1,721,434 research outputs found
Clockwise and anticlockwise P-T path in the axial zone of the Variscan Sardinia-Corsica block
No full textRecent petrological studies on the Axial Zone of the Variscan chain have revealed the occurrence of anticlockwise P-T trajectories in metamorphic rocks from Corsica and Sardinia. These new findings, at first glance, appears to be in contrast with the clockwise P-T paths classically described for several high-pressure metamorphic rocks from the Variscan Sardinia - Corsica block in the last decades.
For the first time in Variscan Corsica, Massonne et al. (2018) described the anticlockwise P-T path of micaschists from the Porto Vecchio region. These rocks underwent prograde metamorphism starting at 3 kbar/600°C and then reached peak metamorphic conditions at 7 kbar/600-630°C. The Porto Vecchio micaschists are bordered to the north by the Solenzara-Fautea metamorphic rock unit that is, in turn, characterized by migmatites and granulites that underwent a clockwise P-T loop related to burial of continental crust to high-pressure (1.8–1.4 GPa) followed by high- to ultrahigh-temperature conditions (900–1000 °C; Giacomini et al., 2008). New, preliminary results on these rocks reveal that these latter peak temperatures could be probably significantly lower than those above mentioned, but there is no doubt about the clockwise shape of their P-T path. In the neighbouring Variscan terrains of north-eastern Sardinia, Scodina et al. (2019) reconstructed the P-T history of an amphibolite body whose burial path started at 0.8 GPa and was then followed by a slight increase in temperature up to peak metamorphic conditions of 1.3–1.4 GPa/690–740 °C. These amphibolites are hosted by migmatites for which a clockwise P-T path, related to a nearly isothermal exhumation, is widely accepted in the literature and confirmed by the most recent studies (Massonne et al., 2013 and references therein). In conclusion, it thus appears that during the Variscan collision, rock slices of different crustal levels were brought together and metamorphic rocks from upper and lower plates were involved in a particle path along an exhumation channel.
Giacomini F., Dallai L., Carminati E., Tiepolo M. & Ghezzo C. 2008. Exhumation of a Variscan orogenic complex: Insights into the composite granulitic–amphibolitic metamorphic basement of southeast Corsica (France).J. Metam. Geol., 26, 403–436.
Massonne H.-J., Cruciani G. & Franceschelli M. 2013. Geothermobarometry on anatectic melts - a high-pressure Variscan migmatite from northeast Sardinia. Int. Geol. Rev., 55, 1490–1505.
Massonne H.-J., Cruciani G., Franceschelli M. & Musumeci G. 2018. Anticlockwise pressure–temperature paths record Variscan upper-plate exhumation: example from micaschists of the Porto Vecchio region, Corsica. J. Metam. Geol., 36, 55–77.
Scodina M., Cruciani G., Franceschelli M. & Massonne H.-J. 2019. Anticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block. Lithos, 324–325, 763–775
Cruciani G., Orlandi P., Pasero M. and Russo M.: First italian occurrence of cumengeite from Vesuvius: crystal-structure refinement and revision of the chemical formula
No full textThe ferroelastic phase transition in ZSM-5 zeolites: chemistry vs. thermodynamic
No full textNo abstract availabl
The ferroelastic phase transition in ZSM-5 zeolites: chemistry vs. thermodynamic
No full textNo abstract availabl
Two stage of garnet growth in mylonitic micaschist from NE Sardinia: evidence from major and trace elements in garnet
No full textMylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan consist of garnet porphyroblasts associated with plagioclase, quartz, biotite, staurolite, white mica, and chloritoid. The garnet porphyroblasts, enveloped by the S2 schistosity, preserve an internal foliation identified by the alignment of quartz, sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth. Major element compositional variation follows a bell-shaped zoning profile with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core (Alm45Grs25 Prp1Sps29) to the outer rim domain (Alm86Grs3Prp11Sps1; Cruciani et al., 2022). LA-ICPMS trace element mapping revealed
that the boundary between core and rim is marked by a thin annulus enriched in Y and HREE. The garnet core consists of an inner and an outer zone where the maximum concentration of elements from Lu to Tb progressively moves outwards with decreasing atomic number. This trend continues in the rim outside the annulus, where a further distinction between a Sm -, Eu-, Gd-, Tb-rich inner rim and a REE-poor outer rim was
observed (Franceschelli et al., 2023). The P-T path of the mylonitic micaschist, which was reconstructed from major element zoning in garnet, and from K-white mica composition and mineral assemblage preserved in garnet, is clockwise, subdivided into two different stages. The P-T trajectory was refined by the Compositional Zoning in Garnet and its Modification by diffusion software by Faryad and Ježek (2019). The garnet growth occurred into two stages, marked by partial resorption of the garnet core. The first stage of the P-T path is a prograde segment up to the peak pressure (T 430–490°C, P 1.3–1.4 GPa) whereas the second one reflects garnet rim growth and staurolite formation at peak metamorphism (560–630 °C/0.6–1.1 GPa) followed by exhumation
Dehydration dynamics of analcime by in situ synchrotron powder diffraction
