Roberto Giustetto

Crystal structure refinement of palygorskite from neutron powder diffraction

Palygorskite is a Mg-rich fibrous clay, present in nature as a mixture of two intricately intertwined polymorphs – monoclinic ( C2/m ) and orthorhombic ( Pbmn ) – characterized by the presence of channels along Z -axis, filled by weakly bound zeolitic water. A neutron powder diffraction study was carried out by full Rietveld refinement on a deuterated sample, measured at ISIS on the HRPD beam-line. The positions of oxygen and deuterium atoms of the zeolitic water were located ab initio through cyclically repeated Difference Fourier maps, and their atomic coordinates and occupancy factors were refined. The frameworks of both monoclinic and orthorhombic palygorskite do not differ significantly from the models reported in the literature, although they are more distorted. The arrangement of the zeolitic water molecules is highly disordered and different in the two polymorphs. Given the coexistence of several deuterium sets, two different H-bonding schemes are proposed for each polymorph. Further H-bonding alternatives could be derived by considering the location of oxygen atoms in partially occupied symmetry related sites. The links between the zeolitic water and the clay framework appear to be weaker in orthorhombic than in monoclinic palygorskite, as shown by the lower number and different strength of H-bonds. The detailed knowledge of the zeolitic water arrangement may help in better understanding the structural features and production techniques of palygorskite-based compounds of great interest, such as the Maya Blue pigment.

Crystal structure refinement of Maya Blue pigment prepared with deuterated indigo, using neutron powder diffraction

Maya Blue, a synthetic pigment produced by the ancient Mayas in pre-Columbian America, is the combination of a fibrous clay (palygorskite) and an organic blue dye (indigo). The main features of the structure of Maya Blue are already known, although the specific interactions occurring between clay and dye have yet to be completely explained. The details of the structure were studied using the Rietveld method on neutron powder diffraction patterns collected at POLARIS (Spallation Neutron Source ISIS) on a Maya Blue sample freshly synthesized using deuterated indigo. The position of the dye molecule in the structure of Maya Blue and the nature of its interactions with the clay framework were described. In the pigment, indigo lies within the palygorskite channels partially substituting the zeolitic water previously expelled during the synthesis. The dye and the water mutually compete to occupy the clay microchannels, although some portions may be empty, containing neither indigo nor water. Furthermore, the encapsulation of indigo increases the disorder in the disposition of the residual zeolitic water. The occupancy of indigo results to be higher in the orthorhombic than in the monoclinic polymorph, presumably for the faster loss of zeolitic water in the former polymorph during heating. The quantity of indigo in the pigment is rather low ( b axis. The stability of Maya Blue is guaranteed by strong H-bonds formed between the clay structural water and the indigo C=O group. All the present results are coherent with data obtained in previous studies.

Crystal structure refinement of a sepiolite/indigo Maya Blue pigment using molecular modelling and synchrotron diffraction

Maya Blue is an artificial pigment used in Pre-Columbian America, renowned for its chemical stability. The pigment can be considered a precursor of modern inclusion compounds as a hosting microporous clay (palygorskite or sepiolite) shelters the guest indigo dye (≤2 wt%) within its micro-channels. While most papers on Maya Blue are focused on the interaction between indigo and palygorskite, this study describes the pigment structural features when sepiolite is the host structure. Synchrotron X-ray powder diffraction patterns were collected on both pristine sepiolite and sepiolite + indigo (2 wt%) pigment. The pigment structure was investigated with the Rietveld method, basing on both molecular mechanics and the refined structure of sepiolite. The evidence obtained shows that: (i) indigo molecules, encapsulated within the micro-tunnels, stay close to a TOT strip in order to receive H-bonds from the structural OH 2 ; (ii) there is no evidence for direct metal-oxygen bonds between the sepiolite Mg and the indigo C=O groups, as the applied heating (≤190 °C) does not remove structural OH 2 ; (iii) the indigo molecule is affected by 4-fold disorder, as it occupies only one of four partially superposed equivalent sites; (iv) indigo and the zeolitic H 2 O compete to occupy the channels; refined occupancies showed that the dye fills 27 vol% of the channels whereas 73 vol% is occupied by H 2 O. Calculated indigo weight % (1.9) is in close agreement with experimental data; (v) indigo encapsulation modifies zeolitic H 2 O sites, increasing the number and strength of mutual hydrogen bonds; (vi) difference-Fourier maps computed removing indigo contribution confirmed the position of the molecule inside the channels.

An easy non-invasive X-ray diffraction method to determine the composition of Na-pyroxenes from high-density `greenstone' implements

A large number of polished stone implements from Palaeolithic to Bronze Age sites of Northern Italy and Southern France are made of high-pressure (HP) metamorphic rocks (eclogite and related rocks), mainly consisting of Na-pyroxene (jadeite to omphacite) from the metamorphic belt of the Western Alps. The standard archaeometric study of prehistoric stone implements follows a procedure that is invasive, expensive and time-consuming. Since Na-pyroxenes may show a large compositional range, a thorough study of the variations affecting the dhkl values, obtained by X-ray diffraction, of three selected reflections as a function of different chemical composition was carried out, in order to determine the chemistry of Na-pyroxene isomorphic mixtures and roughly evaluate their relative amounts. These reflections (\bar221, 310, 002) are sharp, intense and sensitive to the variation of pyroxene chemical composition. Using such dhkl values measured on pyroxenes of known chemistry, a Ca-pyroxene(Di)-jadeite(Jd)-aegirine(Ae) compositional diagram was constructed, from which the composition of an unknown pyroxene can be estimated within an error of about 5%. When the size of the object is relatively small and a flat polished surface is present, the proposed analytical procedure becomes totally non-invasive. The data obtained shed light on the provenance sources of such implements and the prehistoric trade routes.

