What it Means to Name a Mineral
Professor George Rossman holding a sample of rossmanite, as well as others from his collection. Photo: Lance Hayashida
by Andrew Moseman
Just a decade ago, Earth’s most plentiful mineral did not have a name. Not a good one, anyway. In fact, it wasn’t officially a mineral at all.
The material that makes up the majority of the planet’s lower mantle, the layer just above the core, had the formal scientific designation of magnesium silicate perovskite. It had a chemical formula, MgSiO3. But it did not have a simple name that rolled off the tongue, like quartz, topaz, or diamond, the stalwarts of mineralogy.
Chalk it up to a technicality. For a mineral to be officially recognized in the modern era, its discoverers must prove its existence, verify its chemical makeup, and identify its three-dimensional crystalline structure. And, crucially, they must provide a naturally occurring sample of the stuff. That is no easy task when a mineral mostly lives deep down in the earth, says Paul Asimow (PhD ’97), the Eleanor and John R. McMillan Professor of Geology and Geochemistry at Caltech.
Microscopic sample of bridgmanite. Courtesy Chi Ma.
“There were no natural specimens, so it didn't have its own mineral name,” Asimow says. “It just went by this awkward hybrid name, magnesium silicate perovskite.”
Asimow wasn’t the only one to find this situation dissatisfying. Chi Ma, Caltech’s director of analytical facilities in the Division of Geological and Planetary Sciences, is a serial discoverer of new minerals with dozens to his credit, and he was determined to find a physical specimen of MgSiO3 in the perovskite structure. By 2014, Ma, along with Oliver Tschauner, then a visiting associate at the Institute and now a research professor at the University of Nevada Las Vegas, had spent years scouring the globe, looking for traces of the material when, at last, they found examples of magnesium silicate perovskite in fragments of a meteorite that fell in Australia.
Material in hand, Ma and Tschauner had the honor, or perhaps challenge, of bestowing a name. “I thought, ‘We are going to name the most abundant mineral on Earth. How can we do that?’" Ma says. “So, we just named it after Percy Bridgman.” Bridgman, whom Ma calls the father of high-pressure physics, is the only person to have won a Nobel Prize in Physics for pioneering work in high-pressure experiments.
All of a sudden, the nonmineral MgSiO3-perovskite became bridgmanite, the most abundant mineral in the Earth.
What’s in a (Mineral’s) Name
Caltech scientists have a long history of naming minerals and of having minerals named for them. There is feynmanite (for Richard Feynman), paulingite (for Linus Pauling (PhD ‘25)), lipscombite (for William Lipscomb (PhD ‘46)), and hendricksite (for Sterling Brown Hendricks (PhD ‘46)). There is housleyite (for Bob Housley), stolperite (for Ed Stolper), rossmanite (for George Rossman (PhD ‘71)), and rosemaryite (for Rosemary “Romy” Wyllie).
“It’s something that is bothering me,” Asimow says of that last one. “When people name things for men, they use their last name. And when they name things for women, they use their first name.” In sum, 30 people associated with Caltech have minerals named in their honor, a count that includes Ma and Asimow.
None of these researchers labeled the finds eponymously. That, Ma explains, would be a violation of the rules set out by the keepers of the names: the esteemed members of the Commission on New Minerals, Nomenclature and Classification at the International Mineralogical Association (IMA).
This group assesses proposals for new minerals in two stages. Its first step is the crucial task of peer review, in which the scientists validate that the material truly is a new mineral and not a known mineral in disguise. In 2021, for example, their diligence helped Asimow catch a mistake that would have required a retraction.
“I was using the electron probe, which is a very sensitive microanalytical method for characterizing minerals, and I found a mica that I did not recognize,” he says. “It had a lot of fluorine in it, otherwise it looked like muscovite. I thought I had discovered fluoromuscovite. I got very excited about it, I characterized it, and I wrote up a proposal.” In the verification stage, however, reviewers noticed Asimow had not analyzed the specimen for lithium because the element, when bombarded with an electron beam, emits an X-ray with energy too low for the electron probe to detect. Once he used a laser-ablation technique to measure the lithium, his candidate turned out to be a well-known mineral instead: not fluoromuscovite or even muscovite, but trilithionite.
Once a candidate mineral secures its confirmation, the committee takes its second step: to OK the proposed name. That approval is no rubber stamp. In fields such as biology, scientists enjoy great latitude when they add a name to the scientific lexicon. This has led to new species named for TV host Stephen Colbert and Swedish pop icons ABBA. “If you find a new kind of insect from the Amazon region, then basically you can do whatever you want,” Ma says. “You can name it after a movie star or something. But not in the field of mineralogy.”
