Tetrahedral ferric iron in phlogopite: Xanes and Mossbauer compared to single crystal X-ray data

This paper reports the results of a combined methodological study performed on tetraferriphlogopite from the Araxa carbonatite. We compare Mossbauer spectroscopy and X-ray absorption near-edge spectroscopy (XANES) results to previously reported single-crystal X-ray and electron microprobe analysis data (Cruciani & Zanazzi, 1994). The combined techniques provide evidence for the presence of a remarkable amount of tetrahedrally coordinated Fe3+ in this sample, and show that high OH content in tetraferriphlogopite is compatible with the large Fe2O3/FeO ratio inferred from the chemical and spectroscopic data

High-pressure structural behaviour of scolecite

The HP structural evolution of a natural scolecite from Iceland (space group Cc) was studied up to 5 GPa using in situ single-crystal X-ray diffraction data from a diamond-anvil cell (DAC) with silicon oil as non penetrating pressure transmitting medium. Linear regressions yielded mean axial compressibilities for a, b and c axes of \u3b2a = 4.4(2).10-3, \u3b2b = 6.1(2).10-3, \u3b2c = 6.0(1).10-3 GPa-1. K0, refined with a second-order Birch-Murnagham equation, fixing K0 at 4, is 54.6(7) GPa. The bulk scolecite structure compression was the result of the "soft" behaviour of the channels (K 45 17 GPa for [100]-channels; K 45 50 GPa for [001]-channels) and the more rigid behaviour of the tetrahedral framework (K 45 96 GPa), which underwent kinking of the Secondary Building Unit (SBU) along [100]-chains. The angle between the SBUs (\u3c6), increased from 20.80(2)\ub0 at 0.0001 GPa, to 22.00(6)\ub0 at 3.38 GPa. Within the investigated pressure range, the position of the extra-framework cations and water molecules remained almost unchanged. Up to 4.2 GPa no phase transition was observed

Effects of pressure on the structure of bikitaite

The structural behaviour of bikitaite, Li2(Al2Si4O12). 2H2O, was investigated under hydrostatic pressure using X-ray single-crystal diffraction data. A Merrill-Bassett diamond anvil cell was mounted with glycerol, as non penetrating pressure-transmitting medium, ruby chips and a small crystal of quartz as the calibrant. A strong anisotropic compression was observed by linear regressions of lattice parameters against P, bikitaite being softer along the c axis (\u3b2c=9.3(1) 10-3 GPa-1), than along b (\u3b2b = 6.6(1) 10-3 GPa-1) and a (\u3b2a = 2.4(1) 10-3 GPa-1) (\u3b2a: \u3b2b: \u3b2c=1: 2.75 :3.9 . Fitting the cell-volume - pressure data to a second order Birch-Murnaghan equation of state, as indicated by the finite strain-stress plot, yielded K0 = 44.2(4) GPa, with K' = 4 and V0 = 295.58(2) \uc53. The evolution of the bikitaite structure with P was studied by comparing the results of refinements with data collected at room conditions, at 3.2 GPa and after decompression. The structure can be described as sheets of six-membered rings parallel to (001), connected by pyroxene-like chains. 8-ring and 5-ring channels run along [010] and inside the 8-ring channel there is a onedimensional chain of water molecules, which is linked to the framework through the extra-framework Li atoms. Under pressure, the kinking of the pyroxene-like chain decreased the free diameters of the 5-ring channels, strongly reducing the distance between the ab planes. On the contrary, the tridymite-like planes with 6-membered rings were more rigid. The positions of the extra-framework cations and water were maintained at HP even though the configuration of the water chains changed slightly: the distances between the water molecules decreased, whereas the kinking angle of the chain increased

High-pressure structural behaviour of heulandite

The structural evolution up to 5 GPa of a natural heulandite was studied using in situ single-crystal X-ray diffraction data from a diamond-anvil cell (DAC) with glycerol as the pressure transmitting medium. Linear regressions yielded mean axial compressibilities for a, b and c axes of \u3b2a = 1.02(1). 10-2, \u3b2b = 7.6(2). 10-3 GPa-1. The largest strain vector (\u3b21 = 1.16 10-2 GPa-1)lies approximately on the diagonal of the system of channels along [100] and [001]. V0, K0, and K0' refined with a third-order Birch-Murnaghan equation are: V0 = 2121(2) \uc53, K0 = 26.4(1.0) GPa, K0' = 4.9(8). If fitted with second-order Birch-Murnaghan equation of state, fixing K0' = 4, K0 becomes 27.5(2) GPa. The bulk heulandite structure compression was the result of the "soft" behaviour of the channels (K = 10-19 GPa) and the more rigid behaviour of the tetrahedral framework (K 45 60 GPa), which underwent tilting of the fundamental polyhedral unit (FPU) chains. The T5-T5-T5 angles, between the FPUs, decreased from 162.4\ub0 at 0.0001 GPa to 156.2\ub0 at 3.4 GPa. The position of extra-framework cations and water molecules was almost maintained within the investigated pressure range. Up to 3.7 GPa no phase transition was observed. Amorphization was clearly observed at pressure above 4 GPa. The transition to the amorphous phase was still reversible up to 5 GPa

