Some blue, greenish blue, and pink Andean opals that were collected from the copper mining area of Acari, near Nazca in the Arequipa Department of Peru, have been examined for their physical properties, macroscopic and microscopic characteristics, X-ray powder diffraction patterns, EDS-MP chemical analyses, and Raman, FTIR and ESR spectra. All samples showed no play-of-colour. Determined refractive index and specific gravity were within the range of opal, except for the most transparent green samples with higher values for both properties that were probably influenced by the presence of small percentages of chalcedony. X-ray diffraction patterns detected variable amounts of cristobalite and tridymite in both the blue and pink opal varieties, allowing them to be classified as opal-CT. The cristobalite/tridymite (C/T) ratio was generally higher in blue than pink opals. Both palygorskite and sepiolite mineral phases were identified in yellowish opaque areas of the blue opals, whereas only variable amounts (25 to 46 wt%) of palygorskite always were found in pink opals. Small amounts of quartz (chalcedony) were detected in both types of opal. The FTIR spectra of both blue and pink opals highlighted the typical features of four-coordinated silica, water in opal and water of crystallisation in phyllosilicates. EDS-MP chemical analyses revealed that SiO2 is actually the major component in both blue and pink opals. Black dendrites in these opals proved to be a complex Ba and Mn oxide of romanechite and/or hollandite composition. Phyllosilicates, in variable amounts, were always present in both types of opal. Small amounts of CuO, below the sensitivity limit of the instrument, were detected in blue opal. Nevertheless, the presence of copper was confirmed by ESR spectra. Therefore, the Cu²+ ion can be considered to be the colouring agent of blue opal. Chromophoric contributions to the colour of pink opals were detected by Raman and ESR spectroscopy and tentatively ascribed to biogenic compounds, such as carotenoids and quinones (which can be fixed by phyllosilicates), and/or to Fe(III) centres. Based on local geology, and the determined structure and composition of the samples, the pink and blue opals-CT from Acari can be associated with volcanism and genetically related to chemical and volcanic events.
Brajkovic, A., Rolandi, V., Vignola, P., Grizzetti, R. (2007). Blue and Pink Opals from Acari, Peru - Their optical, structural and spectroscopic features. AUSTRALIAN GEMMOLOGIST, 23(1), 3-15.
Blue and Pink Opals from Acari, Peru - Their optical, structural and spectroscopic features
BRAJKOVIC, ANNA;
2007
Abstract
Some blue, greenish blue, and pink Andean opals that were collected from the copper mining area of Acari, near Nazca in the Arequipa Department of Peru, have been examined for their physical properties, macroscopic and microscopic characteristics, X-ray powder diffraction patterns, EDS-MP chemical analyses, and Raman, FTIR and ESR spectra. All samples showed no play-of-colour. Determined refractive index and specific gravity were within the range of opal, except for the most transparent green samples with higher values for both properties that were probably influenced by the presence of small percentages of chalcedony. X-ray diffraction patterns detected variable amounts of cristobalite and tridymite in both the blue and pink opal varieties, allowing them to be classified as opal-CT. The cristobalite/tridymite (C/T) ratio was generally higher in blue than pink opals. Both palygorskite and sepiolite mineral phases were identified in yellowish opaque areas of the blue opals, whereas only variable amounts (25 to 46 wt%) of palygorskite always were found in pink opals. Small amounts of quartz (chalcedony) were detected in both types of opal. The FTIR spectra of both blue and pink opals highlighted the typical features of four-coordinated silica, water in opal and water of crystallisation in phyllosilicates. EDS-MP chemical analyses revealed that SiO2 is actually the major component in both blue and pink opals. Black dendrites in these opals proved to be a complex Ba and Mn oxide of romanechite and/or hollandite composition. Phyllosilicates, in variable amounts, were always present in both types of opal. Small amounts of CuO, below the sensitivity limit of the instrument, were detected in blue opal. Nevertheless, the presence of copper was confirmed by ESR spectra. Therefore, the Cu²+ ion can be considered to be the colouring agent of blue opal. Chromophoric contributions to the colour of pink opals were detected by Raman and ESR spectroscopy and tentatively ascribed to biogenic compounds, such as carotenoids and quinones (which can be fixed by phyllosilicates), and/or to Fe(III) centres. Based on local geology, and the determined structure and composition of the samples, the pink and blue opals-CT from Acari can be associated with volcanism and genetically related to chemical and volcanic events.
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