The mineral calcite is the most stable form of calcium carbonate and is the main mineral found in limestone.

It can be found  in the ocean, in shells or hard parts of marine organisms like plankton, coral reefs, types of red algae, oysters and sponges. There is even a stalactitic form of calcite found in limestone caves. These elongated, icicle-shaped formations hang from cave ceilings, formed over time by the slow drip of water.

The many faces of calcite

Compared to most minerals, calcite is not very hard or dense. However, with more than 1000 crystallographic forms,  it can crystallise in a variety of unique shapes and structures.

In its pure form, calcite can be white or colourless, but often contains impurities or trace elements. These can affect its colour, giving it a range of hues, including blue, green, red, lavender, black, orange, and even yellow. Calcite can be transparent, translucent or opaque.

Renee Birchall is a Senior Research Scientist with the Discovery Program at Australia’s national science agency, CSIRO,  and works in the cross-team CarbonLock Future Science Platform.

“Calcite is one of the more exciting minerals to identify as it effervesces (or gives off bubbles) when exposed to dilute hydrochloric acid,” Birchall says.

“Aside from how readily it reacts with hydrochloric acid, another cool way to identify calcite is by scratching it on an unglazed ceramic plate to find its streak. This is always white, regardless of the colour of the calcite.”

Calcite also fluoresces in many different colours under ultraviolet (UV) light. Fluorescence is the phenomenon where materials emit light when they are exposed to certain electromagnetic radiation, such as UV light.

Calcite key in race to net zero

Calcite’s versatility means it’s used across many industries and industrial applications, such as construction, water treatment, agriculture, pharmaceuticals and cosmetics.

The list also includes iconic works of art. Michelangelo’s “David” is made of marble. Marble is limestone made from metamorphosed calcite which has been subjected to high pressure and high temperature.

“However, it’s cement and concrete production for construction that uses more calcite (from limestone) than any other industry,” Birchall says.

“Unfortunately, this process is one of the world’s biggest sources of carbon dioxide (CO₂) emissions, contributing more than 10 per cent of our global emissions annually.”

Rapid decarbonisation in the cement and concrete industry is necessary to limit further climate change.

The CSIRO is working with industry to help navigate some of these challenges and opportunities.

“Alongside decarbonisation, we are also working on novel carbon dioxide removal (CDR) technologies that remove CO₂ directly from the atmosphere,” says Birchall..

“Without novel CDR technologies, we cannot reach net zero.”

Mineral carbonation – locking away CO₂

Until recently, Birchall’s primary research involved understanding mineral systems of ore deposits for exploration. She now spends most of her time on CDR projects that involve mineral carbonation.

“I’m trying to do my bit in the race to solve the climate crisis.”

Mineral carbonation processes occur in nature and have been occurring for millions of years. It’s a geochemical process that involves CO₂ dissolved in rainwater reacting with (or weathering) certain minerals at the surface. It also occurs when CO₂-rich hydrothermal fluids react with certain minerals deep in the crust.

The CO₂ gets locked into carbonate minerals like calcite and magnesite (magnesium carbonate). Still, the process is very slow and does not quickly reduce the atmospheric CO₂.

However, the whole process can be accelerated through novel technologies.

The technologies are diverse and span many industries. Importantly, they can use waste materials, such as mine tailings.

Coordinating global CDR efforts

Birchall’s work with her colleagues includes leading Australia’s engagement in Mission Innovation’s CDR mission to help connect Australian CDR research internationally.

“It’s the greatest challenge of our time, and we can’t do it alone – to achieve the outcome we need, we all need to work together,” says Renee.

This article first appeared at CSIRO news.

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