Marcello Mellini

Antigorite polysomatism : behaviour during progressive metamorphism

Antigorite forms a polysomatic series of discrete compositions that are chemographically colinear with chrysotile/lizardite, Mg3Si2O5(OH)4 and talc, Mg3Si4O10(OH)2. The compositional variations of antigorite correspond to discrete changes in the lattice parameter, a. A complete suite of antigorites, collected from a cross-section representing increasing metamorphic grade through the Swiss and Italian Alps, has been studied by optical and transmission electron microscopy (TEM). The specimens within this suite range from those formed near the lower stability limit of antigorite (250 °C) through to those formed near its breakdown temperature (550 °C). The lower grade samples belong to the regionally metamorphosed upper Pennine Ophiolites of the Oberhalbstein-Malenco area, while higher grade antigorites were obtained from regionally metamorphosed Malenco serpentinites. The highest grade samples are also from Malenco. They underwent a later contact metamorphism within the thermal aureole of the Bregaglia Intrusive. The lattice parameter a of antigorites evolves from longer (60 Å) to shorter (35 Å) values with increasing metamorphic grade. However, individual antigorites almost invariably show a heterogeneous distribution of a periodicities with higher values close to grain boundaries or reaction fronts and lower values towards the grain centers. The crystal-chemical evolution of antigorite, expressed by reduction in a, is usually accompanied by increased crystallinity. With the TEM, this is seen as an increase in crystallite size and a decrease in the number of crystal defects (twinning, polysomatic disorder, modulation dislocations, wobbling, offset). The structural and compositional evolution of antigorite requires intracrystalline diffusion and reconstructive transformations at relatively low temperatures. Therefore, the process of evolution is sluggish. Equilibrium is frequently not attained, and relics of longer a periodicities can be observed. In addition, relics of chrysotile may be observed in high-grade metamorphic rocks of the Malenco area, in which antigorite coexists with new-formed olivine. Only at one locality is there evidence of “equilibrium”: antigorite formed at 435 °C has a=43 Å; it shows very little variation in the a periodicity, and it is characterized by a homogeneous annealing texture. A geothermometer based upon a periodicities, as proposed by Kunze (1961) has limited practical applicability.

Heterogeneous distribution of metal nanocrystals in glazes of historical pottery

It has been recently shown that lustre decoration of medieval and renaissance pottery consists of silver and copper nanocrystals, dispersed within the glassy matrix of the ceramic glaze. Lustre surfaces show peculiar optical effects such as metallic reflection and iridescence. In many cases, lustre appears overlapped to colored drawings. Here we report the findings of a study on glazes, pigments and lustre of several shards belonging to Deruta and Gubbio pottery of XVI century. The components of glazes and pigments have been identified. Lustre is confirmed to be characterised by silver and copper metal nanocrystals inhomogeneously dispersed in the glassy matrix of the glaze. In the case of lustre overlapped to colored decorations, we found two contradictory cases. The first consists of a lustre surface successfully applied over a blue smalt geometrical drawing. The second consists of a lustre surface, unsuccessfully applied over a yellow lead-antimonate pigment. The yellow pigment hinders the formation of lustre and removes crystals of tin dioxide, normally present in the glaze as opacifier.

The modulated crystal structure of antigorite: The m = 17 polysome

Abstract The modulated crystal structure of an antigorite polysome with m = 17 was refined by single crystal X-ray diffraction in the Pm space group, using highly ordered single crystals from Val Malenco, Italy. The chemical composition is (Mg2.673Fe2+0.098Fe3+0.015Al0.035Cr0.007Ni0.003Mn0.002)Σ=0.823(Si1.997Al0.003)Σ=2O5(OH)3.639. Lattice parameters [a = 43.505(6), b = 9.251(1), c = 7.263(1) Å, β = 91.32(1)°] were determined using a single-crystal diffractometer equipped with an area detector. The structure was refined using 9242 independent reflections, obtaining a final R4σ factor of 0.0577. A continuous, wavy octahedral sheet is linked to a tetrahedral sheet with tetrahedral apices alternatively pointing +c and -c. This sheet is located on the concave side of the octahedral-sheet wave. The octahedral sheet shows normal thickness for a serpentine of this composition, and does not have any internal offset. The tetrahedral sheet inverts its polarity through six- and eight-membered tetrahedral rings (6- and 8-reversals). Between reversals, 6-membered rings are distorted toward ditrigonal configuration, with tetrahedral rotation, α values, ranging along the wave from 4 to 13.6°. The two half-waves have curvature radii of 99.4 and 110.9 Å. Variable interlayer O-O distances occur, indicating the absence of homogeneous, continuous hydrogen bonding. The bond geometry, very similar to that of lizardite, suggests common crystal chemical and geochemical properties. The larger stability field of antigorite compared to lizardite is interpreted to arise from the occurrence of three-dimensionally connected chemical bonds.

