Luca Medici

Interaction between montmorillonite and pollutants from industrial waste-waters: exchange of Zn2+ and Pb2+ from aqueous solutions

Abstract The interactions between montmorillonite (sample SAz-1 from the Source Clay Repository of the Clay Minerals Society) and Zn2+ and Pb2+ solutions of different ionic strengths (from about 10−5 to 1 M) are studied in order to observe changes in the clay-solution system and mineral crystal chemistry. In both types of solution the stationary state of exchange, attained within 20 min, depends on the solutions ionic strength. The uptake can be interpreted by means of a Langmuir-type equation with Zn2+ somewhat more easily exchanged than Pb2+. The shifting of the d(001) spacing from 15.3 A (natural sample) to 12.8 A (Zn2+-exchanged montmorillonite) and 12.4 A (Pb2+-exchanged montmorillonite) suggests that the kind of interlayer cation affects the c dimension of the layer. Upon heating, the exchanged montmorillonite layer collapses (d(001),Zn = 9.5 A; d(001),Pb = 10.0 A) and the closest packing is attained at T = 200°C for Pb2+ - and at 380°C for Zn2+-exchanged samples. In addition to reactions observed for natural and Pb2+-exchanged samples in the temperature ranges 20–250°C and 500–700°C, the thermal analyses of Zn2+-exchanged montmorillonite show a reaction at about 380°C which can be related either to strongly bonded water molecules or to “brucitic-like” interlayers or to Zn2+-exchanged octahedra.

Crystal chemistry and petrologic significance of Fe3+-rich phlogopite from the Tapira carbonatite complex, Brazil

Abstract This contribution deals with the crystal chemistry of phlogopite and Fe3+-rich phlogo- pite from the Tapira alkaline-carbonatite complex (Brazil) to assess the petrological significance and genetic conditions of these rocks. The Tapira complex consists of a layered intrusion composed mainly of ultramafic rocks (dunite, wehrlite, clinopyroxenite, bebedourite, gamet-magnetitite, perovskite-magnetitite, and glimmerite) with subordinate car- bonatite. The wide range of textural, optical, and crystal-chemical characteristics of phlogopite is related to the variation of fO₂, aH₂O, and aCO₂ as well as magma bulk-chemical composition during fractional crystallization. Phlogopite from alkaline-silicate rocks (ranging from dunite to bebedourite) is characterized by fairly constant Al content, moderate [4]Fe3+ substitution, and variable amounts of Ti. The [4]Fe3+ substitution, accompanied by crystals showing reverse pleochroism, increases during fractional crystallization. These features correspond to crystallization at low pressure and high and aH₂O in the presence of moderate saturation in Ti-bearing phases, Al2O3 in the magma, or both. Phlogopite from silicate-carbonatite rocks, classified as ferriphlogopite on the basis of strong reverse pleochroism related to [4]Fe3+ tetrahedral substitution, also presents low to very low Al, Fe2+, and Ti contents. These features suggest very high fO₂, H2O, and CO2 conditions in the presence of strong saturation in Ti-bearing phases as well as very low Al2O3 content in the liquid. The crystal-structure refinements of Tapira phlogopite show that Fe3+ substitutes for Si in tetrahedral sites; Fe distribution is completely disordered, so the resulting space group is C2/m. The octahedral-site composition is similar to that of phlogopite, the octahedral sites being preferentially occupied by Mg. The presence of Fe3+ in the tetrahedral sheet enlarges the whole structure. This enlargement is reflected by an increase in cell-edge lengths and a decrease in β-angle values. The increase in distortion of the tetrahedral ring (a angle up to ~ 11°) is necessary for the tetrahedral and octahedral sheets to fit together.

Interaction between aqueous chromium solutions and layer silicates

Abstract The interactions between Cr in aqueous solutions and phyllosilicates were studied to determine: (a) the amount of Cr(VI) to Cr(III) reduction in aqueous solutions by Fe(II)-bearing phyllosilicates; (b) the removal of the Cr species from solution by interaction with phyllosilicates, as a function of Cr(III) concentration and anionic environment. Chlorite, corrensite and montmorillonite were reacted with solutions containing Cr(VI) (1.62×10 −3 N, 5.77×10 −3 N and 1.32×10 −1 N, respectively). The sorption/desorption of Cr(III) by saponite was investigated in different anionic environments (Cl − , NO − 3 and CH 3 COO − ) and at different initial Cr(III) concentrations (3.21×10 −3 N, 5.49×10 −3 N and 8.49×10 −3 N). The extent of Cr(VI) reduction and the amount of Cr removed by phyllosilicates were measured by analysis of the liquid portion separated by centrifugation after controled periods of exposure. The minerals were studied by chemical, thermal and X-ray powder diffraction analyses. The results show that: (i) Fe(II)-bearing phyllosilicates sorb Cr and reduce Cr(VI) to Cr(III); (ii) the extent of reduction depends on the solution concentration and on mineral crystal chemistry; (iii) Cr(III) sorption isotherms show that the degree of uptake depends both on the initial concentration of metal in solution and on the anionic environment, the order of effectiveness being Cl − ≅NO − 3 >CH 3 COO − ; (iii) Cr(III) is retained in the mineral substrate and its release is difficult.

