Mineral
evolution posits that Earth’s near-surface mineral diversity gradually
increased through an array of chemical and biological processes. A dozen
different species in interstellar dust particles that formed the solar
system have evolved to more than 4500 species today. Previous work from
Carnegie’s Bob Hazen demonstrated that up to two thirds of the known
types of minerals on Earth can be directly or indirectly linked to
biological activity. Now Hazen has turned his focus specifically on
minerals containing the element mercury and their evolution on our
planet as a result of geological and biological activity. His work,
published in American Mineralogist,
demonstrates that the creation of most minerals containing mercury is
fundamentally linked to several episodes of supercontinent assembly over
the last 3 billion years.

   

Mineral
evolution is an approach to understanding Earth’s changing near-surface
geochemistry. All chemical elements were present from the start of our
Solar System, but at first they formed comparatively few
minerals—perhaps no more than 500 different species in the first billion
years. As time passed on the planet, novel combinations of elements led
to new minerals. Although as much as 50% of the mercury that
contributed to Earth’s accretion was lost to volatile chemical
processing, 4.5 billion years of mineral evolution has led to at least
90 different mercury-containing minerals now found on Earth.

   

Hazen
and his team examined the first-documented appearances of these 90
different mercury-containing minerals on Earth. They were able to
correlate much of this new mineral creation with episodes of
supercontinent formation—periods when most of Earth’s dry land converged
into single landmasses. They found that of the 60 mercury-containing
minerals that first appeared on Earth between 2.8 billion and 65 million
years ago, 50 were created during three periods of supercontinent
assembly. Their analysis suggests that the evolution of new
mercury-containing minerals followed periods of continental collision
and mineralization associated with mountain formation.

   

By
contrast, far fewer types of mercury-containing minerals formed during
periods when these supercontinents were stable, or when they were
breaking apart. And in one striking exception to this trend, the
billion-year-long interval that included the assembly of the Rodinian
supercontinent (approximately 1.8 to 0.8 billion years ago) saw no
mercury mineralization anywhere on Earth. Hazen and his colleagues
speculate that this hiatus could have been due to a sulfide-rich ocean,
which quickly reacted with any available mercury and thus prevented
mercury from interacting chemically with other elements.

   

The
role of biology is also critical in understanding the mineral evolution
of mercury. Although mercury is rarely directly involved in biological
processe—except in some rare bacteria—its interactions with oxygen came
about entirely due to the appearance of the photosynthetic process,
which plants and certain bacteria use to convert sunlight into chemical
energy. Mercury also has a strong affinity for carbon-based compounds
that come from biological material, such as coal, shale, petroleum, and
natural gas products.

   

“Our
work shows that in the case of mercury, evolution seems to have been
driven by hydrothermal activity associated with continents colliding and
forming mountain ranges, as well as by the drastic increase in oxygen
caused by the rise of life on Earth,” Hazen said. “Future work will have
to correlate specific mineral occurrences to specific tectonic events.”

   

Future
work will also focus on the minerals of other elements to see how they
differ and correlate with mercury’s mineral evolution, and to new
strategies for locating as yet undiscovered deposits of critical
resources.

   

“It’s
important to keep honing in on the ways that minerals have evolved on
our planet from their simple elemental origins to the vast array in
existence today,” Hazen said.

Source: Carnegie Institution