Federico Zorzi

Dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates

To investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite (“gouge”) at subseismic to seismic slip rates, we conducted single-velocity step and multiple-velocity step friction experiments on an antigorite-rich and lizardite-rich serpentinite at slip rates (V) from 0.003 m/s to 6.5 m/s, sliding displacements up to 1.6 m, and normal stresses (σn) up to 22 MPa for gouge and 97 MPa for bare surfaces. Nominal steady state friction values (μnss) in gouge at V = 1 m/s are larger than in bare surfaces for all σn tested and demonstrate a strong σn dependence; μnss decreased from 0.51 at 4.0 MPa to 0.39 at 22.4 MPa. Conversely, μnss values for bare surfaces remained ∼0.1 with increasing σn and V. Additionally, the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces. Extrapolation of the normal stress dependence for μnss suggests that the behavior of antigorite gouge approaches that of bare surfaces at σn ≥ 60 MPa. X-ray diffraction revealed dehydration reaction products in samples that frictionally weakened. Microstructural analysis revealed highly localized slip zones with melt-like textures in some cases gouge experiments and in all bare surfaces experiments for V ≥ 1 m/s. One-dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge. Friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed. Key Points Gouge friction approaches that of bare surfaces at high normal stress Dehydration reactions and bulk melting in serpentinite in < 1 m of slip Flash heating causes dynamic frictional weakening in gouge and bare surfaces

Evidence of dmisteinbergite (hexagonal form of CaAl2Si2O8) in pseudotachylyte: A tool to constrain the thermal history of a seismic event

Abstract The determination of the maximum temperature achieved by friction melt (Tmelt) in pseudotachylytebearing faults is crucial to estimate earthquake source parameters (e.g., earthquake energy budgets, coseismic fault strength) on a geological basis. Here we investigated the mineralogy of a pseudotachylyte from the Gole Larghe Fault (Italian Alps) by using X-ray powder diffraction, micro-Raman spectroscopy, and EDS-equipped field emission scanning electron microscopy. In particular, we report the presence of the hexagonal polymorph of CaAl2Si2O8 (dmisteinbergite) in a pseudotachylyte. Published experimental work shows dmisteinbergite can crystallize at 1200-1400 °C by rapid quenching. Therefore, the presence of dmisteinbergite in pseudotachylyte could be a reliable geothermometer for friction melts for which Tmelt has only as yet been estimated.

In-situ high-temperature emissivity spectra and thermal expansion of C2/c pyroxenes: Implications for the surface of Mercury

Abstract This work was carried out within the framework of the European Space Agency and Japanese Aerospace Exploration Agency BepiColombo space mission to Mercury and intends to provide valid tools for the interpretation of spectra acquired by the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board of BepiColombo. Two C2/c augitic pyroxenes, with different Mg/Fe ratios and constant Ca contents, were investigated by in situ high-temperature thermal infrared spectroscopy and in situ high-temperature single-crystal X-ray diffraction up to temperatures of about 750 and 770 K, respectively. The emissivity spectra of the two samples show similar band center shifts of the main three bands toward lower wavenumbers with increasing temperature. In detail, with increasing temperature bands 1 and 2 of both samples show a much stronger shift with respect to band 3, which remains almost unchanged. Our results indicate that the center positions of bands 1 and 2 are strong functions of the temperature, whereas the center position of band 3 is a strong function of the Mg# [with Mg# = Mg/ (Mg + Fe2+) atomic ratio]. The analysis of the thermal behavior gives similar thermal expansion volume coefficients, αV, for the Mg-rich and Fe-rich samples, with αV = 2.72(8) and 2.72(7) × 10-5 K-1, respectively, using the Berman (1988) equation. This correspondence totally explains the band center shifts similarity between the two samples. Our data suggest that MERTIS spectra will be able to provide indications of C2/c augitic pyroxene Mg# and will allow a correct interpretation that is independent on the spectra acquisition temperature.

