Role of aluminium in the structure of Brazilian opals

Brazilian precious opals were studied with mineralogical and chemical analyses, thermal analysis, infrared spectroscopy and X-ray diffraction. For the group of opals investigated, the Al (sub 2) O (sub 3) content appears to be the major control on the close-packed structure of silica spheres, surface properties and colour. The Al-poor opals (0.6 to 1.3 wt.% Al (sub 2) O (sub 3) ) are white-yellow to dark yellow, are relatively well ordered and are associated with sandstone host rocks. The amount of physically adsorbed water is lower (2.6 to 3.4 wt.%) than in Al-rich opals. In contrast, the Al-rich opals (1.4 to 1.8 wt.% Al (sub 2) O (sub 3) ) and hydrated (3.8 to 4.9 wt.% physically adsorbed water) opals present blue iridescences due to a less dense packing of the silica spheres. They are found in claystone host rocks. For the whole range of studied opals, isomorphous substitution of Al for Si in the tetrahedral network is a function of their Al (sub 2) O (sub 3) content ; this feature can be seen both in the infrared spectra and in the X-ray diffraction patterns.