67 research outputs found
The behavior at non-ambient conditions of colemanite: a hydrous Ca-borate
No full textColemanite, CaB3O4(OH)3·H2O, is a common hydrous borate of large economic relevance, as it is one of the major commodities of boron with applications in the fields of glass and ceramic industries. Colemanite-rich layers are usually found in stratigraphic successions related to lacustrine basins in semi-arid to arid environments, associated to a local volcanic activity, which provides the source for boron. Despite the large economic relevance, the behavior of this mineral at non-ambient conditions of temperature and pressure was almost unexplored, which can provide a basis for understanding its stability during diagenetic and metamorphic processes. In this contribution, we report the highpressure behavior of colemanite (Lotti et al., 2017), based on in situ single-crystal synchrotron X-ray diffraction data up to 24 GPa, and its low-T behavior by in situ X-ray and neutron single-crystal diffraction. Colemanite was found to be stable up ~ 14.5 GPa, where a reconstructive phase transition towards a high-pressure polymorph (colemanite-II) with same symmetry (space group P21/a), but a six times larger unit cell volume, occurs. The elastic behavior of colemanite was described by fitting the experimental data with a III-order Birchurnaghan equation of state, yielding the following refined elastic parameters: KV0 = 64(4) GPa and KV' 5.5(7). The colemanite-tocolemanite-II phase transition induces an increase in the average coordination number of both the Ca and B cations. In particular, a fraction of the boron sites increases its coordination from triangular to tetrahedral by making a further bond with a H2O-oxygen atom. Although the phase transition occurs (at ambient temperature) at pressures far from those associated with the usual geologic environments of colemanite, the reported results disclosed flexible deformation mechanisms that borate compounds may adopt to accommodate pressure, thus providing new insights on the behavior of borate minerals at non-ambient conditions. The complex hydrogen-bonding network of olemanite has also been characterized, at ambient and low temperature conditions, by means of in situ single-crystal synchrotron X-ray and neutron diffraction experiments. A positional disorder, related to the presence of two mutually exclusive configurations of the H2O-molecule hydrogen atoms, was found both above and below ~ 0°C, where a displacive phase transition from the P21/a to the P21 space group occurs.
Lotti, P., Gatta, G.D., Comboni, D., Guastella, G, Merlini, M., Guastoni, A., Liermann, H.-P. (2017): High-pressure behavior and Pinduced phase transition of CaB3O4(OH)3*H2O (colemanite), J. Am. Ceram. Soc., in press, DOI: 10.1111/jace.14730
High-temperature behavior and dehydration of the natural borate colemanite
No full textColemanite, CaB3O4(OH)3*H2O, is the most common natural hydrous calcium borate and one of the most important mineral commodities for boron. It is mainly extracted from Turkish deposits, which form for the chemical precipitation of colemanite from relatively diluted waters of lacustrine basins, hosted in continental semi-arid to arid environments and fed by B-rich hydrothermal springs related to a local volcanic activity (e.g. Helvaci & Alonso, 2000). Due to its common occurrence in waste rocks at extraction sites, several studies have been focused in the recent past on the exploitation of colemanite as raw material, for example as an additive in the production of lightweight cements (Targan et al., 2003) or radiation shielding materials (Glinicki et al., 2018). However, despite the relevance beared in the applications of colemanite, we still have only a partial knowledge of the high-temperature behavior of this mineral compound.
In this study we investigated the high-temperature behavior of a natural sample of colemanite from the Bigadiç deposit (Turkey), by means of in situ powder synchrotron X-ray diffraction, performed at the MCX beamline of Elettra (Trieste, Italy), using a hot blower device.
The refined unit-cell parameters and structures show a significantly anisotropic thermal expansion, which is only accommodated along the b and c crystallographic axes, whereas the a axis, corresponding to the direction of the chains of B-coordination polyhedra, is almost unaffected. Between 275 and 325 °C, a reduction in the unit-cell volume and a decrease in the refined occupancy of the H2O-oxygen site, reveal the occurrence of a dehydration mechanism, which is followed by a complete and irreversible amorphization between 325 and 375 °C.
Merging the V-T data of this experiment with those obtained in a previous low-T experiment on the same natural sample (Lotti et al., 2018), a thermal Berman equation of state was refined in the range from -171 to 250 °C (104-525 K), yielding a thermal expansion coefficient at ambient-T (αV298K) of 4.5(1)*10-5 K-1 and α1 = 5.7(8)*10-8 K-2.
Glinicki M.A., Antolik A. & Gawlicki M. 2018. Evaluation of compatibility of neutron-shielding boron aggregates with Portland cement in mortar. Constr. Build. Mater., 164, 731-738.
Helvaci C. & Alonso R.N. 2000. Borates Deposits of Turkey and Argentina: A Summary and Geological Comparison. Turkish J. Earth Sci., 9, 1-27.