Ultra-thin clay layers facilitate seismic slip in carbonate faults

Many earthquakes propagate up to the Earth’s surface producing surface ruptures. Seismic slip propagation is facilitated by along-fault low dynamic frictional resistance, which is controlled by a number of physico-chemical lubrication mechanisms. In particular, rotary shear experiments conducted at seismic slip rates (1 ms−1) show that phyllosilicates can facilitate co-seismic slip along faults during earthquakes. This evidence is crucial for hazard assessment along oceanic subduction zones, where pelagic clays participate in seismic slip propagation. Conversely, the reason why, in continental domains, co-seismic slip along faults can propagate up to the Earth’s surface is still poorly understood. We document the occurrence of micrometer-thick phyllosilicate-bearing layers along a carbonate-hosted seismogenic extensional fault in the central Apennines, Italy. Using friction experiments, we demonstrate that, at seismic slip rates (1 ms−1), similar calcite gouges with pre-existing phyllosilicate-bearing (clay content ≤3 wt.%) micro-layers weaken faster than calcite gouges or mixed calcite-phyllosilicate gouges. We thus propose that, within calcite gouge, ultra-low clay content (≤3 wt.%) localized along micrometer-thick layers can facilitate seismic slip propagation during earthquakes in continental domains, possibly enhancing surface displacement.

Crystal form selectivity by humidity control: the case of the ionic co-crystals of nicotinamide and CaCl2

Post-synthesis (de)hydration techniques were used here to explore further hydrated forms of ionic co-crystals (ICCs) of nicotinamide with CaCl2. Humidity is shown to be a crucial factor for ICCs, which cannot be ignored for a complete polymorph screening of this class of compounds. The exposure of nicotinamide·CaCl2·H2O obtained by kneading reaction to a controlled relative humidity of 75% led to the formation of a new hydrated phase: nicotinamide·CaCl2·4H2O. When nicotinamide·CaCl2·H2O was exposed to a relative humidity between 32% and 54%, nicotinamide2·CaCl2·2H2O was obtained. The anhydrous form was achieved as the result of overnight dehydration of nicotinamide·CaCl2·H2O at 150 °C. The tetrahydrate form cannot be obtained as a first product for kinetic (or thermodynamic) reasons. Control of the relative humidity has proven to be an efficient way to selectively isolate and stabilize powders of pure hydrous ICC phases, which is fundamental for industrial applications. Crystalline structures of nicotinamide·CaCl2·4H2O and anhydrous nicotinamide·CaCl2 were determined by powder diffraction.

Isolated spicules of Demospongiae from Mt. Duello (Eocene, Lessini Mts., northern Italy): preservation, taxonomy, and depositional environment

Today, class Demospongiae is the largest of phylum Porifera but its fossil record, especially for “soft” demosponges, is rather scarce. This study documents exceptionally preserved isolated opaline spicules, unique for the Bartonian of Italy. Interpretation of morphological types of spicules by comparison with living species lead to their attribution to five orders (Astrophorida, Hadromerida, Haplosclerida, Poecilosclerida, “Lithistida”), seven families (Geodiidae, Placospongiidae, Tethyidae, Petrosiidae, Acarnidae, ?Corallistidae, Theonellidae) and five genera (Geodia, Erylus, Placospongia, Chondrilla, Petrosia,?Zyzzya). All the described genera are first reported from the Eocene of Europe. This study expands the geographical range of these taxa and fills a chronological gap in their fossil record. The spicules are often fragmented and bear signs of corrosion. They show two types of preservation: glassy and translucent. X-ray powder diffraction analysis confirms that both types are opal-CT with probable presence of original opal-A. Despite this, using a scanning electron microscope the texture of freshly broken surfaces is different. Milky spicules show a porous structure with incipient lepispheres. This feature, together with surface corrosion and the constant presence of the zeolite heulandite/clinoptilolite, point to a certain degree of diagenetic transformation. Macro and micro facies analysis define the sedimentary environment as a rocky shore succession, deepening upward within the photic zone. The spicule-rich sandy grainstone represents the deepest facies and was deposited in a middle-outer carbonate ramp environment, in part in a fairly high energy environment close to storm wave base.