Solving mercury (Hg) speciation in soil samples by synchrotron X-ray microspectroscopic techniques

Direct mercury (Hg) speciation was assessed for soil samples with a Hg concentration ranging from 7 up to 240 mg kg(-1). Hg chemical forms were identified and quantified by sequential extractions and bulk- and micro-analytical techniques exploiting synchrotron generated X-rays. In particular, microspectroscopic techniques such as mu-XRF, mu-XRD and mu-XANES were necessary to solve bulk Hg speciation, in both soil fractions <2 mm and <2 microm. The main Hg-species found in the soil samples were metacinnabar (beta-HgS), cinnabar (alpha-HgS), corderoite (Hg(3)S(2)Cl(2)), and an amorphous phase containing Hg bound to chlorine and sulfur. The amount of metacinnabar and amorphous phases increased in the fraction <2 microm. No interaction among Hg-species and soil components was observed. All the observed Hg-species originated from the slow weathering of an inert Hg-containing waste material (K106, U.S. EPA) dumped in the area several years ago, which is changing into a relatively more dangerous source of pollution.

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.

Refinement of the structure of natural ferriphlogopite

Two ferriphlogopite-1M crystals with a composition (K0.99Na0.01)Σ=1.00(Mg2.73Fe2+0.l7Fe3+0.08-Ti0.01)Σ=2.299[(Fe3+0.95Si3.05)Σ=4.00O10.17](OH)1.79F0.04 (sample S1) and (K102)Σ=1.02(Mg2.68Fe2+0.20Fe3+0.11-Mn0.01)Σ=3.00[(Fe3+0.95Si3.05)Σ=4.00O10.18](OH)1.75F0.07 (sample S2) occur within an alkali-carbonatic complex near Tapira, Belo Horizonte, Minas Gerais, Brazil. Each crystal was studied by single-crystal X-ray diffraction. The least-squares refinements of space group C2/m resulted in R values of 0.031 for S1 and 0.025 for S2. Results showed that Fe3+ substitutes for Si within the tetrahedral sites and that the Fe distribution is fully disordered. The octahedral sites are preferentially occupied by Mg. The presence of Fe3+ within the tetrahedral sheet produces increased cell edge lengths. For sample S1, a = 5.362 Å, b = 9.288 Å, c = 10.321 Å and the monoclinic β angle was: β = 99.99°. For sample S2, a = 5.3649 Å, b = 9.2924 Å, c = 10.3255 Å and the monoclinic β angle was: β = 99.988°. The tetrahedral rotation angle of a = 11.5° is necessary for tetrahedral and octahedral sheet congruency. The enlarged tetrahedral sites are regular, with cations close to their geometric center. Ferriphlogopites have identical mean bond lengths for M1 and M2 sites within standard deviation. The M1-O3 and M2-O3 bond lengths are longer than the mean so that O3 may articulate with the tetrahedra.

The influence of layer charge on Zn (super 2+) and Pb (super 2+) sorption by smectites

Abstract Two smectites with different layer charge localization were used to characterize the chemical and structural aspects of minerals treated with 1.9 × 10-2 м and 1 м heavy metal solutions (Zn2+ and Pb2+). Natural and treated smectites were analysed by: (1) X-ray powder diffraction first at 25℃ and relative humidity of 60% and then at increasing temperature up to 400℃ (2) thermogravimetric and differential thermal analysis; (3) X-ray fluorescence; (4) atomic absorption and UV visible spectrophotometries. The sorption of both metals depends on 2:1 layer features and is enhanced when the total layer charge is mostly due to substitution in the octahedral sheet. The variability between Zn2+- and Pb2+-treated smectites in the interlayer water content depends both upon cation electrostatic features and upon smectite layer charge localization. The greatest water content was observed in [Zn2+]1 м-treated smectite showing charge imbalance in the octahedral sheet. In smectites treated with [Zn2+]1 м solutions, an additional thermal reaction occurs at a temperature of ~380℃ and the d(001) spacing, which is 10 ,~ at a temperature of ~200℃, gradually decreases in the temperature range 200-400℃. In smectites treated with 1 м [Pb2+] solutions this value is reached at a temperature of ~150℃ and remains quite constant up to 400℃.