Lotti P., Gatta G.D., Demitri N., Guastella G., Rizzato S., Ortenzi M.A., Magrini F., Comboni D., Guastoni A. & Fernandez-Diaz M.T. 2018. Crystal chemistry and temperature behavior of the natural hydrous borate colemanite, a mineral commodity of boron. Phys. Chem. Minerals, in press DOI: https://doi.org/10.1007/s00269-017-0929-7.
Targan S., Olgun A., Erdogan Y., & Sevinc V. 2003. Influence of natural pozzolan, colemanite ore waste, bottom ash, and fly ash on the properties of Portland cement. Cement Concrete Res., 33, 1175-1182
Tertiary LCT and NYF pegmatites of the central Alps
No full textA large field (about 100 km in E-W length and 15 km in N-S thickness) of Oligocene pegmatites extends in the
central Alps from the Bergell pluton (to the east) to the Ossola Valley (to the west) within the Alpine nappes north
of the Periadriatic Lineament. The pegmatite field geographically overlaps (i) the highest temperature domain
of the Lepontine Barrovian metamorphic dome, and (ii) the zone of Alpine migmatization. Most pegmatites
have a simple mineralogy consisting of K-feldspar, quartz and muscovite, but a minor amount (< 5%) includes
Sn-Nb-Ta-Y-REE-U oxides, Y-REE phosphates, Mn-Fe-phosphates, Ti-Zr-silicates, Be-Y-REE-silicates, garnet
(almandine-spessartine), and schorl-dravite-fluorelbaite tourmaline.
Major and trace elements geochemistry of pegmatite bulk rock, rock-forming and accessory minerals al-
lows the distinction of different pegmatite populations ranging from NYF (niobium, yttrium, fluorine) to LCT
(lithium, cesium, tantalum) pegmatites, or mixed LCT-NYF ones. Actually, LCT pegmatites of the Central
Alps did not reach a high degree of geochemical evolution. In the Codera Valley (on the western side of the
Bergell pluton) LCT and NYF pegmatites are respectively hosted in tonalites and granodiorites; these pegmatites
include the most evolved types which contain Mn-fluorelbaite, Mn-phosphates, pink-beryl and Cs-Rb-rich feldspar.
From the structural point of view 2 main types of pegmatites can be distinguished: (i) pegmatites that were
involved in ductile deformation, and (ii) pegmatite crosscutting the ductile structures of the SSB. Many peg-
matites from Codera Valley belong to the first structural type: they were at emplaced at relatively high ambient
temperatures (> 450
◦
C) and locally show pervasive recrystallization of quartz. More to the east (Mesolcina and
Bodengo Valleys) the main set of pegmatites crosscut the ductile deformation structures of the SSB, but the area
also includes an earlier generation of boudinaged and folded dykes. The undeformed pegmatites from this area
may contain large miarolitic pockets. There is no systematic difference in mineralogy and geochemistry between
the 2 structural types of pegmatites.
Structural data and the few existing radiometric ages suggest that pegmatites were emplaced over a time
span between 29 and 25 Ma with the youngest dykes postdating the ductile deformations of the Alpine nappes.
The present work presents a first comprehensive geochemical and mineralogical classification of the Oligocene
pegmatite field of the central Alps. In order to constrain the timing of pegmatite formation monazite and xenotime
have been sampled from the different generations of pegmatite
Xenotime-(Y) and Sn-rich thortveitite in miarolitic pegmatites at Baveno (Southern Alps, Italy).