New insights on high-pressure behaviour of microporous materials from X-ray single-crystal data

The main deformation mechanisms induced by pressure on different structural types of zeolites were analysed by comparing experimental data and theoretical models. Data of single-crystal X-ray diffraction obtained with the sample in a Merrill-Bassett diamond anvil cell on a four-circle diffractometer were collected at different pressures for samples of heulandite, scolecite and bikitaite, using non-penetrating pressure transmitting media (glycerol or silicon oil), up to 5 GPa. The results indicated that, at first approximation, the theoretical approach reproduces the structural evolution of zeolites under pressure. However, the flexibility possessed by framework microporous silicates resulted more complex than that which can be modelled by undeformable "rigid-unit modes", being completely flexible in the oxygen hinges. Moreover, the compressibility of the zeolites under study does not appear to be directly related to the microporosity represented by the framework density (FD): The bulk moduli (simply defined as the inverse of volume compressibility coefficients) of heulandite (27.5(2) GPa) and scolecite (54.6(3) GPa) were different even though their FD's were quite similar. Single crystal data have shown that the structural evolution of the open-framework silicates, is strongly controlled by the framework, whereas the role of the extra-framework content was less important. In all three zeolites the position of the extra-framework water molecules and cations was maintained approximately and their coordination numbers remained unchanged within the pressure range investigated

Isothermal equation of state and compressional behavior of tetragonal edingtonite

The high-pressure (HP) structural evolution of a natural tetragonal edingtonite from Ice River, Canada, was investigated up to 5.1 GPa using in situ single-crystal X-ray diffraction and a diamond-anvil cell. The isothermal equation of state was determined. The values of V0, K10, and K1 refined with a third-order Birch-Murnaghan equation of state (BM-EoS) are V0 = 601.6(3) \uc53, K:10 = 59(2) GPa, and K1 = 3.4(8). Under high-pressure conditions the main deformation mechanisms can be described by rotation/kinking of "rigid units," represented by the 4 = 1 secondary building unit (SBU), due to the tetrahedra tilting. The angle between the SBUs (\u3c6) increased from 17.15(8)\ub0 at 0.0001 GPa to 20.03(9)\ub0 at 4.61 GPa. The bulk structural compression results from the combination of the "soft" behavior of the not fully occupied channel [K10 = 19(1) GPa for [100]-channels; K10 = 21 (1) GPa for [110]-channels] and of the rigid behavior of the tetrahedral framework. The extra-framework cations do not increase in coordination number within the pressure range investigated. The barium occupancy factors for the Ba1 and Ba2 sites change with increasing pressure. For P > 2.3 GPa the Ba2 site is completely empty, and only the position Ba1 is occupied

A single-crystal study on the pressure behavior of phlogopite and petrological implications

Here we report the results of the e rst three-dimensional re e nement of the 10 \uc5 phase performed with single-crystal X-ray data. The 10 \uc5 phase, Mg3Si4O10(OH)2H2O, is monoclinic, space group C2/m, a = 5.323(1)\uc5, b = 9.203(1)\uc5, c = 10.216(1)\uc5, \u3b2 = 99.98(1)\ub0, V = 492.9(2) \uc53; the calculated density, assuming Z = 2, is 2.676 g.cm\u963. The structure has been solved by direct methods and re e ned by least-squares method with anisotropic displacement parameters. The e nal agreement index (R1) was 0.088 for 54 re e ned parameters and 499 unique observed re f ections collected with a diffractometer with a CCD detector. The structure of the 10 \uc5 phase is very similar to that of a homo-octahedral, 1 M trioctahedral mica: it is a silicate consisting of 2:1 tetrahedral-octahedral layers parallel to (001). The mean Si-O, Mg1-O, and Mg2-O bond lengths are 1.626, 2.082, and 2.081 \uc5, respectively. The ditrigonal rotation angle \u3b1 is 0.53\ub0. The interlayer of the 10 \uc5 phase is occupied by water molecules. According to the oxygen occupancy, 1 H2O p.f.u. is assumed in the investigated sample. Although the average water oxygen position is in the mid-plane, structural re e nement suggests disorder along c*. Twelve hydrogen bonds are located between the water molecule and the 6 + 6 oxygen atoms of the basal rings of adjacent tetrahedral sheets (water-oxygen distances averaging 3.19 \uc5). Therefore there are six possible orientations for the water molecule, with six hydrogen bonds pointing toward the upper basal ring and six pointing toward the lower ring of tetrahedral sheets. The orientational disorder of water, in agreement with previous Raman spectroscopy data, is a feature relevant to the evaluation of thermodynamic functions and thermal stability of the 10 \uc5 phase, which is a possible water carrier (9.1 wt%) in subducting slabs at high pressure