Subrata Ghose

X-Ray Photoelectron Spectroscopic Studies of Valence States Produced by Ion-Sputtering Reduction:

X-ray photoelectron spectroscopic measurements of various valence states of Fe and Cu in several compounds using ion-sputtering reduction are presented, and the potential usefulness of this time-dependent and in situ method is discussed.

Core Binding Energy Difference between Bridging and Nonbridging Oxygen Atoms in a Silicate Chain

The x-ray photoelectron spectra of the oxygen 1s level of olivines contain a single component whereas those of pyroxenes contain two components with an intensity ratio of 2:1 and an energy separation of about 1 electron volt. We interpret these two components to be the result of the binding energy differences between nonbridging and bridging oxygen atoms within a silicate chain in the pyroxene structure.

Mg2+-Fe2+ order-disorder in cummingtonite, (Mg, Fe)7Si8O22(OH)2: A new geothermometer

Abstract Using the 57Fe Mossbauer resonance spectroscopy the temperature dependence of the Mg2+ - Fe2+ distribution between the M4 site and M1, M2, M3 sites in magnesium-rich cummingtonites has been determined. Assuming the ideal solution model at each set of sites, the Gibbs free energy, ΔG° for the ion-exchange reaction: Fe2+(M4) + Mg2+ (M1, M2, M3) ⇄ Mg2+ (M4) + Fe2+ (M1, M2, M3) has been estimated to be 3.6 – 4.1 kcal/mole. The temperature Tq, below which no cation migration takes place, lies between 400° and 500°C. The temperature of crystallization of two chlorite-amphibolite (metamorphosed ultrabasic) rocks crystallizing below Tq have been estimated to be 265° and 290°C based on the observed Mg2+ - Fe2+ distribution.

Ordering of V2+, Mn2+, and Fe3+ Ions in Zoisite, Ca2Al3Si3O12(OH)

The presence of very small amounts of Mn2+, V2+, and Fe3+ ions in zoisite can be easily detected by the electron paramagnetic resonance technique at room temperature. The Mn2+ and Fe3+ ions are completely ordered and are probably located in the Ca(1)- and Al(II)-sites, respectively, whereas the V2+ ions probably occupy both Ca(1)- and Ca(2)-sites, with a preference for the Ca(1)-site.

Electron Paramagnetic Resonance of V2+, Mn2+, Fe3+, and Optical Spectra of V3+ in Blue Zoisite, Ca2Al3Si3O12(OH)

Electron paramagnetic resonance and optical absorption spectra have been used to clarify the local environments of transition metal ions and their distributions among various Al and Ca sites in a gem quality zoisite crystal from Tanzania. The EPR spectra due to Mn2+, Fe3+, and two types of V2+ have been interpreted by the spin Hamiltonian βH·g·S + I·A·S + D[Sz2 − 13S(S + 1)] + E(Sx2 − Sy2) in the principal axes system x, y, z. The Mn2+ ions occupy one of the Ca sites, probably Ca(1), with point group symmetry m; g = 2.003 ± 0.005 and | A | = (85 ± 2) × 10−4cm−1, both isotropic, and also | D | = (103 ± 5) × 10−4cm−1, | E | = (34 ± 2) × 10−4cm−1, in general agreement with other oxides and hydrates. Highly anisotropic hyperfine and small zero‐field splittings (| D | < 0.014 cm−1) are observed for V2+, which is the opposite of the usual situation. One of the two types of V2+ apparently occupies the same site as Mn2+; the other V2+ ion is in a general position, and probably occupies a double minima potential a...

Nuclear Magnetic Resonance of 1H and 27Al and Al–Si Order in Low Cordierite, Mg2Al4Si5O18 · nH2O

Proton resonance data in the low‐temperature form of cordierite indicate the presence of isolated water molecules in the channel formed of the AlSi5O18 rings, with H–H directions along the crystallographic c axis and with large vibration amplitudes. Two crystallographically nonequivalent aluminum sites have been observed. The quadrupole coupling constant e2qQ/h, the asymmetry parameter η, and the principal axes directions are: AlI: | e2qQ/h |=10.6 MHz; η=0.38; X ‖ c; Z and Y, 30° away from a and b.AlII: | e2qQ/h |=5.6 MHz; η=0.34; Y ‖ c; Z and X, 20° away from a and b. The AlI and AlII sites correspond to the distorted tetrahedral T1 site outside the six‐membered ring and T5 site within the ring in the cordierite structure. The AlI NMR data also agree with Fe3+ paramagnetic resonance data, indicating that the Fe3+ ions preferentially occupy the larger T1 sites. The 27Al NMR results indicate a high degree of Al–Si order in low cordierite.

Partitioning of cations between coexisting single- and multi-site phases: A discussion

Abstract Chemical potentials of atoms or ions on nonequivalent lattice sites have no validity because these particles cannot be considered independently variable components. Orthopyroxene does not in general exhibit ideal mixing on individual sites. Also a corrected form of the mean molar free energy is presented.

Nuclear Magnetic Resonance of 27Al in Topaz, Al2SiO4(F, OH)2

The 27Al quadrupolar coupling constant, | e2qQ/h | and asymmetry parameter η in topaz have been determined to be 1.67 ± 0.03 MHz and 0.38 ± 0.05. These values and the orientations of the principal axes are consistent with the Fe3+ paramagnetic resonance data and with the symmetry of the AlO4F2 octahedron.

Oriented transformation of grunerite to clinoferrosilite at 775° C and 500 bars Argon pressure

A member of the grunerite series (Fe6.685Mn0.142Ca0.110Mg0.096) (Si7.968Al0.016)O22(OH)2, has been transformed to clinoferrosilite, amorphous silica and water at 775° C and 500 bars Argon pressure. Single crystal photographs of an oriented intergrowth of the amphibole and pyroxene show that the clinoferrosilite retains the cristallographic a, b and c axes of the original grunerite in the I2/m orientation and has the cell dimensions: a=9.77, b=9.08, c=5.30 Å, β=109.5°; space group P21/c. Mössbauer resonance spectroscopy confirmed the identity of the olinoferrosilite.A transformation mechanism requires a minimum of ionic movements in an acceptor region (7/8 of the total crystal), where pyroxene is formed by expulsion of protons and acceptance of Fe2+ ions; simultaneous destruction of donor regions provide the Fe2+ ions, the residue being silica and water. The formation of metastable clinoferrosilite rather than fayalite and quartz indicates the strong structural control imposed by the host grunerite structure on the nature of the transformation products.

Nuclear magnetic resonance of 27Al in epidote, Ca2Al2(Fe,Al)Si3O12(OH)

Two types of nuclear electric quadrupole tensors have been observed for 27Al in epidote. The quadrupole coupling constant C and asymmetry parameter η are AlI |C|=11.9 MHz, η=0.3; AlII, |C|=4.6 MHz, η=0.34. They correspond to the Al1 and Al2 sites, respectively, of the epidote crystal structure. The electric field gradient tensors, with principal contributions from its nearest neighbors in the highly distorted AlO6 tetrahedra, are consistent with the epidote structure and with other aluminosilicate minerals.