Olivine, A Common Earth Mineral

Olivine is a common green mineral, present in the Earth’s subsurface.  The mineral is also called ‘peridot’ and ‘chrysolite’ when mined as a semi-precious stone.

When exposed at the Earth’s surface, olivine weathers quickly.  The serpentine mineral that results is of increasing interest to scientists looking for a viable solution to clean the atmosphere from CO₂. 

Olivine occurs as a primary mineral in certain metamorphic rocks.  Chemically-speaking, olivine is a magnesium iron silicate.  Magnesium-rich olivine crystallises from volcanic magma that is rich in magnesium and low in silica.  That magma crystallises to mafic rocks, such as gabbro and basalt.

Thought to be the most abundant mineral present in the Earth’s at shallower depths, olivine and its high-pressure variants constitute over 50% of the Earth’s upper mantle.  The physical properties of olivine have a dominant influence upon the solid flow that drives plate tectonics.  Olivine is stable to pressures equivalent to depths of about 410 km.

 

Squeezing Hydrogen from Stone: A New Source of Green Energy

Scientists from the University of Lyon (France) have discovered a new way to split hydrogen gas from water, using the mineral olivine.  Their method promises a new ‘green’ energy source that could provide copious amounts of hydrogen from a simple mixture of rock and water.

During the chemical reaction, the mineral olivine strips one oxygen atom and one hydrogen atom from an H₂O water molecule to form a mineral called serpentine, thereby releasing the spare hydrogen atom. 

The hydrogen produced is believed to either react with carbon to form methane, or is absorbed by the microbes that manage to survive into rock strata, to sustain life deep beneath the surface of the Earth.  However, it is still unclear exactly how much of this minerally-generated methane is actually being produced within the rocks across the entire planet.

The process occurs naturally in the rocks that form the ocean floors around the globe, particularly along mid-ocean ridges, over geological timescales.  It is greatly speeded up in the laboratory environment.

To replicate the geological conditions, the research team, led by Dr Muriel Andréani, heated olivine minerals in water to about 200°C, and added a little bit of ruby (aluminium oxide) to the mix to provide a source of aluminium atoms.  The whole mix was then placed into a miniature pressure cooker, formed of two diamonds, that squeezed the mixture to a pressure equivalent to 2,000 atmospheres.

The mineralogists of the University of Lyon were expecting the chemical reaction to take several weeks, if not months, but they were shocked to realise that half of the olivine crystals they had used had reacted overnight, in a matter of hours!  They discovered that the addition of aluminium, from the dissolved ruby crystal, was essential to speed up the rate at which olivine dissolved in water, and new serpentine minerals were beginning to grow.

At the moment, the production of hydrogen is mostly performed by a process called steam reforming, combining hydro-carbons, like gas or oïl, with water at around 700°C to 900°C.  The newly-found method uses much lower temperatures, and it involves no fossil fuel: a type of “green chemistry” that opens the way to new sources of carbon-free energy at a low environmental cost.

Experiments suggest that serpentinisation could provide a possible long-term sustainable energy source.  “If you were to process around 10% of the current hydrogen production by this method it would require a volume of rock similar to that used for cement production today,” commented Dr Isabelle Daniel in a BBC interview.

 

Olivine: Solving the Problem of Atmospheric Carbon Capture

A remarkable further application of the serpentine mineral produced in this reaction, is the capture of atmospheric carbon.  Serpentine is already known to actively scour CO₂ by carbonation, and it has been suggested as a feedstock for sequestering atmospheric carbon on a global scale.  Indeed, there is evidence that the weathering of olivine to serpentine in nature has, in the geological past, played an important role in controlling atmospheric CO₂.

A worldwide search is already on for cheap processes to capture CO₂ by mineral reactions, called ‘enhanced weathering’. Removal by reactions  with olivine is an attractive option, because it is so widely available, and because it reacts so easily with the carbon dioxyde (CO₂) in the atmosphere.

When the mineral olivine is crushed, it weathers (or ‘rots’) completely within a few years, depending on the grain size.

All the CO₂ that is produced by burning 1 litre of oil could theoretically be sequestered by less than 1 litre of olivine.

 

The results of the study were published in the journal American Mineralogist.

Read the Scientific Paper: http://dx.doi.org/10.2138/am.2013.4469