An unusual occurrence of palygorskite from Montestrutto, Sesia-Lanzo zone, internal Western Alps (Italy)

Abstract An unusual occurrence of palygorskite was found near Montestrutto, lower Val d’Aosta (Italy), within an eclogite-facies leucogneiss of the Sesia-Lanzo zone. Under the optical microscope the mineral appears as a felt of fibres hundreds of mm long and a few μm thick. SEM observation proved each filament to be a bundle of smaller but remarkably long fibres (length mostly 15−20 μm and thickness <1 μm). EDS and TGA showed a water content [zeolitic H2O (6%) + structural OH2 (≅8%) + framework OH (≅2%) ≅16 wt.%] lower than average (≅20 wt.%). Structure refinement by the Rietveld method confirmed the clay dioctahedral character and coexistence of two polymorphs, monoclinic [77.4(4)%] and orthorhombic [22.6(6)%], the latter showing anomalous variations of some cell parameters [c shortens to 5.138(3) and a lengthens to 12.903(7) Å]. The high crystallinity of this palygorskite suggests that it formed under low-T hydrothermal conditions due to interaction between Mg-rich hydrous fluids, derived from mantle ultramafics, and Al-rich continental rocks. Predominance of unusually long fibres (>5 μm) causes the Montestrutto palygorskite to be potentially carcinogenic.

Are the large filamentous microfossils preserved in Messinian gypsum colorless sulfide-oxidizing bacteria?

The thick gypsum deposits formed in the Mediterranean Basin during the Messinian salinity crisis incorporate dense mazes of filamentous fossils, which were interpreted as algae or cyanobacteria, thus pointing to a shallow-marine subtidal or intertidal environment. The data presented here reveal that these filaments represent remains of colorless, vacuolated sulfide-oxidizing bacteria. This interpretation is supported by the presence of small crystal aggregates of iron sulfide (pyrite) and associated polysulfide within the filamentous fossils. Pyrite and polysulfide are considered to result from early diagenetic transformation of original zero-valent sulfur globules stored within the cells, which is a clade-diagnostic feature of living and degraded sulfur bacteria. In addition to filamentous fossils, the studied gypsum crystals contain remains of euryhaline and stenohaline diatoms and clay-rich aggregates interpreted as alteration products of marine snow floccules. This peculiar fossil assemblage reflects conditions of increased productivity in the water column, triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and freshwater discharge. This study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells, shedding light on the paleoecology of ancient hypersaline environments.

Petrographic classification of unusual high-pressure metamorphic rocks

A great number of prehistoric stone implements from Western Europe are made of high-pressure (H P ) metamorphic lithotypes, such as Na-pyroxenites and fine-grained eclogites. These rocks represent minor “accessory” lithologies that occur as either small primary outcrops or secondary clastic deposits derived from erosion of the former. Due to their scarcity, these lithologies have been partly neglected by the Subcommission on the Systematics of Metamorphic Rocks (SCMR) of the International Union of Geological Sciences (IUGS). Their archaeometric relevance, however, is important. A new petrographic classification of these H P metamorphic lithotypes is presented here, which is consistent with the recommendations of the SCMR. Two simple diagrams are suggested, which are useful to classify “ jades ”, mainly consisting of Na-pyroxenes, and rocks mainly consisting of Na-pyroxenes and garnets. Other unusual H P rocks, sometimes used to make archaeological implements, are also considered and classified according to a serious petrographic approach.

Deterioration of building materials and artworks in the ‘Santa Maria della Stella’ church, Saluzzo (Italy): Causes of decay and possible remedies

An in-depth scientific survey revealed the deterioration mechanisms affecting the ‘Santa Maria della Stella’ church in Saluzzo, Italy, where various salt crystallization processes are strongly damaging the building materials and artworks. Rainwater seepage permeates the vault and interior, causing: (1) epsomite growth as interstitial columnar crystals (resulting in pictorial coating detachment) or superficial, powdery efflorescence; (2) formation of nesquehonite/hydromagnesite crusts on wall paintings; and (3) nitratine growth causing pigment staining and detachment. These processes involve selective Mg2+ mobilization from magnesian-lime mortars and bacterial-induced formation of nitrates from guano, with consequent precipitation of degrading salts. The study confirms how characterization of all deterioration agents is fundamental to planning a viable cultural heritage conservation and restoration programme.

Calcite passivation by gypsum: the role of the cooperative effect

The overgrowth of gypsum (CaSO4·2H2O) on calcite (CaCO3), used as a substrate, has been obtained in order to verify the hypothesis of a crystallographic fit between the two phases. Lattice coincidences show that many orientations of gypsum with respect to calcite can occur. Experimentally, only two among these are fulfilled during the early stages of growth, the selection criterion reasonably being the adhesion energy. Moreover, it has been shown that the presence of calcium carbonate as a specific surface impurity in solution modifies the growth habit of gypsum acting on the growth rate and surface morphology of some gypsum forms. At high concentration, the calcium carbonate in solution promotes the twinning of gypsum according to the Montmartre law.