Annibale Mottana

X-ray absorption study of Ti-bearing silicate glasses

Ti K-edge XANES spectra have been collected on a series of Ti-bearing silicate glasses with metasilicate and tetrasilicate compositions. The intensity of the preedge feature in these spectra has been found to change with glass composition and varies from 29 to 58% (normalized intensity) suggesting a variation in structural environent around the absorbing atom. The pre-edge peak intensity increases for the alkali titanium tetrasilicate glasses from 35% to 58% in the order Li < Na < K < Rb, Cs whereas for the metasilicate compositions there is a maximum for the K-bearing glass. The pre-edge peak intensity remains constant for the alkaline earth titanium metasilicate glasses, Ca and Sr (34%) but increases slightly for Ba (41%). As the intensity of this feature is inversely correlated with coordination number, a comparison of the pre-edge intensity data for the investigated glasses with those of materials of known coordination number leads us to establish a regression equation and to infer that the average coordination number of Ti in these glasses ranges from 4.8 to 5.8. Large alkali cations appear to stabilize a relatively low average coordination number for Ti in silicate melts. The Ti structural environment results appear also to vary as a function of SiO2 content within the K2O-TiO2-SiO2 system. A number of physical properties of the melts from which these glasses were quenched and of other Ti-bearing silicate melts, have been determined in recent years. Clear evidence of a variable coordination number of Ti, consistent with the interpretation of the present XANES data is available from density measurements. These and other property determinations are compared with the present spectroscopic observations in an attempt to relate structure and properties in these melts which contain a major component with variable coordination number.

REDUCTION AND SORPTION OF CHROMIUM BY Fe(II)-BEARING PHYLLOSILICATES: CHEMICAL TREATMENTS AND X-RAY ABSORPTION SPECTROSCOPY (XAS) STUDIES

The reduction of hexavalent chromium species in aqueous solutions by interaction with Fe(II)-bearing solid surfaces was studied using a 0.96 × l0−3 M Cr(VI) solution and iron-rich clays with different Fe(II)/Fe(III) ratios, layer charge, and exchange properties, i.e., chlorite, corrensite, and montmorillonite. Experimental studies demonstrated that Fe(II)-bearing phyllosilicates reduce aqueous Cr(VI) ions at acidic pH. Chlorite and corrensite, owing to the high Fe(II)/Fe(III) ratio, are electrochemically reactive, as rapid Cr(VI) reduction indicated. In contrast, montmorillonite showed minimum to nil reactivity towards Cr(VI). Furthermore, corrensite, which is high in both Fe(II)/Fe(III) ratio and exchange capacity, adsorbs the greatest amount of chromium.X-ray absorption spectroscopy at Al, Mg, Fe, and Cr K-edges was used to investigate the adsorbed chromium species. The montmorillonite sample, unaffected by treatment with Cr(VI) solution, displays no change at any investigated edge. Edge shape and energy also do not change for the Mg and Al spectra in corrensite, and changes are minor in chlorite. By contrast, the Fe K-edge changes both in chlorite and corrensite, and indicates an increase of Fe(III) in treated samples at the expense of pre-existing Fe(II). Cr K-edge spectra show that chlorite and corrensite sorb Cr(III), which implies its reduction from Cr(VI) in the interacting solution.

Effect of aluminum on Ti-coordination in silicate glasses: A XANES study

Abstract The structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Changes in the relative intensities of peaks in the Al K-edge, however, suggest variation in the intertetrahedral (T-O-T) angle. We associate the decrease of this angle for the glasses of peraluminous composition with the presence of triclusters of tetrahedra. The pre-edge peak absorption features in the Ti K-edge XANES spectra indicate that the average Ti coordination decreases with the addition of Al2O3. We infer depletion of fivefold-coordinated titanium (possibly as alkali titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated Ti and alkali aluminate complexes (up to a concentration of 40% in the most peraluminous glass). Significant amounts of [V]Ti remain present, even at peraluminous compositions, in further support of tricluster formation as a mechanism for Al incorporation.