Crystal structure refinement of aluminian lizardite-2H 2

Abstract Well-crystallized euhedral crystals of aluminian lizardite-2H2 (Mg2.35Fe2+0.06Fe3+0.07Al0.52) (Si1.41Al0.59)O5.00(OH)4.00 were found near Schio (Vicenza, Italy). To gain insight into the role of a high Al content lizardite, chemical analyses and single-crystal X-ray data collection were conducted. Structure refinement, completed in space group P63 (agreement factor R = 0.034), gives mean T-O values of 1.654 Å and 1.664 Å for T1 and T2 sites, respectively. The ditrigonal distortion of the six-membered tetrahedral ring is positive (α = +9.7°), as expected for the 2H2 polytype. The octahedral site has a mean bond length similar to that of the Mg-rich octahedra of amesite and distortion parameters similar to those of Al-rich octahedra.

Characterization of Cu-complexes in smectite with different layer charge location: chemical, thermal and EXAFS studies

Abstract The retention of Cu and Cu-amino acid complexes by montmorillonite and beidellite, before and after repeated acidified aqueous solution treatments, was studied using X-ray diffraction, chemical and thermal analyses, mass spectrometry and synchrotron-based X-ray absorption spectroscopy (XAS). The results indicate that the extraction of metal complexes from smectites depends on the nature of the layer charge and on the kind of organic species. Cu-cysteine complexes are strongly retained in the interlayer position, whereas Cu-glycine complexes are mostly adsorbed in cationic form which can be easily removed from the silicate layer. The layer periodicity for Cu-smectites treated with glycine shows little or no layer expansion, whereas significant swelling of the layer periodicity is observed in smectites treated with cysteine. Thermal decomposition of both smectites with sorbed Cu-amino acid species shows the evolution of H 2 O, NO, CH 3 CH 3 , and CO 2 . In Cu-cysteine treated smectites, the release of H 2 S, NO 2 , SO 2 , and N 2 O 3 also occurs. X-ray absorption spectroscopy (XAS) was used to assess the relationships between the structure of the Cu complexes and their desorption from smectites. In Cu-exchanged smectites, the first coordination shell agrees with the hypothesis that the Cu coordinates to oxygen atoms to form monomer and/or dimer complexes. The first coordination shell of Cu in smectites treated with glycine shows four atoms at distances of ∼2 A. Two of these bonds are with nitrogen and two with oxygen atoms. For copper-cysteine complexes XAS data are compatible with the existence of Cu-N clusters, thus suggesting that Cu links to the amino acid by the aminic group.

Clinohydroxylapatite: a new apatite-group mineral from northwestern Ontario (Canada), and new data on the extent of Na-S substitution in natural apatites

A monoclinic analogue of the mineral hydroxylapatite was found in altered leucogabbro making up one of the intrusive units in the Mesoproterozoic Seagull pluton in northwestern Ontario, Canada. The mineral is part of a replacement assemblage developed metasomatically after igneous plagioclase. It occurs as masses of coalescent spherulites < 30 μm in diameter, and is paragenetically associated with prehnite, hibschite, titanite, rutile and an unidentified Ca silicate. The new mineral is white and chalky in appearance, has a Mohs hardness of 5 and a measured density of 3.07(2) g/cm3 ( D calc 3.13 g/cm3), and shows little deviation from a uniaxial optical behavior (α 1.632, γ 1.649). Compositionally, it is an intermediate member of the ternary system Ca5(PO4)3(OH) - Ca5(PO4)3Cl - Na3Ca2(SO4)3(OH). The average content of the latter two end-members is ca. 20 mol. % each. The combination of high Na and S contents (up to 0.63 and 0.71 apfu, respectively) is unparalleled by any previously reported natural composition, and indicates significant solubility of these elements in natural apatites even at low crystallization temperatures (< 400 °C). The monoclinic symmetry of this mineral, determined from microbeam X-ray diffraction patterns, distinguishes it from hydroxylapatite. By analogy with synthetic Ca5(PO4)3OH, its space group is P 21/ b [ a 9.445(2) A, b 18.853(4) A, c 6.8783(6) A, γ 120.00(2)°], and the deviation of symmetry from the archetypal (space group P 63/ m , a ∼9.4 A, c ∼6.9 A) probably results from ordering of (OH) anions in [00z] anionic columns and consequent doubling of periodicity along [010]. In keeping with the nomenclature of apatites, this new mineral was named clinohydroxylapatite.