Raman spectroscopy investigations of the weathering alteration of a predazzite marble mouflon of the Indus Valley Culture

A recumbent ram carved in the style of Mohenjo Daro was examined by a variety of scientific methods in order to clarify questions regarding its condition and the possible recarving of certain details. Testing consisted of microstructural and compositional analysis of the stone and mineral accretions found on it utilizing the complementary methods of Raman spectroscopy, scanning electron microscopy/X‐ray spectrometry, Fourier‐transform infrared spectroscopy and optical petrography. The case demonstrates both the strengths and weaknesses of Raman spectroscopy, and argues for the need for an integrated analytical approach. Significant deviations from the nominal Raman emission wavenumbers were found for both the primary minerals and their weathering products. Consistent differences in Raman spectra between primary and secondary forms of calcite allowed their differentiation in the weathering zone. Exceptionally clear phases led to occasional difficulties in the analysis of petrographic sections due to the excitation of Raman emission from grains below, or adjacent to, the grain of interest. The ram, previously misidentified as a sandstone, was found to have been carved from a marble‐like stone of mixed mineral composition known as predazzite . Despite a deceptively simple appearance, the ram possesses abundant evidence of natural alteration of the carved surface. This alteration takes several forms, including transformation of brucite into hydromagnesite by hydration and carbonation; the loss, by dissolution, of hydromagnesite from the subsurface with resulting void formation; the replacement of subsurface hydromagnesite by void‐filling aragonite or calcite; and the formation of calcite incrustations atop parts of the stone surface. Surface pores produced by weathering create the appearance of sandstone or porous limestone, rock types that are quite different from the much rarer predazzite. The identification of similar artworks as sandstone or limestone should be viewed with skepticism, as published identifications are often not the result of actual mineralogical study. New avenues of archaeometric research, including the determination of the geological source of the stone, should be possible for sculptures of this type on the basis of the fact that predazzites are relatively rare and limited in geological extent. If multiple quarry sites exist in the Indus Valley region, they should be amenable to differentiation by techniques that have proven successful for marbles of the Mediterranean, such as electron paramagnetic resonance spectroscopy, grain size analysis, and carbonate stable isotope analysis. Copyright © 2006 John Wiley & Sons, Ltd.