Geologists reveal trends in mineral diversity

By classifying minerals by how they were made, scientists could better study the complex chemistry on other worlds.

Geology reference texts define mineral species based on chemical composition and crystal structure. But mineral samples collected in the field often contain trace elements and structural defects that distinguish most specimens from a textbook example. “If you’re interested in comparing Earth with other planets, both within our solar system and beyond, then you really need to think beyond those very simple, idealized structures,” says Robert Hazen, a mineralogist and astrobiologist at the Carnegie Institution for Science. So Hazen and his colleagues devised a new way to categorize minerals using the imperfections that tell stories about each mineral’s geologic past.

Because minerals acquire defects as they form, the isotopic ratios and foreign inclusions trapped within their crystal lattices can help researchers understand the context in which they were made. After combing the literature, Hazen and his team identified approximately 60 processes, such as lightning and oxidation events, that contribute to the formation of the more than 5,000 minerals known on Earth. They found that about a quarter of Earth’s minerals could be made by multiple routes that used different processes (Am. Mineral. 2022, DOI: 10.2138/am-2022-8099 and 10.2138/am-2022-8105). For example, the study found that there have been at least 21 different recipes in Earth’s history for making pyrite, also known as fool’s gold.

The team showed that more than 81% of minerals on Earth require water to form and 34% form exclusively from biological processes. That’s good news for scientists trying to uncover the ancient history of planets that might have been habitable, Hazen says. Rocks and minerals can provide tangible evidence for life when you consider that a given mineral can form in multiple ways, including by biological processes, he says.

The findings show the profound impact that water and life have had on the diversity of minerals on Earth, says Linda Kah, a geologist at the University of Tennessee Knoxville. “This gives us a tool for seeing how widespread liquid water was in the universe,” she says.