Mineral monikers skew formal. A name must end in “-ite,” though historic names like feldspar and quartz were grandfathered in. Asimow says a new mineral is often named for its place of discovery (such as benitoite, found in San Benito County, California) or after its chemical makeup. For instance, he is working up a proposal for candidate mineral made of yttrium (Y), arsenic (As), oxygen (O), and fluorine (F), so he plans to offer up “yasofite” as its potential name as a nod to its constituent elements. Chi Ma and Professor of Mineralogy George Rossman named one sample “monipite” after its molybdenum (Mo)–nickel (Ni)–phosphorus (P) makeup.
The other common avenue is to name the new mineral in honor of another contributor to the mineral sciences, which is how so many Caltech-affiliated scientists have come to be mineral namesakes. Rossman notes that Anthony Kampf, retired curator at the LA County Museum of Natural History, who has collaborated with Institute scientists on more than 50 mineral discoveries, was the one who named new minerals for Caltech luminaries such as Richard Feynman and Linus Pauling.
When Sasha Krot from the University of Hawaii needed help with an odd aluminum-bearing titanium oxide he found, he called in his frequent collaborator, Ma, who used Caltech’s electron-beam analysis tools to unravel its identity. “I realized, ‘This is new mineral,’” Ma says. “Of course, it's not easy to crack it at the beginning. It took me like half a year to ultimately understand its chemical composition and the crystal structure.” As its discoverer, Krot submitted the proposal to the IMA and chose to honor his Caltech colleague with its proposed name. This form of aluminum-titanium oxide is now called machiite.
“I'm very humbled,” Ma says. “There’s no higher honor than that for me.”
Tiny sample of machiite. Courtesy Chi Ma.
It Came From Outer Space
You might think all the accessible minerals scattered across the countryside or buried just beneath Earth’s surface would have been found by now, scooped up and categorized by some hell-bent rockhound or unearthed in the course of mining operations. Not quite, Asimow says.
While, indeed, a few hundred of the most common “classical” minerals have been known seemingly forever, around a hundred new minerals are added to the official tally each year, and most, according to Asimow, still come from Earth’s crust. Rossman has three varieties of rossmanite named for him, and those minerals have been found all over the world, including a meter-long crystal that turned up in a Manitoba mine used to extract rare elements for modern electronics.
Although the new entries keep coming, they do tend to be complex and come from environments with unusual chemistries that can concoct the peculiar minerals. That is not to say there are no simple elegant minerals left to be found. There are. But many of the new discoveries come from outer space.
Ma has discovered 60 minerals to date—a full 1 percent of the approximately 6,000 known minerals, he likes to point out—and a lot of those came from out of this world. The samples that allowed bridgmanite to become a real mineral, for example, came from fragments of a meteorite called Tenham that fell in Australia sometime in 1879. Ma found allendeite, and 19 others, in the Allende meteorite that crashed into Mexico in 1969.
Meteorites are fertile hunting for scientists like Ma because they experienced extreme conditions like nothing that occurs near the surface of the Earth. Some of these minerals condensed directly from gas in the solar nebula present at the dawn of the solar system. Others were forged by collisions among the careening, tumbling rocks that make up the solar system’s asteroid belt, which is tucked between Mars and Jupiter. The ferocity of these impacts unleashes hellish temperatures and pressures that squeeze molecules until heretofore-unknown minerals can materialize. Fragments of asteroids bearing these high-temperature and high-pressure products get bumped off their orbital track, and a few of those detours end up on a collision course with Earth. Upon arrival, they become meteorites.
Although it took the hot wrath of the universe to bring those minerals into existence, the processes that birthed them, like gas-phase condensation and shock compression and release, do not favor formation of complex chains of molecules with protracted chemical formulas. “What's nice to me about a lot of these asteroidal nano-minerals is they're compositionally very simple,” Asimow says. “You can write down their mineral formula with just a few elements, like monipite, which is just molybdenum, nickel, phosphorus. Done.”
The shocked meteorites made his own namesake mineral possible. The story starts with olivine, a group of green-tinted minerals that comes in magnesium-rich and iron-rich varieties. The normal structure of olivine, called alpha, is a relatively low-density material by geological standards, and the alpha version of the magnesium-rich side, called forsterite, makes up much of the planet’s upper mantle. (It was the most abundant mineral on Earth before bridgmanite became official.) Under the right high-pressure conditions, olivine can be squashed and compressed into denser structures: first the beta structure, called wadsleyite, and then the gamma structure, ringwoodite.