Hydrogen positions and thermal expansion in lizardite-1T from Elba; a low-temperature study using Rietveld refinement of neutron diffraction data

Abstract The structure of lizardite-1T from Monte Fico, Elba, was refined in space group P31m using neutron diffraction data, measured at 8, 150, and 294 K, and full-profile Rietveld refinement techniques. The lattice parameters at 8 K [a = 5.3267(2), c = 7.2539(6) Å], 150 K [a = 5.3260(2), c = 7.2574(6) Å], and 294 K [a = 5.3332(2), c = 7.2718(6) Å] show nonlinear expansion, with nearly all volume change above 150 K. H positions were precisely refined at 8 K. The inner H4 atom deviates from the idealized O,O,z positions and is disordered over three symmetry-related positions 0.24 Å away from the ternary axis. The outer H3 atom location is consistent with the previous single-crystal X-ray structure refinement. On the basis of the present thermal expansion data and previous compressibility measurements, the following equation of state for lizardite-1T is proposed: VP,T = V0[1 + 32.8 × 10-6(T - 294) - 15.5 × 10-4(P - 0.001)]. Accordingly, the constant volume condition requires geothermal gradients on the order of 15 °C/km.

The crystal structure of a second antigorite polysome (m = 16), by single-crystal synchrotron diffraction

Abstract A model for the modulated crystal structure of an antigorite polysome with m = 16 (where m is related to the number of tetrahedra spanning a wavelength along a) was refined by single-crystal synchrotron diffraction data in C2/m, using crystals coexisting with the m = 17 polysome from Val Malenco, Italy, which was previously determined structurally. Lattice parameters [a = 81.664(10), b = 9.255(5), c = 7.261(5) Å, β = 91.409(5)°] were determined using a single-crystal diffractometer equipped with an area detector at the Desy synchrotron (Hamburg). The structure was solved by direct methods, and the model refined using 19 222 symmetry-related reflections. The final R4σ factor was 0.0951, calculated for 7246 reflections. The structure of the m = 16 antigorite polysome strongly resembles that of the m = 17 polysome. A continuous, wavy octahedral sheet is linked to a tetrahedral sheet, reversing its polarity through sixfold tetrahedral and eightfold tetrahedral rings. The half-wave has a curvature radius of 80.1 Å. Polyhedral geometry, ditrigonalization angles, and interlayer O-O distances are similar in the two polysomes. The only differences concern the number of tetrahedra for the m = 16 polysome (an even number which leads to symmetric half-waves) and the periodic b/2 shift involving the eightfold rings (to produce the doubling of the a parameter and a C-centered cell).

Chlorine in the Elba, Monti Livornesi and Murlo serpentines evidence for sea-water interaction

The nature of the water source for serpentinization has been previously addressed mostly using oxygen isotopes, that document interaction with sea-water and point out that lizardite and chrysotile originated within the oceanic domain. Mesh textures and bastites characterize the Elba, Monti Livornesi and Murlo (Central Italy) retrograde serpentinites. These rocks, formed by hydration of harzburgitic peridotites, were sampled and analyzed to find further evidence for the origin of serpentinization fluids. Based on chemical and mineralogical compositions, we conclude that the serpentinites from Elba, Monti Livornesi and Murlo contain important amounts of chlorine. In particular, bulk analyses indicate amounts ranging from 182 to 950 ppm; contents as high as 0.6 wt.% have been observed within the serpentine pseudomorphs. Chlorine is not present as a specific phase, even in nanometer-size domains; instead, chlorine isomorphically replaces hydroxyl groups. Even if widespread, chlorine is not completely homogeneously distributed, but increases from mesh rims to mesh cores, bastites and chrysotile veins. The heterogeneous chlorine distribution matches the previously reported two-stage serpentinization process, based upon thermal fracturing of the peridotite first (formation of the mesh rim) and massive water penetration into the weakened peridotite (formation of the mesh core). Geochemical balance of the chlorine content and the textural chlorine distribution are in agreement with sea-water origin of chlorine during serpentinization.