Crystal chemistry of kimzeyite from Anguillara, Mts. Sabatini, Italy

A single garnet crystal of kimzeyite from Anguillara Sabazia, Sabatini Volcanic District (Rome), has been studied by electron probe microanalysis (EPMA), single crystal X-ray diffraction (XRD) and Raman spectroscopy (RS). X-ray intensities for the structure refinement were collected by means of both a conventional four circle diffractometer (CD) and an area detector (AD) diffractometer. Salient crystallographic data from AD measurements are: a = 12.397(4) A, V = 1905.24 A 3 , μ= 25 cm −1 , crystal dimension ∼ 40 μm, D x =3.83 g cm −3 . The chemical analyses allowed to ascertain that Ca, Ba, Fe, Al, Mg, REE, Zr, Si and Ti are present in the structure. The transition element distribution is: Fe 3+ (Y) ≪ Fe 3+ (Z), and Ti as Ti 4+ occurs only at Y. No hydrogen atoms were detected. The problem of the correct distribution of Ba and REE (essentially Eu and Gd) is dealt with, but not fully solved in the present study. Because of the very small amount of sample available and of the strong dilution of Ba and REE (∼0.03 and 0.02 atoms per formula unit respectively) the correct distribution of these atoms over the X and Y sites is problematical. The radius ratio rule and electrostatic requirements suggest Ba at X and REE at Y. From these principles, the following site distribution can be deduced: Ca 2+ and Ba 2+ at X; Mg 2 +, REE 3+ , Zr 4+ , Ti 4+ , Fe 3+ at Y; Si 4+ , Al 3+ , Fe 3+ at Z. On the other hand, structural details and the best fit between EPMA and XRD site populations indicate REE at X and Ba at Y. Bond strength theory arguments support this hypothesis. The main substitution mechanisms can be summed up as follows: 1) Ti 4+ (Y) + Fe 3+ (Z) ↔ Si 4+ (Z) + M 3+ (Y) [schorlomite substitution]; 2) Zr 4+ (Y) + M 3+ (Z) ↔ Si 4+ (Z) + M 3+ (Y) [kimzeyite substitution]; 3) Mg 2+ (Y) + Zr 4+ (Y) ↔ 2M 3+ (Y) [morimotoite-like substitution], where M 3+ = (Al, Fe). In addition, following the bond strength theory indications, another mechanism which possibly involves the X and Y sites is also taken into account: 4) REE 3+ (X) + Ba 2+ (Y) ↔ Ca 2+ (X) + M 3+ (Y).

Octahedral versus tetrahedral coordination of Al in synthetic micas determined by XANES

Abstract We used the JUMBO monochromator at SSRL to measure the Al K-edge X-ray absorption spectra of synthetic micas having variable Al content and occupancy, from 0 to 2/4 in the octahedral M positions, and 0 to ⅔ in the tetrahedral T positions. The measured Al K edges differ markedly, but the differences may have a common explanation: (1) Micas containing ⅓ Al in M or ¼ Al in T have K edges that differ in the energy and intensity of the first two features, which are related to interaction of Al with its first-shell nearest neighbors (O and OH or F). They are nearly identical to the K edges of reference minerals such as albite (tetrahedral Al only) or grossular (octahedral Al only). (2) Micas containing Al in both M and T have K edges that can be interpreted as a weighed combination of the simple edges.

Chromium-containing muscovite crystal chemistry and XANES spectroscopy

To verify chromium enrichment of the muscovite layer, a crystal chemical and XANES study on chromium-containing muscovite crystals from the South Island of New Zealand was carried out. The crystals studied differ from those of end-member muscovite in that they display variable levels of octahedral substitutions and homovalent and heterovalent substitution of K in interlayer sites. Single-crystal X-ray diffraction data were collected for three crystals in the space group C 2/ c to an agreement factor (R obs ) from 0.025 to 0.033. Tetrahedral cation disorder was found for each sample and the values of mean bond length for both tetrahedra do not depart significantly from that of the end-member muscovite-2 M 1 . Electron density at the M2 site is greater than that required for the ideal muscovite-2 M 1 structure, and a small excess of electron density is found for two crystals in M1. As the octahedral substitution of larger cations for Al increases in the octahedral sites, the match between tetrahedral and octahedral sheets improves and tetrahedral rotation angle, α, decreases. XANES spectra at the Cr K-edge in these chromium-containing muscovite samples exhibit octahedral symmetry. Moreover, a careful analysis of the pre-edge region shows at least two features. A qualitative fitting procedure of the pre-edge region indicates that no more than 0.5% of total Cr(III), if any, may occupy the tetrahedral site.

