Alain Baronnet

A XRD and TEM study on the transformation of amorphous calcium phosphate in the presence of magnesium

The evolution of the amorphous calcium phosphate (ACP), precipitated at 25°C from solutions of concentration [Ca] = [P] = 5 mM at pH = 7.94, was followed at fixed time intervals by combined X-ray, electron diffraction and TEM imaging. The same runs were carried out in the presence of magnesium ions (C = 1 mM). Only one kind of amorphous compound was observed (ACP1), which seems to change directly into apatite (HAP) without crystalline intermediate. Edge dislocations and grain boundaries were found by HRTEM analysis in HAP. The effects of magnesium on conversion of ACP into HAP and on the crystal structure and habit of HAP are interpreted in terms of incorporation and surface adsorption.

Occurrence, composition and growth of polyhedral serpentine

Occurrences, compositions and crystallization of polyhedral serpentine were investigated by SEM, AEM/TEM and μ-XANES analysis of samples from different ultramafic units. Polyhedral serpentines are identified in all of these contexts and form as an alteration product of orthopyroxene (enstatite) and as late veining events. They are always the last serpentine type to crystallize. Their formation requires a combination of three factors: 1) open space, 2) a relatively low temperature ( T < 200–300 °C), and 3) the presence of trivalent cations (Al3+ in this study, ⩾ 0.1 atoms per serpentine formula unit). μ-XANES data at the iron K -edge indicate that Fe is predominantly ferrous and octahedrally coordinated in our Al-rich samples. This microstructure therefore cannot be systematically used as a marker of oxidizing conditions. Textural and microstructural criteria suggest that polyhedral serpentine crystallizes via a “gel” precursor first reorganized into a poorly-crystallized proto-serpentine, in which onion nucleation takes place as nested, discontinuous sheets. Grains expand radially, inwards and outwards, by a layer-by-layer mechanism. Thick layers, made of tens of serpentine sheets, propagate laterally in the (001) plane and result in a pseudo-spherical “onion-like” morphology. By analogy with available clay synthesis experiments, the relatively low temperature conditions under which polyhedral serpentine form may favor a segregation of trivalent cations in the structure. This could create locally dioctahedral components in the structure that may explain the peculiar bending along 〈010〉 responsible for the faceted morphology of polyhedral serpentine.

Serpentinites from Central Cuba petrology and HRTEM study

We have sampled serpentinites and the closely associated metabasites in the paleosubduction zone context of Central Cuba in order to characterise the microstructures of serpentine minerals as a function of metamorphic grade. The samples were collected in the eclogitic unit of the Escambray Massif and in the Zaza Zone, where eclogitic conditions were locally attained. Serpentinites are associated to the metabasites in lenses embedded in a metasediment matrix (Escambray) or form the matrix that embeds the metabasites (Zaza Zone). Field and petrological evidence suggests that serpentinites and associated metabasites underwent the same metamorphic history. Serpentinites from the Escambray Massif have preserved high-grade structures. In the Zaza Zone, most of the sampled serpentinites only underwent low-grade metamorphism and the sample that underwent eclogitic conditions is strongly retrogressed. Low-grade serpentinites show pseudomorphic textures in thin section, a mixture of chrysotile, poorly crystallised serpentine and minor lizardite. High-grade samples, characterised by non-pseudomorphic textures, mainly consist of antigorite, associated to minor chrysotile. Transmission electron microscopy shows that antigorite from preserved samples (Escambray Massif) displays few microstructural defects such as stacking faults or modulation dislocations, in contrast to the higher density of defects encountered in highly retrogressed serpentinites (Zaza Zone). We thus propose that the record of high metamorphic grade in matrix antigorite is best characterised by the elimination of structural defects.

