Volume 23, Number 1, January - March 2007
|
|
(Follow this link for abstracts of past issues)
BLUE AND PINK OPALS FROM ACARI, PERU
Anna Brajkovic1,
Vanda Rolandi2, Pietro Vignola3
and Rosa Grizzetti1
1Italian College of Gemmologists, (CIG),
External collaborator at the University of Milano-Biocca, Italy
2Department of Geological Sciences and Geotechnologies,
University
of Milano-Biocca, Italy
3CNR - Institute for the Dynamics of Environmental Processes, Milano,
Italy
|
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.
|
 Pink opal Beads from Acari, Peru |
|
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. |
FROM THE LABORATORY
Tay Thye Sun
Far East Gemological Laboratory, Singapore
ABSTRACT
This From the Laboratory presents a selection of sometimes challenging
gemstone identifications that have recently been made in a commercial gem
testing laboratory in Singapore. The author hopes that the content of these
brief reports may be of some assistance to Australian Gemmologists.
LETTER TO THE EDITOR and reply re cut grading
From: - The GIA Diamond Cut Group and The Cut Group,
ABSTRACT
Letters of reply to an article published in
Volume 22 Number 10, April-June 2006
KRÜSS HA10 HEARTS AND ARROWS LOUPE
GAA Instrument Evaluation Committee Report
Trevor Linton, Kym Hughes, Roy Beattie
|
ABSTRACT For fifty years the display of a Hearts and Arrows pattern has been known as a technique for displaying the cut quality of a gemstone, yet it has not been used by many jewellers fro stone assessment. Krϋss Optronics of Germany supply a simple, effective instrument, the HA10 Hearts and Arrows loupe, that is easy to use and effectively displays the cut percentages found in Tolkowski cut diamonds. |
|
| 'Hearts' and 'Arrows' viewed through a Krϋss HA10 Loupe |
