Of the more than 5,000 minerals recognized by geologists, fewer than 100 are thought to constitute 99 percent of the Earth’s crust. Much more than that—over half of all known minerals, in fact—are considered rare, meaning they appear in five or fewer locations on Earth.
And then there are the rarest of the rare: the minerals that have a total known volume of less than one cubic centimeter, or smaller than the size of a sugar cube. Those are the ones that Robert Hazen, a researcher at the Carnegie Institution, and Jesse Ausubel, an environmental scientist at Rockefeller University, like to study.
“Many of those minerals are known from a single microscopic find, sometimes only one crystal,” Hanzen wrote in an email. “And in some cases those crystals wash away when it rains. So those are really rare minerals!”
Minerals include gemstones, like diamonds (a form of carbon) and rubies (a form of corundum tainted with chromium), as well as more mundane things, like calcite (chalk) and halite (salt). Any naturally occurring substance of a defined chemical composition that can form into a crystal is considered a mineral. (Ordinary rocks are not minerals in and of themselves, but some are a mishmash of several different minerals.)
In a forthcoming paper in the journal American Minerologist, Hazen and Ausubel outlined a new mineral-classification system to help geologists better understand the designation of “rare.” They based their work on a similar system by the biologist Deborah Rabinowitz, who studied rare biological species. According to Rabinowitz, a species can be considered rare if it meets at least one of three criteria: a small geographic range, highly specific habitat requirements, or a small population size.
Hazen and Ausbuel similarly argue that in a rare mineral must meet at least one of four criteria. The first is a narrow range of stability, meaning that it’s found only in very specific conditions. The mineral hazenite, for instance, can only form on the decomposed remains of cyanobacteria in super-salty, highly alkaline environments. Mono Lake, California, is currently the only known source of hazenite.
The second criterion is a composition that includes rare elements, or elements that rarely occur together in nature. Senaite, with the chemical formula Pb(Mn,Y, U)(Fe,Zn)2(Ti,Fe,Cr,V)18(O,OH)38, requires 11 different chemical elements—lead, manganese, yttrium, uranium, iron, zinc, titanium, chromium, vanadium, oxygen, and hydrogen—organized in a highly precise manner.
The third is a mineral’s transience in ambient conditions. Plenty of minerals are unable to withstand extreme environments, but some can be destroyed by pressures and temperatures that humans find tolerable. One such mineral is methane hydrate (a.k.a. methane clathrate), which can form in cold, high-pressure environments like the ocean floor. A potentially important source of energy or greenhouse gas, methane hydrate evaporates when it is brought to the surface. And researchers have identified a few vampire-like minerals, like the mercury-containing edoylerite, that are destroyed by sunlight.
The fourth and final criterion is sampling bias, meaning a mineral may be considered rare simply because scientists have a hard time finding it. Minerals that can only be seen with a microscope or are beyond geologists’ reach–such as those deep in Earth’s crust or mantle–will logically seem rare, even if they may actually be quite common.
Understanding rare minerals can also help scientists identify new ones. But more intriguingly, the study of rare minerals may shed some light on the origins of life. Hazen and Ausubel argue in their paper that Earth appears to be more mineralogically diverse than the other planets and moons in the solar system, meaning that rare minerals may have played a role in helping the earliest life forms to emerge. On the flip side, some rare minerals only form due to biological activity. Either way, the two scientists believe, the presence of rare minerals on other planets—ones we’ve seen here on Earth in tiny doses, or ones we’ve never even encountered—could possibly be a sign of extraterrestrial life.