Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low-temperature to hydrothermal alteration

International audienceHydroschorlomite, a Ti-, Ca-, Fe-rich andraditic arnet present in the deepest cores of basalts (661?749 bsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro-Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5?50 ?m) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100?C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5?26.5 wt%), TiO2 (22.0?28.6 wt%), and FeOt (6.2?12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2?97.3 wt%) is confirmed by the OH vibration at 3557 cm?1 in the micro-Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny (<10 ?m) skeletal titanomagnetite. The occurrence of hydroschorlomite in Hole 1256D basalts coincides with a general downward increase in temperatures and overall intensity of alteration manifest by the alteration of plagioclase and the occurrence of small amounts of mixed-layer chlorite-smectite. The titanium necessary to form hydroschorlomite is provided by the breakdown of primary tiny (<10 ?m) titanomagnetite, while calcium is provided by the replacement of plagioclase by albite. Hydroschorlomite is thus an indicator of alteration of titanomagnetite under conditions transitional from low-temperature alteration to hydrothermal metamorphism with formation of titanite and may affect magnetic properties of the rocks

Onion morphology and microstructure of polyhedral serpentine

We describe the shape and internal structure of polyhedral spheroids found in serpentinized peridotites which correspond to a new serpentine microstructure. Serpentine spheroids resemble geodesic domes made of c. 160 to 180 triangular facets. At facet edges, the nested layers bend by c. 14° along their three crystallographic directions, resulting in an onion-like structure with lateral continuity of the layers. The stacking of the serpentine layers within sectors is controlled by interlayer bonding. These polyhedral onions are made of a novel type of spherical nanostructure for layered materials

The spalling decay of building bioclastic limestones of Provence (South East of France): From clay minerals swelling to hydric dilation

International audienceMedieval historical monuments of the Provence region (South East of France) were erected with bioclastic limestones and display different sensitivities to spalling decay. The present study aims at understanding the physical processes at play as well as the internal properties governing its intensity. Limestones of contrasting sensitivity to spalling were compared to a reference type, unaffected by this decay, by means of petrography, petrophysic, mineralogy, and hydromechanics. The obtained results highlighted that the various sensitivities can be explained by the deformation recorded during water content variation (hydric dilation). A clay fraction was systematically detected within the mineralogical composition except for the reference material, and some swelling layers were identified in ontmorillonite/glauconite mixed layer minerals. A specific quantification procedure based on the combination of transmission electron microscopy coupled to an energy-dispersive X-ray spectrometer (TEM–EDX) and profile modeling of X-ray diffraction patterns was applied. A strong relationship between swelling layer content and hydric dilation of limestones was evidenced and corroborated the spalling sensitivity. Further interpretation of results showed that swelling layers localization within the texture significantly influence hydric dilation kinetics. Eventually, a mechanical softening was measured after water saturation. This behavior seems unrelated to the clay mineral content and its relative influence on spalling should be examined

Structural and chemical changes in kaolinite caused by flash calcination: Formation of spherical particles

WOS:000360772000032International audienceTo understand the morphological changes of three commercial kaolins during flash calcination and to compare them with those obtained during traditional heat treatments in the laboratory (an electric furnace at 700 degrees C for 5 h), this paper presents the physical and chemical characteristics of metakaolins obtained from an industrial flash calciner. In the metakaolin products, kaolinite was not completed dehydroxylated during calcination, and the proportion of untransformed kaolinite was greater in flashed metakaolins than in traditional rotary-calcined metakaolins. Several particle morphologies were discernible in the metakaolins, including spherical particles that were formed in flash calcinations. These spherical particles were cut with a focused ion beam (FIB) and were revealed to contain a vitrified aluminum silicate phase with traces of mullite and various gases. These spherical particles were produced from the direct calcination of several submicron kaolinites near the flame of the calciner. (C) 2015 Elsevier B.V. All rights reserved

