Quantification of amorphous phases in the silt fraction of Mexican pre-Hispanic adobe earth bricks

This study is focused on quantifying the amorphous phases in soil-related material samples by quantitative phase analysis (QPA) with a carefully designed methodological strategy in the X-ray diffraction sample measurement and in the refinement procedure. The configuration applied was Debye-Scherrer geometry with Mo K alpha radiation which, together with meticulous instrumental profile modelling using an LaB6 standard, allowed the refinement of a minimum of sample-related parameters. This method was applied to two micro-samples of adobe earth bricks from The Great Pyramid of Cholula, Mexico, and one local soil micro-sample containing several amorphous and semi-crystalline phases such as allophane, volcanic glass and opal. The results obtained by the QPA method, complemented by elemental particle induced X ray emission (PIXE) spectrometry analysis, were compared with a silicon chemical environment analysis by Si-29 MAS-NMR. An average amorphous content of 40 wt% was calculated with the QPA/PIXE results, which is in agreement within 10% with the NMR experiments. Consequently, the methodology proposed could be of interest for further studies of cultural heritage geomaterials, which usually contain amorphous phases in their composition and allow only micro-sampling.