No full textThe continuous structural transformation of tetragonal analcime (Na15.87Al15.20Si32.64 O-96. 16.3H(2)O) upon dehydration was studied, using Rietveld structure analysis of temperature-resolved powder diffraction data collected using synchrotron radiation. The variation of the a-e axis length difference and normalized intensity of the (200) reflection as a function of temperature suggest that tetragonal analcime evolves toward a cubic structure at high temperature. The removal of water was accompanied by a spreading of the initial Na sites into many positions bonded to the framework O atoms. The migration of H2O molecules through the [111] channels during dehydration caused the six-member ring apertures to open as widely as possible: this was accompanied by a twisting of the tetragonal prism, constituting the analcime framework, which led to an opposite tilting of tetrahedra connecting the prisms. These modifications induced by water diffusion an not energetically favored because they would increase the elastic energy of the system, and require a substantial thermal activation energy. The analcime framework reached a maximum distortion at about 650 K, the temperature of complete water loss, then underwent a relaxation process during which the T-O-T angles were restored to the starting value. The relative variation of cell volume associated with the opening of wide six-member ring channels during water migration, and then due to the framework relaxation process after complete dehydration, provides an explanation of the negative thermal expansion (i.e., volume contraction) effect in dehydrated analcime, which is complementary to that based on the Rigid Unit Modes theory
P-T conditions of garnet-staurolite-bearing schists from Variscan NE Sardinia (Italy)
No full textIn north-eastern Sardinia, in the southern side of the Posada-Valley which is a km-wide shear zone of regional relevance, we collected garnet-staurolite schists with plurimillimetric garnet and staurolite porphyroblasts. At the outcrop scale the schists appears as silver-coloured, strongly foliated rocks rich in mica characterized by the occurrence of reddish to brownish garnet porphyroblasts up to 0.8-1 cm in size. Garnet crystals are enveloped by the main S2 regional schistosity which is identified by the orientation of white mica and chlorite. The garnet porphyroblasts are characterized by an inclusion-rich nucleus (garnet core) surrounded by a garnet mantle almost free of inclusions. The
inclusions within the garnet crystals, which mainly consist of quartz, rutile, ilmenite, chloritoid, mica and zircon, often follow an oriented spiral-shaped and/or sigmoidal arrangement recording an earlier S1 schistosity. Some smaller (less than 0.5 cm) and/or anhedral garnet grains lack the inclusion rich nucleus. The yellowish staurolite porphyroblasts (up to 0.5 cm in size) are often characterized by opaque minerals oriented inclusions. Chloritoid is found as oriented and isolated single crystals within the rock matrix. Paragonite locally grows on chloritoid, when this latter is included in the
garnet. Accessory minerals in the schists are apatite and tourmaline. EMP X-ray mapping and rim-core-rim garnet compositional profiles reveal a strong compositional zoning of the garnet components and the occurrence of a very thin (only a few dozens of μm in size) outern garnet rim, not detectable with the polarizing microscope. The garnet core composition ranges between XCa (Ca/(Ca+Mg+Fe+Mn): 0.21-0.27; XMg (Mg/(Ca+Mg+Fe+Mn): 0.01-0.02; XFe (Fe/(Ca+Mg+Fe+Mn): 0.45-0.50; XMn (Mn/(Ca+Mg+Fe+Mn): 0.25-0.30. The garnet mantle is enriched in almandine (XFe
0.55-85), pyrope (XMg 0.018-0.075) and depleted in grossular (XCa 0.06-0.18) and spessartine (XMn 0.05-0.24) components as compared to the garnet core. The narrow outern rim shows an abrupt decrease in grossular (XCa 0.02-0.03) and increase in pyrope content (XMg 0.11-0.13). Plagioclase single crystals in the matrix are oligoclase/albite (Ab ca. 90 mol.%). Staurolite porphyroblasts show XMg ratio of 0.14-0.13 in the core and 0.11-0.10 in the rim. Ti and Mn
contents in staurolite is 0.1 and 0.02-0.05 a.p.f.u, respectively. Chloritoid, independently from its microstructural position (i.e. inside the garnet or in the rock matrix) shows XMg ratio of 0.12-0.13. Biotite in the matrix has XMg ratio of 0.47. White mica shows different composition based on the microstructural position: white mica included in garnet has Si (a.p.f.u.) 6.38 and XMg 0.54, white mica single crystals in the matrix shows Si 6.11 a.p.f.u. and XMg 0.47, white mica in the micaceous layers has Si 6.04 a.p.f.u. and XMg 0.40. Chlorite shows XMg 0.4. Monazite in situ dating with
EMP yielded ages comprised between 417 and 322 Ma (average 377 Ma) for monazite included in garnet and 402-330 Ma (average 357 Ma) for monazite in the rock matrix. Small rutile grains preserved in garnet contain up to 0.02 wt.% zirconium, whereas the rare rutile grains in the rock matrix have undetectable zirconium content. Application of the Zr-in-rutile thermometer, following the calibration after [1], to 35 rutile grains included in garnet gives an average temperature of 557°C. Preliminary results on thermodynamic modelling suggest that the schist underwent a clockwise P-T path with a prograde portion that reached ca. 1.0 GPa peak pressure and temperature up to 600°C. The rock underwent later low-grade re-equilibration and mylonitization.
