Chinese team cracks a rare earth mystery

Recent research by Chinese scientists has shed new light on the formation of rare earth mineral deposits, a subject that has long been a thorny geological puzzle.

The research showed that the depth of a certain type of volcanic rock — carbonatitic magma, which cooled under pressure — is a key factor in determining the concentration levels of rare earths.

Rare earth minerals are essential to many high-tech products.

While more than half the world's rare earth reserves are derived from carbonatite, less than 10 percent of carbonatite masses actually contain economically viable rare earth deposits.

Associate researcher Xue Shuo and researcher Yang Wubin from the Guangzhou Institute of Geochemistry, along with a team of collaborators, found an explanation: The depth of ancient hot, liquid rock, known as magma, determines the potential for rare earths to form. The deeper the magma, the slower it cools so that the scarce minerals can emerge.

The discovery elegantly explains the distribution pattern of global rare earth deposits in carbonatite. World-class deposits, including China's Bayan Obo and Maoniuping, were all formed from magma intrusions at depths greater than 10 kilometers, Yang said.

The research report was published in the international academic journal Nature Communications on Feb 3.

Through experiments involving high temperatures and pressures, the team simulated the cooling and crystallization process of carbonatitic magma in the upper-middle crust of the earth, about 6 km to 20 km underground.

Researchers found that with a boundary at around 10 km below the surface, the magma's evolution followed two distinct evolutionary pathways.

"When carbonatitic magma has entered at shallow depths, apatite crystallizes earlier," Xue said.

The apatite formed under such conditions is rich in silicon and sodium, and its crystal structure acts as a specialized "cage" that traps rare earth elements within the lattice.This leads to the early sequestration of the rare earths, making it difficult for them to migrate and accumulate further, he said.

"Meanwhile, the low-pressure environment promotes the release of large volumes of low-salinity hydrothermal fluids from the magma," Xue said, adding that such fluids have a limited capacity to transport rare earth elements and are unable to effectively concentrate any residual rare earths. This hinders the formation of economically viable ore deposits at later stages.

When carbonatitic magma is intrudes at greater depths, olivine crystallizes first, consuming a significant amount of silicon from the magma, Xue said. It prevents subsequently crystallized apatite from forming the cage structure, making it difficult to accommodate and lock in the rare earth elements.

Meanwhile, the high-pressure environment allows the magma to dissolve more water, which promotes the evolution of the system into an alkali-rich "salt melt", he said. Rare earths are highly soluble in such salt melts, enabling them to get richer.

The process leads to the crystallization of substantial transitional minerals such as huanghoite — named for the Yellow River — laying a solid foundation for the large-scale precipitation of economically valuable rare earth minerals such as bastnaesite in later stages.

The study is the first to establish a complete causal chain linking pressure, mineral crystallization sequence, melt properties and rare earth enrichment. It not only deepens the understanding of the mechanisms behind extraordinary rare earth enrichment but also offers new insights for the exploration of carbonatite-type rare earth deposits, Yang said.

According to the United States Geological Survey, China's reserves totaled 44 million tons, accounting for 48.4 percent of the global total.

Among the areas in China where rare earth minerals are relative abundant, Bayan Obo, located in Baotou, Inner Mongolia, has unique significance. Its vast reserves account for about 90 percent of the country's total rare earth resources and about 40 percent of the world's proven total.

In contrast, many shallow carbonatite bodies, such as Alno in Sweden and Ol Doinyo Lengai in Tanzania, contain rare earth elements but the mineralization is often dispersed and not concentrated. Consequently, they "lack economic viability for mining", Yang said.

Unraveling the origin of the Bayan Obo deposit serves as an essential step in advancing theoretical insights that can guide future ore exploration and sustainable, green extraction practices, Yang said.