More than 5,000 minerals have been identified on Earth, more than half of which are classified as rare, meaning they’ve been found in five or fewer places on the planet. Robert Hazen and Jesse Ausubel recently catalogued these rare minerals for the first time. Some are so rare that the cumulative volume of all known samples is less than a cubic centimeter.
Earth has vastly more types of minerals than any other planet or moon in our solar system, because of the diversity of environments here and biological processes that directly or indirectly led to the creation of new minerals. Rare minerals are an indicator of the pressure and temperature conditions that prevailed when they formed, which in turn gives us insight into how Earth’s crust formed and how geology and biology co-evolved.
Hazen and Ausubel compared the rarity of mineral species to that of biological species. The latter can become rare or extinct due to several factors, including abundance, geographic range and habitat restrictions. Biological species also can evolve. Minerals, of course, do not evolve, but trace elements and isotopic differences found in common minerals have varied throughout Earth’s history in response to changing conditions. Applying the biological habitat approach, Hazen, who’s at the Carnegie Institution of Science, was able to map minerals and their “mineral ecology” using statistical methods, and to model the diversity and distribution of mineral species in Earth’s near-surface environments.
If we’re lucky, these maps of rare minerals may provide us with some insight into the origin of life on Earth. Many origin-of-life scenarios have rare minerals such as sulfide, borate, and molybdate minerals playing a role. Since biology is involved in forming some minerals on Earth, they may serve as useful biosignatures if we find them on other planets. We may even discover new types of minerals on worlds like Mars and Europa, where temperatures are lower than anywhere on Earth.