Pressure-induced coordination change of Ti in silicate glass: a XANES study

The effect of pressure on titanium coordination in glasses, with composition K2TiSi4O11, quenched isobarically from liquids equilibrated at high pressure (5, 10, 15, 20, 25, 30 kbar respectively) and T=1600° C has been investigated by X-ray absorption spectroscopy (XAS). The XANES spectra collected at the Ti K-edge clearly show a variation with pressure that is related to changes in the geometrical environment around the Ti atoms. By comparison with spectra of standard materials, the XANES spectra of the glasses suggest a relatively low average coordination number (near 5) in samples quenched at low pressure and a higher coordination number (near 6) in samples quenched from the highest pressure. The combination of XANES data with density and compressibility measurements supports the idea that a mixture of 6- and lower coordinated (4- and/ or 5-coordinated) Ti geometries are present in the 1 bar glass, and an increasing proportion of 6-coordinated Ti occurs in the glasses synthesized at progressively higher pressures.

Experimental and theoretical XANES and EXAFS study of tetra-ferriphlogopite

Experimental and theoretical EXAFS and XANES data on Fe coordination and bond distances have been obtained for a natural tetra-ferriphlogopite. XANES data show a predominance of trivalent iron in tetrahedral coordination. The use of theoretical multiple scattering calculations helps distinguish the contributions to the total spectrum coming from the different iron atoms. EXAFS spectra were reduced and analyzed using the GNXAS programs and Fe has been confirmed to be in tetrahedral coordination, with refined structural data Fe-O = 1.86 A, Fe-Si = 3.24 A, Fe-O-Si = 138°. Moreover, a further contribution from Fe is detected corresponding to a Fe-O distance equal to 2.22 A. This latter contribution (20 mol % of the total Fe ca. ) is interpreted to arise from divalent Fe in octahedral coordination, in excellent agreement with the structural and chemical data.

Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy

Abstract The crystal-chemical relationships occurring within a single grain of monazite-(Ce) from Vetralla, Vico Volcanic Complex, north of Rome, are outlined. The sample is from a miarolitic cavity in a holocrystalline ejectum consisting of K-feldspar plus minor plagioclase, mica and Fe-oxides, collected from a pyroclastic explosive level. The Gandolfi film (Cu-Kα radiation) can be indexed in space group P21 /n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Å ,β = 103.63(3)° :V = 299.0(6) Å3. Electron-probe microanalyses plot within the field of monazite along the huttonite-monazite edge of the huttonite-monazite-brabantite triangle. Despite patchy and irregular zoning, the grain shows a clear enrichment towards pure monazite at the outer rim. A constant Th:Si ratio of 1:1 indicates the existence of a simple solid-solution between huttonite and monazite. The substitution can be written as Th4+ + Si4+ → REE3+ + P5+ without requiring any electrostatic compensation by divalent cations, or by anionic groups. The REE distribution pattern is compatible with that of monazites from syenitic rocks.

Crystal chemistry of ferroan phlogopites from the Albano maar lake (Colli Albani volcano, central Italy)

This work considers micas present both as phenocrysts in granular xenoliths and as large xenocrysts in the phreatomag-matic deposits, which characterize the final volcanic activity of Colli Albani volcanic complex, Roman Comagmatic Region (central Italy). In these micas the Fe/(Fe+Mg) ratio spans from 0.06 to 0.55, thus the micas range from phlogopite to magnesian annite through ferroan phlogopite. Heterovalent octahedral substitutions involve Ti, Al and also Cr but in very limited amounts. In the interlayer, Ba for K substitution occurs up to 0.06 apfu. Crystal structure refinements were determined in space groups C 2/m and C 2/c for the 1M and 2M 1 polytypes, respectively (agreement factor, R obs : 0.021 ≤ R obs ≤ 0.035). The crystal structure of these micas is greatly affected by the annitic substitution. An increase in the annitic component is reflected by a decrease in distortion affecting both individual tetrahedra (tetrahedral angle variance, TAV: 0.49 ≤ TAV ≤ 1.13° 2 ) and the entire tetrahedral ring [6.6 ≤ α ≤ 10.34°]. The interlayer topology is affected as well. A strong correlation exists between Fe and Ti; thus, the same correlations found for Fe apply to Ti also. Another strong correlation exists between heterovalent octahedral substitutions and the (OH)- site deprotonation. In contrast, heterovalent interlayer substitutions are accompanied by substitution of F- for (OH)-. Despite their scattered occurrence in pyroclastics, these micas nicely represent the overall crystal chemical trends of micas, which re-crystallized undisturbed in magma before this reached explosive conditions. One crystal of the dataset considered may represent a phenocryst in a hypabissal body disrupted by one of the many explosive phases of the Alban Hills volcanism.