Deveroite-(Ce): a new REE-oxalate from Mount Cervandone, Devero Valley, Western-Central Alps, Italy

Abstract Deveroite-(Ce), ideally Ce2(C2O4)3·10H2O, is a new mineral (IMA 2013-003) found in the alpine fissures of Mount Cervandone, overlooking the Devero Valley, Piedmont, Italy. It occurs as sprays of colourless elongated tabular, acicular prisms only on cervandonite-(Ce). It has a white streak, a vitreous lustre, is not fluorescent and has a hardness of 2-2.5 (Mohs’ scale). The tenacity is brittle and the crystals have a perfect cleavage along {010}. The calculated density is 2.352 g/cm3. Deveroite-(Ce) is biaxial (-) with 2V of ~77º, is not pleochroic and the extinction angle (β ^ c) is ~27º. No twinning was observed. Electron microprobe analyses gave the following chemical formula: (Ce1.01Nd0.33La0.32Pr0.11Y0.11Sm0.01Pb0.04U0.03Th0.01Ca0.04)2.01(C2O4)2.99·9.99H2O. Although synchrotron radiation was not used to solve the structure of deveroite-(Ce) the extremely small size of the sample (13 μm × 3 μm× 1 μm) did not allow us to obtain reliable structural data. However, it was possible to determine the space group (monoclinic, P21/c) and the unit-cell parameters, which are: a = 11.240(8) Å, b = 9.635(11) Å, c = 10.339(12) Å, β = 114.41(10)º, V = 1019.6 Å3. The strongest lines in the powder diffraction pattern [d in Å (I)(hkl)] are: 10.266(100)(100); 4.816(35.26)(211̄); 3.415(27.83)(300); 5.125(24.70)(200); and 4.988(22.98)(111). Deveroite-(Ce) is named in recognition of Devero valley and Devero Natural Park.

Cosmetics and Cosmetology at Shahr-I Sokhta

Abstract A set of X-rays Diffraction and SEM investigations of the contents of thirteen stone flagons at Shahr-i Sokhta (Sistan, Iran, third millennium BC) demonstrates that they held cosmetic substances. A surprisingly varied list of mixtures of mineral phases (natural and possibly synthetic) indicates the use of green, blue, and white pigments. After discussing the typology of the stone flagons, their geographical distribution and morphological changes in time, we show how cosmetic ingredients were moved and mixed from a specific type of elongated vial to miniature vessels and/or finally, before application, to a stone perforated cap, previously interpreted as a “lid” or “spindle whorl”. Bird feathers were probably drawn through these perforated caps holding cosmetic powders to apply the substances to the skin. The economic implications of the cosmetology of Shahr-i Sokhta are briefly outlined in the Conclusions.

Ghiaraite: a new mineral from Vesuvius volcano, Naples (Italy)