Nano to macroscale biomineral architecture of red coral (Corallium rubrum)

Abstract Different techniques have been used to characterize the physical and chemical structure of the red coral calcitic skeleton. A section normal to the axis of the skeleton shows a medullar zone surrounded by a circular domain composed of concentric rings. Growth rings are revealed by the cyclic variation of organic matter (OM) and Mg/Ca ratio. These growth rings are annual; thus, both OM and Mg/Ca ratio can be used to date red coral colonies. Growth rings display wavelets. The internal structure of each wavelet results from the stacking of layers with tortuous interfaces. Tortuosity is due to the presence of microprotuberances. Interfaces between layers may display sharp discontinuities indicative of interruption of the mineralizing process. SEM and TEM studies show that each layer is made of (1) fibers, organized or not in fan-shaped structures; and (2) submicrometer (apparently mono-) crystalline units. Fibers are superstructures made of submicrometer units possibly assembled by an oriented aggregation mechanism. HRTEM studies show that in spite of displaying single-crystal scattering behavior, the submicrometer crystalline units are made of 2-5 nm nanograins again possibly aggregated by a mechanism of oriented attachment. Thus, submicrometer crystalline units and polycrystalline fibers can be both defined as mesocrystals. The red coral skeleton is a hierarchically organized organic-inorganic composite that exhibits porosity and structural and compositional order on length scales from the nanoscale to the macroscale.

Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures.

Alpine antigorite serpentinites associated with eclogites were investigated to determine if they can be used as indicators of the tectono-metamorphic conditions during subduction and exhumation processes. The detailed petrology of serpentinites sampled in the Monviso massif (Western Alps, Italy) was combined with a transmission electron microscopy study. Alpine serpentinites display a degree of serpentinization close to 100%. Antigorite is the main mineral present, forming non-pseudomorphic textures in the various studied samples and exhibiting a homogeneous chemical composition with limited cationic substitutions. Considering its oceanic origin, the Alpine serpentinite in the Monviso massif formed a lizardite + chrysotile assemblage that recrystallized under greenschist-facies conditions into poorly ordered antigorite, with a modulation wavelength showing significant variations at the crystal scale. Under blueschist-facies conditions, the modulation wavelength of antigorite becomes regular. Thus, periodic antigorites can be related to high-grade conditions, while poorly ordered antigorites characterize lower metamorphic grade. In the present study, we failed to observe any elimination of structural defects with increasing metamorphic grade. While around 50% of the antigorite crystals are highly ordered, it seems that this ordering is at least partly obliterated by retrogressive deformation. Antigorite displays strong evidence of deformation-sensitivity, and the observed microstructures can be directly related to the mechanical behaviour of serpentinites in subduction zones. We investigated the deformation-induced microstructures in serpentinites collected in the Erro-Tobbio eclogitic unit (Ligurian Alps, Italy), which appear to preserve prograde and retrograde structures formed during subduction. According to the microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. Collected samples were fractured at different scales (cm to nm) and have a well-developed schistosity characterized by a strong crystallographic fabric. With increasing metamorphic grade, the brittle behaviour gives way to pressure-solution, which persists up to eclogite-facies conditions. The common obliteration of high-grade microstructures in antigorite, as observed in the Monviso serpentinites, results from continuous recrystallization of this mineral during retrogressive deformation.

A microstructural study of a “crack-seal” type serpentine vein using SEM and TEM techniques

Serpentine banded veins are frequently observed in massive serpentinized peridotites. They form by extension or extensional shearing during hydrothermal alteration of peridotites. Serpentine minerals display different structural varieties, the occurrences of which are not well defined in terms of temperature, pressure, and chemistry, but may be controlled by departure from equilibrium and by the local water/rock ratio. Serpentines are therefore potential markers of environmental conditions during vein formation. However, they have never been used to assess the mechanism of banded vein formation. Using multi-scale microscopy techniques, and comparing detailed observations of natural samples from cm to nm scale with available experimental results, we attempt to deduce constraints on growth mechanisms of serpentines in banded veins. The banded internal structure and the filling along the vein-wall contact suggest a crack-seal mechanism of formation. Each crack is homogeneously filled with chrysotile and some rare polygonal serpentines (tubular serpentine varieties) and disseminated patches of gel-type protoserpentine. The tubes are not parallel to each other, but clearly show a preferred orientation perpendicular to the crack wall. Recent synthesis experiments describe a temporal succession of occurrence of these three serpentine microstructures. The observations suggest that such an evolution can occur in natural samples. The geometric peculiarities of macroscopic growth mechanisms in microscopic interstices may account for capillary effects. Based on this consideration, a simple qualitative model of serpentine banded vein formation is proposed. This model provides a possible origin for the enhancement and maintenance of a diffusional mass transfer from the matrix to the crack. This model also predicts the very good tracking of vein opening directions in such veins.

