Roberto Giustetto

Bio: Roberto Giustetto is an academic researcher from University of Turin. The author has contributed to research in topics: Palygorskite & Indigo. The author has an hindex of 17, co-authored 40 publications receiving 1124 citations.

Crystal structure refinements of palygorskite and Maya Blue from molecular modelling and powder synchrotron diffraction

Abstract: Maya Blue, a synthetic pigment produced by the ancient Mayas, is a combination of a specific clay, palygorskite (or sepiolite), containing large channels in the crystal structure and the organic dye indigo. Little is known about the interaction of the two components to give the most stable pigment ever produced. The aim of this work is to obtain a refined model for the Mexican palygorskite used to prepare the pigment and to elucidate the structure of the clay-indigo complex, using both molecular modelling and Rietveld refinement on data collected with synchrotron radiation. Molecular modelling proved that indigo can fit into the channels without steric impediment (forming strong hydrogen bonds between the C=O group of the dye and the structural water of the clay) and produced a model, showing reasonable distances and angles, used as the starting set for the Rietveld refinement. Difference Fourier maps, calculated without indigo, showed a residual of electron density coherent with the expected disordered position of the indigo molecule. A refinement carried out using the model of palygorskite obtained in this work and a 6-fold disordered arrangement of indigo confirmed these findings. The ratio between the two polymorphs of palygorskite (monoclinic and orthorhombic) present in the natural clay was obtained for our sample and for several palygorskite specimens coming from different sites. Samples within the same outcrop show similar ratios, while samples from different locations do not. This may be used to characterize the provenance of ancient specimens, with the goal of determining whether Maya Blue was invented and produced in one place only or if the production technology was widespread in all the Mayan region.

Maya Blue: A Computational and Spectroscopic Study

Abstract: Maya Blue pigment, used in pre-Colombian America by the ancient Mayas, is a complex between the clay palygorskite and the indigo dye. The pigment can be manufactured by mixing palygorskite and indigo and heating to T > 120 °C. The most quoted hypothesis states that the dye molecules enter the microchannels which permeate the clay structure, thus creating a stable complex. Maya Blue shows a remarkable chemical stability, presumably caused by interactions formed between indigo and clay surfaces. This work aims at studying the nature of these interactions by means of computational and spectroscopic techniques. The encapsulation of indigo inside the clay framework was tested by means of molecular modeling techniques. The calculation of the reaction energies confirmed that the formation of the clay−organic complex can occur only if palygorskite is heated at temperatures well above the water desorption step, when the release of water is entropically favored. H-bonds between the clay framework and the indigo wer...

Pre-columbian nanotechnology: reconciling the mysteries of the maya blue pigment

Abstract: The ancient Maya combined skills in organic chemistry and mineralogy to create an important technology – the first permanent organic pigment. The unique color and stability of Maya Blue can be explained by a new model where indigo dye fills the grooves present at the surface of palygorskite clay, forming a hydrogen bonded organic/inorganic complex. Existing theory assumes the dye is dispersed inside the channels of an opaque mineral. Based on data from thermal analysis, synchrotron and neutron diffraction, ESEM and chemical modelling calculations, our new concept of Maya Blue structure resolves this contradiction and suggests some novel possibilities for more durable, environmentally benign pigments.

Structure and Mineralogy of Clay Minerals

Abstract: Phyllosilicates, and among them clay minerals, are of great interest not only for the scientific community but also for their potential applications in many novel and advanced areas. However, the correct application of these minerals requires a thorough knowledge of their crystal chemical properties. This chapter provides crystal chemical and structural details related to phyllosilicates and describes the fundamental features leading to their different behaviour in different natural or technical processes, as also detailed in other chapters of this book. Phyllosilicates, described in this chapter, are minerals of the (i) kaolin-serpentine group (e.g. kaolinite, dickite, nacrite, halloysite, hisingerite, lizardite, antigorite, chrysotile, amesite, carlosturanite, greenalite); (ii) talc and pyrophyllite group (e.g. pyrophyllite, ferripyrophyllite); (iii) mica group, with particular focus to illite; (iv) smectite group (e.g. montmorillonite, beidellite, nontronite, saponite, hectorite, sauconite); (v) vermiculite group; (vi) chlorite group; (vii) some 2:1 layer silicates involving a discontinuous octahedral sheet and a modulated tetrahedral sheet such as kalifersite, palygorskite and sepiolite; (viii) allophane and imogolite and (ix) mixed layer structures with particular focus on illite-smectite.

Tesis de maestria

Abstract: Tesis de ,aestria presentadas en la facultad de ciencia y tecnologia, Universidad Pedagogica Nacional.

Chapter 2 Structures and Mineralogy of Clay Minerals

Abstract: Publisher Summary This chapter describes structures and mineralogy of clay minerals. Phyllosilicates considered in this chapter ideally contain a continuous tetrahedral sheet. Each tetrahedron consists of a cation, T, coordinated to four oxygen atoms and linked to adjacent tetrahedra by sharing three corners (the basal oxygen atoms, Ob) to form an infinite two-dimensional hexagonal mesh pattern along the a, b crystallographic directions. The free corners (the tetrahedral apical oxygen atoms, Oa) of all tetrahedra point to the same side of the sheet and connect the tetrahedral and octahedral sheets to form a common plane with octahedral anionic position Ooct. Ooct anions lie near to the center of each tetrahedral 6-fold ring, but are not shared with tetrahedra. The 1:1 layer structure consists of the repetition of one tetrahedral and one octahedral sheet, while in the 2:1 layer structure one octahedral sheet is sandwiched between two tetrahedral sheets. In the 1:1 layer structure, the unit cell includes six octahedral sites (i.e., four cis and two trans-oriented octahedral) and four tetrahedral sites. Six octahedral sites and eight tetrahedral sites characterize the 2:1 layer unit cell. Structures with all the six octahedral sites occupied are known as “trioctahedral.” If only four of the six octahedra are occupied, the structure is referred to as “dioctahedral.” The structural formula is often reported based on the half unit-cell content—that is, it is based on three octahedral sites.