Abstract In this work we report the first finding of CaCl2β·4H2O, long known as a synthetic phase. The mineral, called ghiaraite, was discovered in 2011 in a sample belonging to the Real Museo Mineralogico di Napoli (Italy), that had been collected in 1872 at Vesuvius volcano and stored in a glass sealed vial. It is associated with chlorocalcite (KCaCl3), hematite, sylvite, and halite. The mineral was found inside an ejecta of 5 m in size transported by a lava flow to the locality of Massa di Somma. Here with the ejecta still hot the sample was collected and rapidly stored in a sealed glass vial to preserve it from the atmospheric conditions. Ghiaraite is triclinic, space group P1̄, with unit-cell parameters: a = 6.3660(5), b = 6.5914(5), c = 8.5568(6) Å, α = 93.504(6)°, β = 97.778(7)°, γ = 110.557(6)°, V = 330.802(9) Å3, Z = 2. The calculated density is 1.838 g/cm3 using the ideal formula and the powder X-ray diffraction data. It occurs as euhedral isometric grains up to 5-6 μm long intimately intermixed with chlorocalcite. The eight strongest reflections in the X-ray powder diffraction pattern [listed as d(Å)(I)(hkl)] are: 2.628(100)(022̄̄̄); 2.717(88)(103̄); 4.600(88)(11̄ 1̄); 2.939(77)(200); 2.204(75)(121), 5.874(73)(100), 6.124(47)(010); 3.569(46)(111̄). Ghiaraite was approved by the Commission on New Minerals, Nomenclature and Classification with IMA number 2012-072. The mineral was named in honor of Maria Rosaria Ghiara (b. 1948), Head of Real Museo Mineralogico of Napoli and Centro Musei delle Scienze Naturali e Fisiche dell’Università degli Studi di Napoli Federico II for her important work in promoting the scientific research focused on the mineralogy of Vesuvius volcano.

Tazzoliite: a new mineral with a pyrochlore-related structure from the Euganei Hills, Padova, Italy

Abstract Tazzoliite, ideally Ba2CaSr0.5Na0.5Ti2Nb3SiO17[PO2(OH)2]0.5, is a new mineral (IMA 2011-018) from Monte delle Basse, Euganei Hills, Galzignano Terme, Padova, Italy. It occurs as lamellar pale orange crystals, which are typically a few μm thick and up to 0.4 mm long, closely associated with a diopsidic pyroxene and titanite. Tazzoliite is transparent. It has a white streak, a pearly lustre, is not fluorescent and has a hardness of 6 (Mohs’ scale). The tenacity is brittle and the crystals have a perfect cleavage along {010}. The calculated density is 4.517 g cm−3. Tazzoliite is biaxial (−) with 2Vmeas of ~50°, it is not pleochroic and the average refractive index is 2.04. No twinning was observed. Electron-microprobe analyses gave the following chemical formula: (Ba1.93Ca1.20Sr0.52Na0.25Fe2+0.10)∑4(Nb2.88Ti2.05Ta0.07Zr0.01V5+0.01)∑5.02SiO17[(P0.13Si0.12S0.07)∑0.32O0.66(OH)0.66][F0.09(OH)0.23]∑0.32. Tazzoliite is orthorhombic, space group Fmmm, with unit-cell parameters a = 7.4116(3), b = 20.0632(8), c = 21.4402(8) Å, V = 3188.2(2) Å3 and Z = 8. The crystal structure, obtained from singlecrystal X-ray diffraction data, was refined to R1(F2) = 0.063. It consists of a framework of Nb(Ti) octahedra and BaO7 polyhedra sharing apexes or edges, and Si tetrahedra sharing apexes with Nb(Ti) octahedra and BaO7 polyhedra. The structure, which is related to the pyrochlore structure, contains three Nb(Ti) octahedra: two are Nb dominant and one is Ti dominant. Chains of A2O8 polyhedra [A2 being occupied by Sr(Ca,Fe)] extend along [100] and are surrounded by Nb octahedra. Channels formed by six Nb(Ti) octahedra and two tetrahedra, or four A1O8(OH) polyhedra (A1 being occupied by Ba), alternate along [100]. The channels are partially occupied by [PO2(OH)2] in two possible mutually exclusive positions, alternating with fully occupied A3O7 polyhedral pairs [A3 being occupied by Ca(Na)]. The seven strongest X-ray powder diffraction lines [d in Å (I/I0) (hkl)] are: 3.66 (60) (044), 3.16 (30) (153), 3.05 (100) (204), 2.98 (25) (240), 2.84 (50) (064), 1.85 (25) (400) and 1.82 (25) (268). Raman spectra of tazzoliite were collected in the range 150−3700 cm−1 and confirm the presence of OH groups. Tazzoliite is named in honour of Vittorio Tazzoli in recognition of his contributions to the fields of mineralogy and crystallography.