1,721,139 research outputs found
Valutazioni preliminari dell’impiego di zeoliti di neosintesi nella bonifica di suoli contaminati da metalli pesanti
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Impiego della sintesi in situ di zeoliti da fly ash per il recupero di suoli contaminati da metalli pesanti e da xenobiotici organici: studi preliminari
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Ba-rich celestine: New data and crystal structure refinement
No full textA Ba-rich celestine (Sr0.87Ba0.13SO4) filling cavities of volcanoclastic rocks from Montecchio Maggiore (Vicenza, Italy) was studied. The role of the Ba content in the mineral was determined using X-ray powder data, single crystal X-ray refinement, thermal and chemical analyses. The unit cell parameters (obtained by single crystal diffraction) are a = 8.408, b = 5.372, c = 6.897 Angstrom, and the refinement in the space group Pnma (Z = 4) gives the final R value of 0.039. The average (Sr,Ba)-oxygen bond length is 2.842 Angstrom and agrees with an occupancy of Sr 87% and Ba 13%. Individual bond lengths (Sr,Ba)-O and bond strength calculations confirm that all twelve interactions are significant and define an irregular array around the cation
Refinement of the structure of natural ferriphlogopite
No full textTwo ferriphlogopite-1M crystals with a composition (K0.99Na0.01)(Sigma=1.00)(Mg2.73Fe0.172+Fe0.083+-Ti-0.01)(Sigma=2.99)[(Fe0.953+Si3.05)(Sigma=4.00)O-10.17] (OH)(1.79)F-0.04 (sample S1) and (K-1.02)(Sigma=1.02)(Mg2.68Fe0.202+Fe0.113+Mn0.01)(Sigma=3.00)(Fe0.953+Si3.05)(Sigma=4.00)O-10.18](OH)(1.75)F-0.07 (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 Angstrom, b = 9.288 Angstrom, c = 10.321 Angstrom and the monoclinic beta angle was: beta = 99.99 degrees. For sample S2, a = 5.3649 Angstrom, b = = 9.2924 Angstrom, c = 10.3255 Angstrom and the monoclinic beta angle was: beta = 99.988 degrees. The tetrahedral rotation angle of alpha = 11.5 degrees 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
V3+, Cr3+ and Mn2+ uptake by sepiolite and sedimentary zeolites.
No full textThe aim of this study was to investigate the potential usefulness of minerals in industrial waste-water treatment. Fixed beds, one of a zeolite-rich rock (zeolitite) and the other of a phyllosilicate (sepiolite), were treated with a soln. of heavy-metal cations (V3+, Cr3+, Mn2+) in equal concns. of 3.33×10-3 N. To ascertain the different rates heavy metal uptake and subsequent release, the beds were eluted with a Na+ soln. (3.33 × 10-3 N) and the cations (V3+, Cr3+, Mn2+, Na+, Mg2+, K+, Ca2+) in soln. measured. At first, the heavy-metal cation level in the effluent was consistently low for both beds; thereafter, it was obsd. that: (i) the ability of sepiolite to retain V3+ and Cr3+ is greater than that of zeolitite, whereas zeolitite reveals greater affinity for Mn2+; (ii) the max. heavy metal release occurs in the initial stage and slows down as the amt. of influent soln. increases; (iii) the residue of heavy metals in the beds after elution does not significantly affect the mineral structure
Sepiolite and industrial waste-water purification: Removal of Zn2+ and Pb2+ from aqueous solutions
No full textThe interaction between sepiolite and Zn2+ and Pb2+ aqueous solutions has been studied in order to verify the heavy-metal cation sorption-desorption rates. To this end, weighed amounts of sepiolite were placed in two conventional chromatographic columns and percolated at constant temperature, flow rate, pH and percolating solution concentration (0.5 x 10(-2) M, with Zn2+ or Pb2+, Mg2+ and 10(-2) M Na+, respectively). The amount of cations retained and released by the sepiolite bed in the effluent solution was analyzed by atomic absorption and UV-Visible spectrophotometries. The experimental results indicate that: (i) heavy-metal cation sorption can occur on the surface of broken edges, in channels and at specific sites according to crystal chemical affinity: Mg2+ sites are mostly affected by Zn2+-treatment and Ca2+ sites by Pb2+- treatment (Zn2+-treated bed, Mg2+/Ca2+ = 0.627; Pb2+-treated bed, Mg2+/Ca2+ = 0.475); (ii) repeated treatments with heavy-metal cation solutions produce a decrease in adsorption and/or exchange capacity of the sepiolite bed; however, even when sepiolite is treated with alkali and earth alkali cations, it is able to retain a heavy-metal cation fraction, the retention of Zn2+ being greater than that of Pb2+. In order to investigate the chemical variation induced in sepiolite crystals by interaction between Zn2+ and Pb2+ solutions, a natural sample was treated with 1 M Zn2+ and 1 M Pb2+ solutions and then analyzed by X-ray powder diffraction and by differential thermal and thermogravimetric techniques
The origin and significance of euhedral apatite crystals on conodonts
Full text linkCrystal overgrowth on fossil remains is well-documented in the literature. Attention has specifically focused on
bioapatite (i.e., an apatite of biochemical origin regardless of post-mortem changes) configurations, in order to
decipher any possible relation to fossilization/diagenesis. This study investigates the Rare Earth Element (REE)
and other High-Field-Strength Element (HFSE) composition of euhedral crystals formed on the surface of conodont elements compared with that of crystal-free surfaces. Euhedral crystals are by definition crystals characterized by sharp faces, developing solids that, for apatite, assume the form of hexagonal prisms, reflecting its crystal symmetry. Late Ordovician (Amorphognathus ordovicicus Zone) conodonts from two localities in Sardinia and the Carnic Alps (Italy) are herein investigated. Conodont elements reveal the occurrence of smooth surfaces and surfaces partially covered with euhedral crystals. Since euhedral crystals did not reasonably grow during the organism’s lifetime, the REE and HFSE analysis can provide important insights into the crystal growth process. The experimental results indicated a substantial contribution of diagenetic imprinting for all the analyzed material, although more evident on euhedral crystals that are significantly enriched in middle and, subordinately, in heavy REE with respect to smooth surfaces. The positive correlations between La + Th vs log[ΣREE] and Ce + Th
vs log[ΣREE] could support the hypothesis that the neoformed euhedral crystals grew also by depleting the pristine bioapatite of the conodont elements. Nevertheless, the occurrence of two types of apatite cannot be ruled out: euhedral crystals as neoformed products of diagenetic processes and smooth surfaces as remains of the pristine conodont bioapatite after diagenesis
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