Oriented, non-topotactic olivine → serpentine replacement in mesh-textured, serpentinized peridotites

Partially serpentinized harzburgites from Southern Tuscany, Italy, show serpentine replacing the peridotitic minerals, as rims around olivine and thin lamellae parallel to pyroxene cleavage. Exempt from post-serpentinization tectonometamorphic overprints, these mesh-textured serpentinites offer a favourable setting for the study of seafloor serpentinization. Studied by HRTEM and AEM, the olivine → serpentine replacement reveals a complex sequence of reaction steps. Initially, olivine dissolves forming a silicon-enriched amorphous domain, where early serpentine nuclei are formed. These nuclei recrystallize producing oriented columnar lizardite. The lizardite in the rim shows silicon excess, due to intermixed amorphous or talc-like layers. No chrysotile fiber occurs at the reaction front. Although the olivine-to-lizardite reaction is clearly not-topotactic, recrystallization of early formed serpentine leads to large lizardite sectors, oriented with (001) almost parallel to the reaction front. As the olivine-to-lizardite reaction is estimated to occur in the upper 300–500°C range, lizardite has to be considered as the high-temperature serpentine phase in retrograde serpentinites.

The role of H3O in the crystal structure of illite

In spite of decades of research on the subject, the crystal structure of illite is still poorly understood. The purpose of this study was to address this problem by investigating the nature of the interlayer content in illite IMt-2 from Silver Hill, Montana, using analytical transmission electron microscopy (ATEM), thermogravimetry (TG), and X-ray powder diffraction (XRPD) analyses. The ATEM data, together with literature and TG results, yielded the formula K0.70a0.01(H2O)0.42 (Al1.53Fe2+0.06Fe3+0.19Mg0.28)Σ−2.06(Si3.44Al0.56)O10(OH)2 or, assuming the presence of H3O+, K0.69Na0.01(H3O)+0.28(Al1.47Fe2+0.06Fe3+0.19Mg0.28)Σ−1.99(Si3.40Al0.60)O10(OH)2. The first formula indicates surplus interlayer and octahedral species, whereas the second shows no excess. The XRPD data were refined by Rietveld techniques, down to an Rp factor of 10.48–13.8%. The mineral composition consists largely of illite-2M1, illite-1M, and minor quartz. Although the refinement accuracy is limited by the intrinsic poor quality diffraction of the illites, the partially refined model is consistent with the chemical composition; in particular, attempts to introduce octahedral cations in excess of 2 were fruitless. All the results support the simple structural model, by which the illite structure strictly corresponds to a dioctahedral mica with H3O+ replacing K. As a consequence, the crystalchemical formula of illites should be calculated on the basis of six tetrahedral plus octahedral cations.

Accurate and precise lattice parameters by selected-area electron diffraction in the transmission electron microscope

Abstract Lattice parameters for gold nanocrystals, quartz, and vesuvianite have been determined by electron diffraction in routine transmission electron microscopy (TEM) work, with precision and accuracy near to 0.1%, after correction for elliptical distortion. The distortion, measured in three different microscopes, is constant for each microscope and may be easily eliminated. Variable camera constants have been avoided by positioning the oriented specimen on the eucentric plane and using parallel illumination. The current flowing in the first intermediate lens was kept fixed, assuring constant conditions of the TEM projecting system, with no further diffraction focus applied. Application of this method to micas from metamorphic rocks produced deviations between measured and expected values up to 0.8%. Although easy species distinction is still possible, minor crystal chemical differences within the sample may be lost. Likely causes of these deviations are the possible heterogeneous samples, as well as beam damage leading to cation loss with subsequent variation in basal spacings.

Antigorite in deformed serpentinites from the Mid-Atlantic Ridge

Deformed, non-psxeudomorphic serpentinites from fault zones in the Rainbow and Menez Hom areas, in the Mid-Atlantic Ridge, contain antigorite associated with variable amounts of chrysotile, while pseudomorphic or non-pseudomorphic lizardite + chrysotile serpentinites are the rule in this and other oceanic environments. A detailed TEM study of these deformed serpentinites shows that antigorite (polysomes m = 12 to 16) replaces chrysotile through dissolution-recrystallization, rather than through solid-state transition. This dissolution-recrystallization process is probably favoured by intense shear stress, the effects of which are preserved in the textures of these rocks. Oxygen isotope temperature estimates for these serpentinites fall well below 300 °C, confirming that Mid-Atlantic Ridge antigorite does not result from high-temperature prograde metamorphism, as it often does in other geological environments. Antigorite-bearing serpentinites, therefore, may occur locally in low-temperature, high-deformation settings, characterized by intense tectonic activity and major shear zones, as frequently found along the slow-spreading Mid-Atlantic Ridge. Technical difficulties may have limited the access to and sample recovery from important deformation settings, such as shear zones and fault scarps, thus explaining the relative scarcity of antigorite-bearing deformed serpentinites recovered from oceanic environments.