SOIL-DWELLING microbes could have produced precious opals in a matter of
months, according to an international team of geologists and microbiologists.
Until now, most geologists thought the gems were produced by physical processes
over thousands of years.
The team used a scanning electron microscope to examine rocks and opals from
mines in Lightning Ridge, New South Wales. Eighty micrographs reveal that every
rock sample that contains opals also seems to hold the closely packed fossils of
more than 20 different types of bacteria and fungi.
The samples they looked at contained feldspar minerals鈥攁 group of
rock-forming aluminosilicates. 鈥淚n some cases, you can clearly see a grain of
feldspar being attacked by bacteria, and at the edge of the bacteria there鈥檚
opal,鈥� says team member John Watkins, a geologist at the New South Wales
Department of Mineral Resources in Sydney. At least nine of the bacterial
fossils look similar to modern soil-dwelling bacteria, says Watkins.
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Most of the world鈥檚 best opals come from Lightning Ridge, where they formed
about 100 million years ago. Microscopic spheres of silica gel, that settle into
open spaces in rock and solidify, create the semi-precious stone鈥檚 iridescent
sheen. As long as the spheres are all the same size and regularly stacked, light
is diffracted at various wavelengths, creating colours of different
intensity.
The majority of researchers believe that Australian opals form when the
weathering of rocks produces silica, an oxide of silicon. This silica dissolves
in ground water, then precipitates against a clay layer blocking the water鈥檚
passage through the rock. This explains why opal stones are usually found at the
interface between surface sandstone and clay. But there鈥檚 one problem: silica
has a very low solubility, so conditions would have to remain stable for
thousands of years for spheres to settle into regular arrays.
Geologist Hans Behr and microbiologist Karen Behr, both of G枚ttingen
University in Germany, and Watkins found fossils of bacteria that appear to have
grown through several layers of silica spheres. Bacteria only live for a few
months, suggesting that the spheres were laid down over the same period. The
team proposes that organic acids secreted by the bacteria rapidly produce silica
from aluminosilicates in the rock. Acid conditions would also encourage the
silica spheres to rapidly settle out, Hans Behr told the 15th Australian
Geological Convention in Sydney last week.
鈥淚t鈥檚 certainly a feasible mechanism,鈥� says geologist Simon Pecover,
executive chairman of the Pan Gem group of mining companies in Sydney. He notes
that bacteria play a role in a variety of mineral-forming processes. But while
Pecover agrees that Australian opals formed over short periods of time, he
suggests an alternative process. He proposes that as pressurised hot water from
the Great Artesian Basin flowed into fractures and cavities in the rock, the
pressure and temperature dropped, allowing silica dissolved within to rapidly
precipitate into opal.