ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The natural occurrence of asbestos (NOA) in rocks and soil has been known for many years in several areas of the world, differently from the natural presence of asbestiform minerals. In Italy, the mapping of NOA is mandatory according to the 2001 and 2003 regulations. An investigation, not yet concluded, has revealed that in Italy, NOA is represented by chrysotile and tremolite asbestos with minor amounts of actinolite asbestos and anthophyllite asbestos. A field survey conducted in the Italian Western Alps (IWA), dealing with the natural occurrence of asbestiform minerals non-asbestos classified and not regulated, started many years ago and is still ongoing. It revealed that the following kinds of asbestiform silicates are present (in decreasing order of frequency): asbestiform polygonal serpentine and asbestiform antigorite, asbestiform diopside, asbestiform carlosturanite, asbestiform forsterite, asbestiform sepiolite, asbestiform balangeroite, and asbestiform talc. The asbestiform non-silicates brugnatellite and brucite have been rarely detected. Outside the IWA, asbestiform zeolite (erionite and offretite), asbestiform sodium amphibole (fluoro-edenite), and a few other asbestiform silicates have been also detected. For some asbestiform minerals, the identification is problematic and needs the use of transmission electron microscopy combining imaging at high magnification and electron diffraction and chemical data. This investigation is particularly important to distinguish four kinds of asbestiform minerals (antigorite, polygonal serpentine, carlosturanite, and balangeroite) from chrysotile since only the last one is regulated. The issue is much more complicated by the intergrowth of different fibrous species on the submicrometer scale.〈/span〉

Description: The olivine macrocrysts found in oceanites, picrites and magnesian basalts erupted at hotspot volcanoes are generally interpreted either as phenocrysts crystallized from the magma or as xenocrysts extracted from a deforming cumulate. To constrain the origin of these crystals we studied their texture and composition at Piton de la Fournaise volcano, La Réunion. We show that macrocrysts are organized and subdivided into parallel units; this suggests a crystallization by dendritic growth and ripening rather than by a complex combination of paired nucleation, crystal aggregation or synneusis. Dendritic growth is also evidenced by the occurrence of hollow faces, P-rich zones, melt and Cr-spinel inclusions formed from the accumulation of slow diffusing impurities (P, Cr, Al) in the liquid at the contact with rapid-growing olivine. We suggest that early dendritic crystallization may even cause branch misorientations and lattice mismatches, yielding subgrain boundaries, dislocation lamellae and to a certain extent undulose extinction, which have all been formerly interpreted in terms of plastic intracrystalline deformation. We interpret olivine macrocrysts as phenocrysts crystallized under a strong degree of undercooling (–T 〉 60°C), and derived from a harrisitic mush formed on the cold walls of the magma reservoir. Given the growth shapes indicated by P zoning patterns and external faces, the olivine macrocrysts (which consist of groups of several subcrystals) have grown in suspension within the liquid and were neither aggregated into a dense cumulate nor corroded, shocked or deformed before or during their transport to the surface. The major consequence of our study is that most olivine macrocrysts are not xenocrysts, and very few of them, if any, have experienced intracrystalline deformation. The importance of deforming (creeping) cumulate bodies, thought to accommodate the spreading of basaltic volcanoes in La Réunion and Hawaii, may hence have been overestimated.

Description: In the Provence region (south-eastern France), a large part of the building heritage was erected using a bioclastic limestone called "Pierre du Midi". Under this common name, the heterogeneity of the material and, specifically, its stability upon climatic exposure depend on the actual extraction location. In some cases, building stones are affected by spalling decay which is commonly related to the presence of clay minerals although without quantitative support. The present study aims at characterizing and quantifying the clay mineralogy of eight "Pierre du Midi" samples showing various spalling degrees. The combination of transmission electron microscopy (TEM) coupled to energy-dispersive X-ray spectrometer (EDX) and of X-ray diffraction (XRD) full-profile modeling of 00 reflections patterns is used for quantitative phase analysis. Identification of clay minerals present in the 〈 4 μm fractions is performed on the basis of qualitative analysis of experimental XRD patterns recorded in both air-dried (AD) and glycolated (EG) states. The crystal-chemistry of clay minerals is then refined from TEM-EDX analyses which are used as an essential constraint for the modeling of both AD and EG XRD patterns. The results highlight that the complex clay mineralogy of limestones can be unraveled and quantified when using the present coupled approach. Finally, the contrasting sensitivity of samples to spalling decay can be explained by the overall proportion of expandable layers, essentially present in Fe-rich mixed-layers, in the clay fraction. A content of expandable layers higher than 0.80 wt. % results in a high sensitivity to spalling decay, whereas rocks with less than 0.20 wt. % of expandable layers remain essentially unaffected.