Equatorial rain forest lateritic mantles: A geomembrane filter

The superimposed weathering layers in equatorial rain forest lateritic mantles from Gabon, AtXca, function as interactive compartments forming a dynamic semipermeable geomembrane filter. Selectivity of the filter is controlled by a progressive downward disappearance Of Connected macropore pathways created by bioturbation and dissolution. The natural balance of root activity, translocation, dissolution, deformation, and pore evolution leads to the development of porous and permeable, mature, open geochemical weathering systems at the expense of the lithosphere. These conclusions can be useful in modeling the fate of lateritic soils, which cover one-third of the emerged area of the world and which are economically important both as metal deposits and agricultural soils.

Topology and crystal growth of natural chrysotile and polygonal serpentine

Abstract Through detailed HRTEM examination of a number of serpentinite specimens, the microstructures of textures and individual chrysotile and polygonal fibers are reviewed and their probable growth processes anticipated. It is recalled that five-fold symmetry is ubiquitous among these fibers and that their axial diffraction pattern is insensitive to their deformation or unequal development. 15- and 30-sectored fibers of polygonal serpentine are described by a fan-like arrangement of flat lizardite sectors, with radiating sector boundaries occupied by arrays of partial dislocations. In each case, the layer stacking sequence varies from one sector to the next. Crystal growth of chrysotile probably occurs by thick, curved sectors closing up as tubules, which subsequently develop to a critical size controlled by local storage of elastic energy. For a strict elastic control we may envision the use of chrysotile geometry as an indicator of departure from equilibrium. A solid-state transformation, chrysotile-to-polygonal serpentine, is likely to be driven by the release of elastic energy stored in chrysotile. Diameter growth of polygonal serpentine is no longer limited by elasticity constraints, so allowing “giant” size and tight intergrowth of those fibers.

Sectors in polygonal serpentine. A model based on dislocations

Based on coexisting rolled chrysotile and polygonal serpentine fibers with 15 or 30 sectors each, a crystallographic model for polygonization of chrysotile is proposed. It is based on an assumed chrysotile-to-lizardite transition. Polygonization of chrysotile requires more likely 15 partial dislocations per turn, as required by polytype translational operators for serpentines. The observed number of sectors corresponds to the two most elastically stable arrays of dislocations. Homogeneous shear of the layer stacking arising from intersector kinking results in a cyclic distribution of twins and/or different polytypes. This makes the fiber axis a fivefold symmetry axis and consequently polygonal serpentine and chrysotile to be both forms of serpentine with local fivefold symmetry. This model is alternative to the recent crystallograpic model by Chisholm (1991, 1992).

The fading of red lead pigment in wall paintings: tracking the physico-chemical transformations by means of complementary micro-analysis techniques

The phenomenon of red lead pigment fading in wall paintings was investigated through the study of an experimental fresco painting. Chemical modifications of the pigment induced a local fading of the pictorial layer after a 25-years natural ageing period. Representative features of the alteration phases, including composition and structural information, were obtained by applying complementary micro-analysis techniques to the study of a single paint sample. Focused X-ray diffraction patterns of small areas were collected using a highly sensitive detector, revealing the transformation of red lead pigment into both cerussite (lead carbonate) and anglesite (lead sulphate). The distribution of Pb, S, O and Ca elements within the cross-section was established using electron micro-probe analysis, and correlated to micro-Raman semi-quantitative mappings of minium (Pb3O4), cerussite (PbCO3), anglesite (PbSO4) and calcite (CaCO3) phases. The micro-structural characteristics of each lead-containing phase were investigated by means of scanning electron microscopy observations of the sample cross-section using backscattered electron imaging. The major role of atmospheric pollutants (SO2, CO2), together with water condensation on such a red pigment fading is emphasised.