No full textWhat is the actual structure of samarskite-(Y)? A TEM investigation of metamict samarskite from the garnet codera dike pegmatite (Central Italian Alps)
No full textWe investigated, by scanning and transmission electron microscopy (SEM, TEM), wavelength-and energy-dispersive spectroscopy (WDS, EDS), and electron diffraction tomography (EDT), several (Y-REE-U-Th)-(Nb-Ta-Ti) oxides from the Garnet Codera dike pegmatite (Central Italian Alps). These oxides have compositions in the samarskite-(Y) field and yield an amorphous response from the single-crystal X-ray diffractometer. Backscattered electron images reveal that the samples are zoned with major substitutions involving (U+Th) with respect to (Y+REE). At the TEM scale, the samples show a continuous range of variability both in terms of composition and in radiation damage, and the amount of radiation damage is directly correlated with the U-content. Areas with high U-content and highly damaged show crystalline, randomly oriented nanoparticles that are interpreted as decomposition products of the metamictization process. On the other hand, areas with lower U-content and radiation dose contained within 0.7 × 1016α-event/mg, although severely damaged, still preserve single-crystal appearance. Such areas, noticeably consisting of relicts of the original samarskite structure, were deeply investigated by electron diffraction techniques. Surprisingly, the retrieved crystal structure of untreated samarskite is consistent with aeschynite and not with ixiolite (or columbite), as believed so far after X-ray diffraction experiments on annealed samples. In particular, the resolved structure is a niobioaeschynite-(Y), with Pnma space group, cell parameters a = 10.804(1), b = 7.680(1), c = 5.103(1) Å, and composition (Y0.53Fe0.22Ca0.10U0.09Mn0.07)Σ=1(Nb1.07Ti0.47Fe0.34Ta0.07W0.06)Σ=2O6. If this finding can be confirmed and extended to the other members of the group [namely samarskite-(Yb), calciosamarskite, and ishikawaite], then the samarskite mineral group should be considered no longer as an independent mineral group but as part of the aeschynite group of minerals. It is finally suggested that the rare crystalline sub-micrometric ixiolite domains, occasionally spotted in the sample by TEM, or the nanoparticles detected in highly metamict areas interpreted as decomposition product of the metamictization process, which may have in fact the ixiolite structure, act as seeds during annealing, leading to the detection of ixiolite peaks in the X-ray powder diffractograms
Cobaltite-rich mineralisation in the iron skarn deposit of Traversella (Western Alps, Italy)
No full textCobaltite-rich mineralisation from the iron skarns of the Traversella magnetite mine (Western Alps, Italy) was studied by reflected-light microscopy, scanning electron microscopy and electron microprobe analysis. Cobaltite is found in carbonate–chlorite-rich rocks at the margins of the main magnetite masses, where it forms disseminations and metasomatic veinlets that postdate the formation of magnetite. The paragenesis includes cobaltite (± arsenopyrite), bismuthinite, pyrrhotite and/or pyrite, chalcopyrite, carbonates, talc, chlorite, and native gold, and is indicative of a low-sulphidation environment. The sulpharsenides show oscillatory and sector zoning, which indicates disequilibrium during crystal growth. Compositional variations are mainly due to variations in the Co/Fe ratio of arsenopyrite and in either the Co/Fe or the Ni/(Fe + Co) ratios of the coexisting cobaltite. The Ni contents are low to very low in the cobaltites (< 2.4 wt%) and very low in the arsenopyrites (< 0.16 wt%). The As/S molar ratios in the cobaltites are highly variable (0.59–1.00) and show a broad negative correlation with the Fe contents. The formation of cobaltite is related to circulation of relatively low-temperature (< ca. 300 °C), (Co, As, Bi)-rich fluids during the retrograde sulphidation stage which followed the formation of magnetite. The apparent restriction of cobaltite (+ bismuthinite ± arsenopyrite) to the margins of the main magnetite columns may reflect the establishment of thermo-chemical gradients around the main direction of infiltration of the retrograde metasomatic fluids
Arsenic-rich fergusonite-beta-(Y) from Mount Cervandone (Western Alps, Italy) : crystal structure and genetic implications
No full textAn As-rich variety of fergusonite-beta-(Y) occurs as greenish yellow pseudo-bipyramidal crystals up to 1 mm in length in centimeter-sized secondary cavities within sub-horizontal pegmatite dikes at Mount Cervandone (Western Alps, Italy). The mineral is associated with quartz, biotite, potassium feldspar, and orange-yellow barrel-shaped hexagonal crystals of synchysite-(Ce) up to 2 mm in length. Fergusonite-beta-(Y) crystallized during the Alpine metamorphism under amphibolite-facies conditions, as a result of interaction between As-enriched hydrothermal fluids, circulating through the pegmatite dikes, and precursor accessory minerals in the pegmatites enriched in high-field-strength elements. These pegmatites are of NYF (niobium-yttrium- fluorine) geochemical type and served as the principal source of Be, Y, Nb, Ta, and rare-earth elements (REE) that were liberated and redeposited as rare Be-As-Y-REE minerals, including the As-rich fergusonite-beta-(Y). The latter mineral crystallizes with monoclinic symmetry [a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V = 293.87(14) Å3, space group I2/a] and has the empirical formula (Y 0.70Dy0.07Er0.05Ca0.05Gd 0.02U0.02Yb0.01 Tb0.01Th 0.01Nd0.01)Σ0.95 (Nb 0.68As0.275+W0.06Ta 0.01Si0.01)Σ1.03O4. The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R1 = 6.6% for 441 observed reflections with Fo/σFo > 4. The incorporation of As in the structure of monoclinic fergusonite-type phases is discussed in the context of the data available for synthetic analogs