Description: Electron-microprobe analysis, single-crystal X-ray diffraction with an area detector, and high-resolution transmission electron microscopy show that minerals related to wagnerite, triplite and triploidite, which are monoclinic Mg, Fe and Mn phosphates with the formula Me 2+ 2 PO 4 (F,OH), constitute a modulated series based on the average triplite structure. Modulation occurs along b and may be commensurate with (2 b periodicity) or incommensurate but generally close to integer values (~3 b , ~5 b , ~7 b , ~9 b ), i.e. close to polytypic behaviour. As a result, the Mg- and F-dominant minerals magniotriplite and wagnerite can no longer be considered polymorphs of Mg 2 PO 4 F, i.e. , there is no basis for recognizing them as distinct species. Given that wagnerite has priority (1821 vs. 1951), the name magniotriplite should be discarded in favour of wagnerite. Hydroxylwagnerite, end-member Mg 2 PO 4 OH, occurs in pyrope megablasts along with talc, clinochlore, kyanite, rutile and secondary apatite in two samples from lenses of pyrope–kyanite–phengite–quartz-schist within metagranite in the coesite-bearing ultrahigh-pressure metamorphic unit of the Dora-Maira Massif, western Alps, Vallone di Gilba, Val Varaita, Piemonte, Italy. Electron microprobe analyses of holotype hydroxylwagnerite and of the crystal with the lowest F content gave in wt%: P 2 O 5 44.14, 43.99; SiO 2 0.28, 0.02; SO 3 –, 0.01; TiO 2 0.20, 0.16; Al 2 O 3 0.06, 0.03; MgO 48.82, 49.12; FeO 0.33, 0.48; MnO 0.01, 0.02; CaO 0.12, 0.10; Na 2 O 0.01, –; F 5.58, 4.67; H 2 O (calc) 2.94, 3.36; –O = F 2.35, 1.97; Sum 100.14, 99.98, corresponding to (Mg 1.954 Fe 0.007 Ca 0.003 Ti 0.004 Al 0.002 Na 0.001 ) =1.971 (P 1.003 Si 0.008 ) =1.011 O 4 (OH 0.526 F 0.474 ) =1 and (Mg 1.971 Fe 0.011 Ca 0.003 Ti 0.003 Al 0.001 ) =1.989 (P 1.002 Si 0.001 ) =1.003 O 4 (OH 0.603 F 0.397 ) =1 , respectively. Due to the paucity of material, H 2 O could not be measured, so OH was calculated from the deficit in F assuming stoichiometry, i.e. , by assuming F + OH = 1 per formula unit. Holotype hydroxylwagnerite is optically biaxial (+), α 1.584(1), β 1.586(1), 1.587(1) (589 nm); 2V Z (meas.) = 43(2)°; orientation Y = b . Single-crystal X-ray diffraction gives monoclinic symmetry, space group P 2 1 / c, a = 9.646(3) Å, b = 12.7314(16) Å, c = 11.980(4) Å, β = 108.38(4)°, V = 1396.2(8) Å 3 , Z = 16, i.e. , hydroxylwagnerite is the OH-dominant analogue of wagnerite [β-Mg 2 PO 4 (OH)] and a high-pressure polymorph of althausite, holtedahlite, and α- and -Mg 2 PO 4 (OH). We suggest that the group of minerals related to wagnerite, triplite and triploidite constitutes a triplite–triploidite supergroup that can be divided into F-dominant phosphates (triplite group), OH-dominant phosphates (triploidite group), O-dominant phosphates (staněkite group) and an OH-dominant arsenate (sarkinite). The distinction among the three groups and a potential fourth group is based only on chemical features, i.e. , occupancy of anion or cation sites. The structures of these minerals are all based on the average triplite structure, with a modulation controlled by the ratio of Mg, Fe 2+ , Fe 3+ and Mn 2+ ionic radii to (O,OH,F) ionic radii.

Description: Rock-forming serpentine minerals form flat, cylindrical, and corrugated crystal microstructures, which reflect energetically efficient layering of alternate tetrahedral and octahedral sheets. Serpentinization of peridotite involves internal buffering of the pore fluid, reduction of oxygen fugacity, and partial oxidation of Fe 2+ to Fe 3+ . Sluggish MgFe diffusion in olivine causes precipitation of magnetite and release of H 2 . The tectonic environment of the serpentinization process dictates the abundance of fluid-mobile elements in serpentinites. Similar enrichment patterns of fluid-mobile elements in mantle-wedge serpentinites and arc magmas suggest a linkage between the dehydration of serpentinite and arc magmatism.

Description: Natural mineral materials such as tabular and spheroidal halloysites have recently been suggested as candidates for intercalating metal ions or organic molecules. Their potential use as nano-adsorbents is related to their porous structure and water content. Although the two morphologies can coexist in natural deposits, spheroidal halloysites remain poorly characterized whereas much literature exists on tubular halloysites. The present study investigates the native morphology, internal porous structure, and behavior upon dehydration of spheroidal halloysite from Opotiki (New Zealand). This mineral was characterized in its natural hydrated state using a transmission electron microscope equipped with an environmental cell (EC-TEM). The sample was placed in a sealed block in which water vapor-saturated air circulated at a pressure of 30 Torr. The observed particles consisted of almost complete spheroids displaying polyhedral external surfaces. 1:1 layers stack concentrically as a pore-free, onion-like structure. The dynamic processes of dehydration created by slow depressurization of the cell resulted in a decrease in the layer-to-layer distance ( d 001 ) from ~10 Å to ~7 Å due to the loss of interlayer water molecules. Irreversible formation of spurious ‘internal pores’ was recorded during this process. These pores were not indigenous to the hydrated 10 Å halloysite and resulted from the collapse of the native layers. They cannot account for the physical chemical properties of spheroidal halloysite. Spheroidal halloysites would have a lower propensity for intercalating ions or molecules than tubular halloysites. Isolated facets were also observed in high-resolution-TEM and displayed a pseudo-hexagonal morphology. The three-dimensional microstructure of the spheroid appeared bent along the three pseudo equivalent y i directions of the kaolinite-like single layers. An analogy with polyhedral serpentine has allowed the proposal of a formation process of hydrated spheroidal halloysite triggered by enrichment in divalent ions in the growth system.