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(Follow this link for abstracts of past issues)
Grahame Brown
ALLGEM Services, Brisbane
ABSTRACT
Australia is the world's largest producer, by
volume, of diamond, precious opal, white South Sea bead nucleated pearls,
and chrysoprase. This paper will provide a status report on these
industries, and suggest reasons for any changes in the economics of these
industries that may have occurred over recent years. The status of other
Australian sources of gemstones, of some economic significance, also will be
briefly described.
COLOUR IN QUARTZ:
FROM ATOMIC SUBSTITUTIONS TO NANOINCLUSIONS
Paulo Vasconcelos, Ben Cohen, Nick Calos
Department of Earth Sciences, The University of Queensland, Brisbane
ABSTRACT
The application of modern high-resolution techniques
permits unambiguous analytical identification of dispersed elements or
mineral inclusions in quartz, thus clarifying some questions with respect to
our understanding of colour in this mineral. The application of trace element analysis, by in
inductively coupled plasma mass spectrometry (ICP-MS) permits the relating
of colour centres to particular elements dispersed throughout the gemstone
structure, as in the case of amethyst and citrine. ICP-MS trace element
analyses of optically clean. gem quality amethyst and citrine sectors in
ametrine crystals from Anahi, Bolivia, and amethyst crystals from Marabá,
Brazil, show that the untwinned z-sectors contain higher trace element
contents than the Brazil-twinned r-sectors, Colour intensity, as determined
by optical absorption spectroscopy, is directly proportional to Fe contents,
thus confirming iron as the major chromophore in these gem varieties. The
analytical results indicate that the untwinned z-sectors in amethyst quartz
have higher iron contents than the twinned r-sectors, exactly the opposite
to the distribution proposed by Sch1össin and Lang (1965) based on x-ray
absorption spectroscopy. Current hypotheses about the causes of colour in
amethyst and, citrine (Rossman, 1994) must be revised.
The application of high-resolution transmission electron microscopy (HRTEM), together with synchroton-based x-ray diffractometry (XRD) and x-ray absorption fine structure spectrostopy (XAFS) is particularly useful for the identification of solid inclusions associated with gem minerals. Transmission electron microscopy (TEM) of chrysoprase from Marlborough, Central Queensland, indicated that the blue-green colour of chrysoprase is associated with the presence of nanoinclusions, of a nickel phyllosilicate, tentatively identified as the Ni-talc, willemseite (Vasconcelos and Singh, 1996). Rietveld refinement of the high resolution synchroton x-ray diffraction patterns, combined with XAFS spectroscopy results, indicated that the phyllosilicate present as nanoincluslons in Marlborough chrysoprase is most likely pimelite, a poorly ordered talc mineral from the kerolite-pimelite series.
(QUEENSLAND) SAPPHIRE ROUGH
Mark Maxwell
Technical Services Manager, Great Northern Mining
ABSTRACT
Great Northern, a subsidiary of CTN Resources, has
conducted a series of experiments with respect to the low cost bulk heat
treatment of lower quality ROM Subera sapphire rough to enhance its colour,
quality and value. Use of heat treatment regimes proprietary to a USA based
consultant company Crystal Chemistry, and an in house heat treatment regime
conducted at lower temperatures and over a shorter time, failed to produce a
successful result, with almost as many sapphires reducing in value as
gaining in value. These experiments and their results suggest that economic
bulk heat treatment of Subera sapphire rough is not feasible.
AUSTRALIAN SEDIMENTARY OPAL
- WHY IS AUSTRALIA UNIQUE
David Horton
Managing Director, Opal Horizon Ltd, Brisbane
ABSTRACT
Australia currently produces about 95 per cent of
the world's precious opal from widely scattered fields throughout central
Australia. No other country on Earth has such an abundance of this rare
precious gemstone.
The sedimentary opal deposits of central Australia occur along generally flat-lying horizontal layers within 30 metres of the earth's surface. They are a product of a unique set of geological events that occurred over a 100 million year period. These events can be summarised as follows:
-
Between about 122 million years ago (Ma) and 91 Ma,
central Australia was covered by a vast shallow epicontinental sea. The
sedimentary rocks that were deposited in this sea were derived from
volcanic rocks and were organic-rich. These formed the principal host
rocks for opal deposits in central Australia.
-
Following surface exposure through lowering of the
sea level, these host rocks were subject to a prolonged sub-tropical
weathering regime until about 40 Ma. Central Australia probably looked not
unlike today's Amazon Basin. During this time, the water table was close
to the surface and was acidic releasing silica and iron from weathering of
the host rocks.
-
The climate became more arid from about 40 Ma and,
as a result, water table levels gradually lowered and the groundwater
became alkaline. Mild tectonism at 24 Ma gave rise to subtle extremely
long wavelength surface folds that facilitated both lateral and vertical
migration under arid conditions of the earlier-released silica. Opal was
preserved in the weathered profiles beneath the crests of the developing
surface folds as water tables here lowered more rapidly due to tectonic
uplift. Siliceous cap rocks discouraged erosion.
- Over the last 10 million years, dissection and scarp erosion exposed the weathering profiles containing the opal.
Geologists believe that the volume of gems that have been produced over the past 150 years in Australia is but a minute fraction of the amount yet to be discovered.
THE MYSTERY ABOUT COLOURED DIAMONDS
Clive Carrington
President,
GAA (Western Australian Division)
ABSTRACT
Over recent decades Australia's AK1 mine has become
famous as a world source of coloured diamonds. These include brown
(champagne and cognac) diamonds, and the much rarer
and very much more expensive pink to red diamonds.
This presentation will discuss the Argyle mine and its diamonds, and will
give particular emphasis to the grading and valuing of Argyle coloured
diamonds.
AGATE CREEK AGATES
Paul Howard,
Elanora, Queensland 4221
ABSTRACT
Agates are one of the most attractive varieties of
the cryptocrystalline quartz, chalcedony. At Agate Creek, in far North
Queensland, some 100 km south of Georgetown, an abundance of agate nodules
can be found in the valley at the head of three creeks-Spring Creek, Black
Soil Creek and Agate Creek. These creeks drain into the Robertson River,
which joins the Gilbert River and eventually empties into the Gulf of
Carpentaria. The Queensland Government gazetted 45 km2 Agate Creek
Fossicking area attracts agate 'buffs', lapidaries and rockhounds each year
d tiring the cooler winter months of May to September. Agates from this
location display every conceivable colour and pattern that is possible to
imagine. The accompanying illustrations display the variety obtainable.
