavese et al (1999) described the thermo-elastic behaviour of a natural 3T-phengite by in-situ synchrotron X-ray powder diffraction with a Paris-Edinburgh cell.…”

“…Pavese et al (1999) described the thermo-elastic behaviour of a natural 3T-phengite by in-situ synchrotron X-ray powder diffraction with a Paris-Edinburgh cell. On the basis of HP single-crystal X-ray diffraction experiments with a diamond anvil cell (DAC), Smyth et al (2000) reported the evolution of the unit-cell parameters under hydrostatic pressure of synthetic 3T and 2M 1 phengite samples. More recently, Curetti et al (2006) studied the elastic properties of coexisting natural 3T and 2M 1 phengite samples from Cima Pal (Sesia Zone, Western Alps, Italy), with similar chemical composition, on the basis of in-situ HP powder diffraction using synchrotron radiation and a DAC.…”

Cation sorption on the muscovite (001) surface in chloride solutions using high-resolution X-ray reflectivity

“…An analytical absorption correction was applied using the indexed shape and measured dimensions of the crystal (Sheldrick, 1997). The structure was refined in space group C2/c using neutral-atom scattering factors from the International X-ray Crystallographic Tables (1973), and initial atom positions of Smyth et al (2000).…”

Equation of state of hydrous Fo90 ringwoodite to 45 GPa by synchrotron powder diffraction

“…The cubic unit cell parameter of ringwoodite was refined using the Rietveld program GSAS (Larson and von Dreele, 2000) and reported in Table 1 . In a single-crystal compression study Smyth et al (2000) observed a K´ value of 9 for two polytypes of phengite mica. We have also obtained a bulk modulus of 146 GPa with K' of 6.1 in a singlecrystal compression study of hydrous wadsleyite II (Smyth et al 2005).…”