Formation of clay minerals on Mars: insights from long- term experimental weathering of olivine

International audienceLaboratory experiments are useful to constrain the environmental parameters that have allowed the formation of the ancient hydrous mineralogical assemblages observed at the surface of Mars, which are dominated by ferric smectites. Weathering under a dense CO2 atmosphere on early Mars is a process frequently invoked to explain their formation but has proven difficult to test in the laboratory due to low reaction rates. Here, we present a long-term weathering experiment (470 days, 45°C) of forsteritic olivine specially designed to increase as much as possible the amount of reaction products and thus allow their detailed SEM/TEM petrological and chemical characterization. Our results show the formation of crystalline smectites both under 1 bar of CO2 and under ambient air. However, important 2 differences are observed between the two types of conditions. The smectite formed under CO2 has an average chemical formula per half unit-cell of Si3.92Al0.16Fe 3+ 0.78Mg1.66 Cr0.01Ni0.06K0.04Ca0.04.O10(OH)2. It is thus intermediate between a trioctahedral Mg-rich saponite and a dioctahedral ferric smectite. It is also clearly enriched in Fe compared its counterpart formed under ambient air (Si3.68Al0.12Fe 3+ 0.37 Mg2.61Cr0.01Ni0.02K0.04Ca0.25.O10(OH)2). This result demonstrates that the enrichment in Fe observed for Martian smectites is to be expected if they were formed by low-temperature weathering under a dense CO2 atmosphere. Another difference is the nature of the accompanying phases, which includes amorphous silica (in the form of opal spheres 10 to 100 nm in diameter) and Mg-carbonates under CO2, but are limited to rare kaolinite under ambient air. The observation of kaolinite particles and the significant amount of Al measured in smectites despite the Al-poor nature of the initial material shows that this element is easily concentrated by low-temperature processes. The concentration of this element by weathering could lead to the formation of Al-rich upper horizons, as frequently observed on Mars

Clay mineralogy of the nickel laterite ore developed from serpentinised peridotites at Murrin Murrin, Western Australia

International audienceSamples from the saprolite and extensive smectite zone (up to 25 m thick) of the Murrin Murrin lateritic nickel deposit, developed from serpentinised peridotites, were studied using optical microscopy, XRD, SEM and TEM-EDX. In the saprolite, zone, the initial mesh-texture of the parent rock is still well preserved. The mineralogy is dominated by serpentine, smectite and maghemite. Two kinds of smectites occur in this zone: sapontes and smectites with chemistries intermediate between two dioctahedral endmembers (Fe nontronite, Fe montmorillonite) and a trioctahedral one (Mg + Ni saponite). In the smectite zone, serpentine and saponite completely disappear, however some relicts of the initial mesh-texture exist. Smectite and maghemite are the dominant minerals and goethite appears at the top of this zone. Smectites from the smectite zone have a complex chemistry which appears intermediate between four dioctahedral end-members (Fe Nontronite, Fe montmorillonite, X beidellite, Al montmorillonite) and a trioctahedral one (Mg + Ni saponite). Smectites in fissures exhibit a greater variability in Al vs Fe + Cr content than smectites from plasma: this suggests a supply of Al by the percolating fluids. Nickel, essentially concentrated in the smectite zone (1.3 wt%), is located in the octahedral sheets of the plasma and fissure-smectites with Ni content up to 0.30 per half unit-cell. The oxygen and deuterium isotopic compositions of the smectites show that they crystallised at low temperature in contact with meteoric fluids. The delta(18)O values of Murrin Murrin smectites are negatively correlated with their Fe + Cr content

Single mineral particles makes an electron point source

International audienceA mineral (celadonite, kaolinite) nanometer-thick particle deposited on a flat carbon film or at the apex of a carbon fiber provides electron emission at low applied fields. Voltage and time dependences of the emission intensity are studied, and a model of the underlying mechanism is proposed. An electron point source providing emission from a single particle is built and characterized