New insights into the crystal chemistry of epididymite and eudidymite from Malosa, Malawi : a single-crystal neutron diffraction study
No full textThe crystal chemistry of two dimorphic hydrated sodium beryllium silicates, epididymite (a =12.7334(4), b=13.6298(5), c=7.3467(3)Å, V=1275.04 Å3, space group Pnma) and eudidymite (a=12.6188(10), b=7.3781(5), c=13.9940(9)Å, β=103.762(5)°, V=1265.47Å3, space group C2/c) from Malosa, Malawi, has been reinvestigated by means of energy dispersive X-ray spectroscopy, thermo-gravimetric analysis, inductively coupled plasma-optical emission spectroscopy and single-crystal neutron diffraction. Two anisotropic structural refinements have been performed with final agreement index R1=0.0317 for 137 refined parameters and 2261 unique reflections with Fo>4σ(Fo) for epididymite and R1=0.0478 for 136 refined parameters and 1732 unique reflections with Fo>4σ(Fo) for eudidymite. The analysis of the difference-Fourier maps of the nuclear density of the two dimorphs confirms the presence of extra-framework water molecules in both the dimorphs, and not hydroxyl groups as wrongly reported in previous studies and in several crystal structure databases. The real chemical formula of edipidymite and eudidymite is Na2Be2Si6O15•H2O (Z=4). The configuration of the water molecules and the hydrogen bonds are fully described for both the dimorphs. The chemical analysis shows that a low, but significant, amount of Al and Fe (most likely substituting Si in the tetrahedral sites) and K (substituting Na as extra-framework cation) occurs in both the two dimorphs
The crystal structure of dalnegroite, Tl5-xPb2x(As,Sb)21-xS34: A masterpiece of structural complexity
No full textThe crystal structure of the rare mineral dalnegroite, Tl5xPb2x(As,Sb)21xS34 with x & 1, was
determined for a crystal from Lengenbach, Binn Valley, Switzerland. The structure is triclinic, space
group P1, with a = 16.218(3), b = 42.546(7), c = 8.558(1) A ̊ , a = 95.70(4), b = 90.18(3), g = 96.38(4)o,
V = 5838.9(9) A ̊ 3, Z = 4. Refinement of an isotropic model led to an R1 index of 0.0536 for 22226
observed reflections and 980 parameters, and R1 = 0.0590 for all 25266 independent reflections.
Although dalnegroite cannot be considered a layered compound, its structure can be usefully described
as a regular alternation of two kinds of layers stacked along the b axis, with four layers in the unit cell:
(1) a layer 7.8 A ̊ thick, at y & 0.15 and 0.65, can be considered as derived from the SnS archetype;
(2) a layer 13.6 A ̊ thick, at y & 0.35 and 0.85, derived from the PbS archetype. Different chemical
compositions, such as Tl:Pb and Sb:As ratios, for different samples belonging to the chabourne ́itedalnegroite
family could play a central role in controlling different degrees of order, leading to
different superstructures
Effects of different peritoneal dialysis fluids on the TH1/TH2 balance
No full textBackground. Peritoneal dialysis (PD) is associated with a depression of T cell function, as suggested by the impaired production of cytokines by Th cells collected from PD patients. Although treatment biocompatibility could be implicated in this immune dysfunction, it has been poorly investigated, thus far. Therefore, we undertook a study aiming to analyze the effects of different peritoneal dialysis fluids on the Th1/Th2 balance in PD patients. Methods. Twenty three patients on continuous ambulatory peritoneal dialysis (CAPD) were evaluated. Seven patients were on CAPD with icodextrin solution (ICO-PD), seven with glucose and lactate/bicarbonate-buffered solution (LAC/BIC-PD), and nine with glucose and lactate-buffered solution (LAC-PD). The Th1/Th2 balance was evaluated by measuring IFN-gamma (Th1 subset) and IL-4 (Th2 subset), both in circulating and peritoneum-derived Th lymphocytes unstimulated or stimulated by phytohemoagglutinin (PHA). Moreover inflammatory, nutritional and dialysis-related parameters were recorded. Eight normal subjects comprised the control group (CON). Results. Circulating T cells: IFN-gamma was significantly lower in the LAC-PD group (p<0.05) compared to the ICO-PD and LAC/BIC-PD groups. The IFN-gamma/IL-4-producing cell ratio was significantly lower in PD patients than in CON. Peritoneal T cells: after 24-h PHA stimulation, IFN-gamma increased in all patients, but the rise was less pronounced in the LAC-PD group (p<0.05) than in the other two PD groups. The Th1/Th2 ratio was significantly lower in the LAC-PD group when compared both to LAC/BIC-PD and ICO-PD groups. In addition, the LAC-PD group presented a significantly higher rate of peritoneal infections compared to the other PD groups. Conclusions. CAPD with lactate-buffered peritoneal fluid has deleterious effects on the Th1 cell subset, while the use of more biocompatible fluids, bicarbonate-buffered and icodextrin, is associated with a more physiologically representative Th1/Th2 balance and a reduced peritonitis rate
- …