On the iron-rich side, the alpha version is called fayalite. But the other structures proved difficult to find: For years, the iron-rich gamma structure was known only from experiments, while no iron-rich beta structure had been observed at all. When Ma and colleagues discovered the gamma structure (created by subjecting fayalite to exceedingly intense pressure) in a meteorite derived from Mars, they chose the name ahrensite to honor Thomas J. Ahrens, the Fletcher Jones Professor of Geophysics, Emeritus, at Caltech until his death in 2010.
“I thought, ‘We are going to name the most abundant mineral on Earth. How can we do that?’”
With a density between fayalite and ahrensite, the beta structure now known as asimowite ought to form at intermediate pressures. But in static experiments, which hold samples at elevated pressure and temperature long enough to reach chemical equilibrium, it turns out that the material skips past the intermediate phase and transforms directly from fayalite to ahrensite. However, in 2018, two groups of scientists led by Luca Bindi of the University of Florence and Fabrizio Nestola of the University of Padua found samples of iron-rich beta-olivine in two different shocked meteorites, opening the door to its official mineralhood.
Before their official proposal, the discoverers asked Asimow if he would like to be the namesake of this weird, disagreeable, elusive mineral. First, he had to make sure the work was solid. If asimowite or any other potential name is rejected during the review process, that name can never be used again. It happened to venerable geologist James B. Thompson Jr., who taught the first Earth science class Asimow took at Harvard that put him on his life’s path.
“The story that I've been told is, [someone] deliberately sent in a proposal for a new mineral called thompsonite that was not valid and was rejected, meaning the name thompsonite can never be used. But when somebody else actually had a legitimate mineral, and they wanted to honor JBT, they just put in the mineral named jimthompsonite. That is actually a mineral now.”
Risks aside, Asimow was elated to be connected with asimowite. Given that Ahrens, his mentor as principal investigator of the Lindhurst Laboratory for Experimental Geophysics, is the namesake of the gamma structure of iron-rich olivine, Asimow says it is appropriate to have the related beta structure of the mineral named for him.
“And I don't mind that it's not stable,” he says.
New Arrivals
The arrival of new minerals, and the Caltech tradition of naming them, is not liable to slow down any time soon. For one thing, Asimow says, the number of available meteorite samples has vastly expanded in recent years thanks to the National Science Foundation’s Antarctic Search for Meteorites (ANSMET), a program that collects meteorites accumulated and concentrated by glaciers as well as the explosion of meteorites discovered in North Africa.
“The nomads that wander the Sahara in Algeria, Tunisia, and Morocco, have become really good at recognizing and finding meteorites. And because it's so dry there, it's almost as good as Antarctica in terms of preservation,” Asimow says.
Caltech also has been instrumental in changing the way candidate minerals can become officially recognized. Formerly, according to Asimow, before a new mineral could be named, it had to have its structure solved by means of X-ray diffraction, in which scientists shoot X-rays at a sample and measure what happens to the light in order to figure out the structure of a material. “One of the things that Chi Ma and George Rossman have changed about this whole business is they've been able to name a number of minerals where the only specimens we have are too small to collect X-ray diffraction data,” Asimow says. He notes that you can, however, get electron diffraction data by bombarding the sample with an electron beam instead. “And that opened the whole field of nano-mineralogy, which is what has allowed us to go from having the gallery outside my office of nice color pictures of 12 Caltech-associated minerals to the 30-some it is now.”
It means the classical idea of a mineral as something you might see in a big chunk behind glass at a museum is changing too. Consider burnettite, Rossman says, which was named in honor of Donald S. Burnett, Caltech professor of nuclear geochemistry, emeritus. The physical sample taken from the Allende meteorite that allowed burnettite to become a mineral measured a mere 2 microns wide. “That’s the entire world supply,” Rossman says.
Asimowite, and many others, share the same problem. “There's a collector somewhere in Europe who is trying to get a specimen of every named mineral in his collection—all 6,000,” Asimow says. “So, when asimowite was named, he wrote to me and said, ‘Can I have a specimen?’ And I had to write back and say, ‘I don't have one. It's only been found in these two meteorites and deposited in these mineral museums, and I don't have any.’ He said, ‘OK, then can you sign a postcard and send it to me? I'll take your autograph instead.”
Asimow bought a postcard from the Caltech Store, signed it, and mailed it across the sea.