Труды российских конференций

Maya Blue, MB, the famous pre-colombian pigment of the ancient Mayas, was made from palygorskite (a modulated 2:1 phyllosilicate with a discontinuous octahedral sheet) and the leaves of the plant Indigofera suffruticosa (Van Olphen, 1966).The pigment can be manufactured by heating a palygorskite-indigo mixture at T=100-150 ^oC for a few hours.Despite serious efforts, the exact nature of MB and the structural origin of its extreme chemical stability are still elusive (Sanchez del Rio et al., 2006).Indigo in MB is described as adsorbed at the external surface of palygorskite, or inserted in the tunnels, or reacting to dehydroindigo, while some authors associate its formation with specific structural defects.On the other hand, a possible influence of indigo on the structure of palygorskite itself has not been considered as an additional proxy for the structure of MB.

In this study we employ near-infrared (NIR) spectroscopy to study the nature of the indigo-palygorskite interactions in MB.NIR can readily identify the structural or surface OH groups, as well as the different types of H₂O in palygorskite, while peaks due to organics do not overlap with the spectrum of the clay (Gionis et al., 2006).

Palygorskite and palygorskite / indigo mixtures with indigo content in the 1-10 wt% range are heated to 130 ^oC (i.e. to conditions suitable for surface and zeolitic dehydration), sealed,left to cool to RT, and measured non invasively.Several types of palygorskite with different compositions have been studied.Contrary to earlier reports, the formation of MB is manifested by changes in the NIR spectrum of indigo that can be rationalized only by interactions between isolated indigo molecules and crystalline H₂O inside the tunnels.These interactions characterize MB regardless of palygorskite composition, and therefore do not require the presence of specific defects in e.g. Al or Fe-rich samples.The majority of crystalline H₂O remains unperturbed, perhaps because one indigo molecule inserted in the tunnel of palygorskite occupies three unit cells along c.Interestingly, the spectrum of the surface Si-OH terminal groups, which is a sensitive sensor for the adsorption of a variety of organics on palygorskite, remains unperturbed by indigo in MB.This result is clearly opposed to models requiring the adsoprtion of indigo onto grooves or channels of the external surface of palygorskite.

Further, opening the sealed palygorskite and MB preparations and monitoring their NIR spectra in real time as they equilibrate to ambient, proved that MB would fully rehydrate their external surface within minutes, similarly to neat palygorskite, but only partially refill their tunnels with zeolitic water.Zeolitic rehydration was found to be inversely proportional to indigo content, and is almost blocked in MB with ca. 10 wt% indigo.Clearly, the main interaction between indigo and palygorskite in MB involves the insertion of indigo molecules inside the (zeolitically dry) tunnels.

Gionis, V., Kacandes, G. H., Kastritis, I. D., and Chryssikos, G. D. (2006).On the structure of palygorskite by mid- and near-infrared spectroscopy.American Mineralogist, 91, 1125-1133.

Sanchez del Rio, M., Picquart, M., Haro-Poniatowski, E., van Elslande, E. and Uc, V.H. (2006). On the Raman spectrum of Maya Blue. Journal of Raman Spectroscopy, 37, 1056-1053.

Van Olphen, H. (1966) Maya Blue: A clay-organic pigment. Science, 154, 3749, 645-646.