[ RadSafe ] Another type of mining in Australia

[John Jacobus]

>From http://physicsworld.com/cws/article/news/31941

Nov 22, 2007

Uranium gives opal its shine


The beautiful optical properties of the gemstone opal
are the result of tiny amounts of uranium present when
the stones were formed, say researchers in Australia.
They claim that their work could lead to the
production of artificial opal and have already shown
that the gamma rays given off by the uranium and its
radioactive daughters can lead geologists to new
underground deposits of the gem. 

Opal is made of amorphous silica and this makes it
very difficult to find because the gem is usually
surrounded by other rocks made of the same material.
“Precious” opal differs from the much less valuable
common opal in that it contains spheres of silica
about 200 nm in diameter that are arranged in a
regular superlattice. This gives the gem its famous
“play of colour” — different colours that appear when
opal is viewed from different angles — which is caused
by the diffraction of light in the superlattice. 

Now, geologist Brian Senior and physicist Lewis
Chadderton have discovered that the superlattice of
spheres forms because of the presence of tiny
quantities of uranium and its decay products. Using
analytical techniques such as electron microscopy and
neutron activation analysis along with theoretical
models of sedimentation processes, the team have shown
that some of the very heavy elements that are made
when uranium decays act as a “seeds” for the formation
of the silica spheres during sedimentation. 

Gamma-ray logging

The uranium also makes precious opal much more
radioactive than surrounding silica-based rocks, which
tend to have low levels of radioactivity. With this in
mind, Senior and Chadderton have adapted a standard
tool of oil exploration called gamma-ray logging to
create a technique for finding precious opal. 

Their technique involves boring a minimum of three
holes in a region where the gem is thought to occur. A
sodium iodide gamma ray detector is lowered by a winch
into each hole and readings are taken every 15 cm.
Data are collected on a laptop computer and the system
is installed in the back of a four-wheel drive
vehicle. If high levels of gamma rays are detected,
the data from the three bores are triangulated to
locate the centre of the deposit. 

 
The team have used their technique to find new
deposits at several locations in Australia. Senior,
who runs his own consulting company, believes that the
technique could be a boon to Australia’s opal mining
industry — which, despite producing over 90% of the
world’s precious opal, he describes as a “cottage
industry”. 

Chadderton, who is at the Australian National
University in Canberra, told physicsworld.com that the
team are using their insight into opal formation to
create artificial gems. As well as being used in
jewellery, Chadderton believe that such opals could be
engineered to be photonic crystals. These materials
can be used to control light in much the same way as
semiconductors are used to control electrical currents
— something that could make them very useful in
fibre-optical communications systems. Senior and
Chadderton will report their results in an upcoming
issue of The Australian Gemmologist .

About the author
Hamish Johnston is editor of physicsworld.com



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"Courage is what it takes to stand up and speak, Courage is also what it takes to sit down and listen." -- Sir Winston Churchill

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com


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