References:
[1] Tomkins et al. (2007) J Metamorphic Geol 25:703–71
Cation partitioning and substitutional mechanism in 1 M phlogopite: a crystal chemical study
No full textThe crystal chemistry of IM phlogopite was studied by single-crystal X-ray diffraction and microprobe analysis. Structural refinement was carried out in space group C2/m (R values between 0.021 and 0.052) on 24 samples from basic lamproites, leucitites, kimberlite, and carbonatites.
The geometric and chemical features of the octahedral sites show the preferential partitioning of high-charge cations (Ti4+, Al3+, Cr3+, Fe3+) in the cis M2 sites. Bond-length and edge-length distortion parameters show the remarkably different behavior of the two octahedral sites when the high-charge cation content increases. The external shape of the trans MI polyhedron is heavily constrained by the average field strength in the M2 sites.
In Ti-rich phlogopite the central cations of M2 sites show a significant off-center shift toward the 04 position. The substitution mechanism [6](R2+) + 2(OH)- = [6](Ti4+) + 2O2- best explains the observed structural modifications. The loss of a proton linked to 04 is mainly recognized from the shortening of the c lattice dimension.
The consequences of octahedral cation partitioning and structural adjustments related to the Ti substitution may be important when dealing with the thermal stability field of phlogopite and mica-based geothermometry or geobarometry and can help in understanding the dependence of Ti solubility on temperature, f(O2), and a(H2O)
Micronization of ceramic colorants. From understanding to energy efficiency of the process
No full textWith over 16 billion m2 worldwide, in 2020 the ceramic industry marked a new record in tile production (Baraldi, 2021). Similarly, the global demand for ceramic pigments and dyes (ceramic colorants), driven by the ceramic tile industry, has also grown rapidly in the last decades leading to a growing need for raw materials and energy.
Nowadays, inkjet printing (IJP) with micronized inks has become the decoration technology par excellence for ceramic tile (Molinari et al., 2020). Through this technology, the finished product is no longer a powdered colorant, but a micronized colorant dispersed in a carrier, i.e., an ink that undergoes a high-energy stirring milling process where particles must have a median size (d50) of 300 nm to fulfill IJP requirements (Hutchings, 2010).
Being mainly dependent on the specific energy input (i.e., the energy supplied to the grinding chamber per mass of product), pigment micronization down to the requested particle size proves to be the most energy-consuming comminution process per unit weight of product (Wang & Forssberg, 2007). It follows that comminution of ceramic colorants is a key issue for ink production, which has strong repercussions on color strength, mechanical properties and resistance to amorphization of the colorants' crystal structure, as well as on the energy management of the entire process. On the other hand, a deep understanding of the comminution dependence on many of these aspects is still lacking.
In this contribution, the micronization effects on a set of five industrial ceramic colorants are thoughtfully investigated through a simulation of the industrial process at a pilot plant, where particle size distribution and energy consumption are monitored during the comminution process.
The combined analytical approach (i.e., X-ray diffraction, scanning electron microscopy, and diffuse reflectance spectroscopy) aided by a physical/semiempirical modelling of the colorants' elastic features versus the energy response of the particle reduction has yielded details on the nature of the micronization-induced microstructural changes in ceramic colorants.
The results obtained represent a fundamental development towards the optimization of the comminution process. A proper energy modulation allows to limit harmful emissions and save raw materials.
Baraldi L. 2021. World production and consumption of ceramic tiles. Ceram. World Rev., 143, 26-40.
Hutchings I. 2010. Ink-jet printing for the decoration of ceramic tiles: technology and opportunities. Proc. 12th World Congress on Ceramic Tile Quality, QUALICER, 1-16.
Molinari C., Conte S., Zanelli C., Ardit M., Cruciani G., & Dondi M. 2020. Ceramic pigments and dyes beyond the inkjet revolution: From technological requirements to constraints in colorant design. Ceram. Int., 46, 21839-21872.
Wang Y. & Forssberg E. 2007. Enhancement of energy efficiency for mechanical production of fine and ultra-fine particles in comminution. China Particuol., 5, 193-201
- …
