WASHINGTON, D.C. — Every rock has a story. Understanding how it formed can tell scientists about the environment that surrounded the rock at its birth. But when scientists classify the minerals that make up rocks, they leave out the details of how those minerals formed. One scientist wants to change that. He wants to give minerals back their biographies. Knowing that history, he says, could help scientists understand more about our planet — and many others.
Minerals are solid substances that occur in nature. Up close, their molecules form regular, three-dimensional crystal patterns. A diamond is one example. Diamonds most often occur as cubic crystals. This means their atoms are stacked in repeating cube shapes. The whole rock (which can be made of one or more minerals) may end up with sharp edges too. It might be a cube, or it could be an octahedron, a shape with eight flat faces.
Diamonds are a good example of how minerals with the same name can have different histories, says Andrew Christy. He’s is the curator of minerology at Queensland Museum in Brisbane, Australia. The big diamonds found in jewelry formed more than 160 kilometers (100 miles) below Earth’s surface, in the mantle. Heat and pressure there smushed carbon atoms together into cubes. That created diamond crystals. The ones we dig up today have been pushed to the surface through violent volcanic eruptions.
But diamonds can form elsewhere in the universe, as well. Tiny diamonds, for instance, form in meteorites crashing to Earth from space. Some of those meteorites have carbon atoms in them, Christy explains. When a meteorite blasts through Earth’s atmosphere, it can generate shockwaves of heat and pressure. “Those shockwaves passing through that carbon [in the meteorite] will create tiny, tiny diamonds,” Christy explains.
Both of these rocks are diamonds. Based on their crystal structure, mineralogists would classify them the same. But Robert Hazen says scientists lose a lot when they take away the story of a mineral’s birth. “Natural minerals are storehouses of information,” he says. “They’re time capsules with vast amounts of information we have not yet explored.” Hazen is a mineralogist at the Carnegie Institution’s Geophysical Laboratory in Washington, D.C.
Stories minerals could tell
Right now, every mineral has a “species.” Diamond is one species, for example. A group called the International Mineralogical Association (IMA) makes sure that mineral species names are consistent. But a rock’s species doesn’t tell you anything about how the minerals in it formed.
Maybe the name of a mineral could say more. Organisms are organized into genus and species. A tiger, for instance, is Panthera tigris. “Panthera” is the genus and “tigris” is the species. Species that belong to the same genus are closely related. Panthera tigris is a tiger, but Panthera leo is a lion. Hazen wants something like that for minerals.
In Hazen’s system, a mineral would still have its species — diamond, for example. But it would also have a “kind” — a way to describe how the mineral formed. Diamonds would no longer be just diamonds. Instead, a diamond formed in a meteorite would be an impact diamond. A diamond in the mantle of the Earth would be Type I or Type II.
That doesn’t mean the IMA would go away. “I’m not trying to overthrow the system,” Hazen says. Instead, he wants to add a “kind” to each mineral. That “kind” would be like a note telling the story of where the rock came from.
This might seem like a small change. But “adding that formation footnote to a mineral opens up new worlds of information,” Hazen explains. “It will allow us to look at ancient minerals and pinpoint more precisely the environment in which they formed.”
If a scientist finds iron with rust inside it, it’s no longer just iron. The rust is important, too. Rust indicates there was oxygen present when the iron formed. So there had to be something like microbes or plants around to fill the air with oxygen. These kinds of details tell scientists more about the world in which the mineral formed. Hazen presented his idea of mineral “kinds” at the American Geophysical Union fall meeting on December 14, 2018.
“I think it’s a good idea” to add these “kinds” to a mineral, says Edward Grew. He’s a mineralogist at the University of Maine in Orono. “It’s no good to consider minerals in isolation — to have a pretty quartz crystal on your table, and not think about how it formed.”
Of course, minerals form in many different places on Earth and in outer space. Hazen hopes that telling the life story of minerals might help scientists learn more about our planet — and other planets, too. “Why is Earth special? Is it special?” he asks. “What’s different about Mars or Mercury or the moon? Minerals are in some cases the most robust lines of evidence we have of the 4.5-billion-year history of our solar system.” He thinks that by noting how minerals formed, scientists might have a better chance of understanding how the solar system — and the rock we call home — came to be.
atmosphere The envelope of gases surrounding Earth or another planet.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
crystal (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.
curator Someone who manages a collection of items, for instance in a museum, library or art gallery. This person’s primary job is to design exhibits, organize and acquire collections and do research on the artifacts included in the collection.
diamond One of the hardest known substances and rarest gems on Earth. Diamonds form deep within the planet when carbon is compressed under incredibly strong pressure.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
genus (plural: genera) A group of closely related species. For example, the genus Canis — which is Latin for “dog” — includes all domestic breeds of dog and their closest wild relatives, including wolves, coyotes, jackals and dingoes.
iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.
mantle (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.
Mars The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.
meteorite A lump of rock or metal from space that passes through Earth’s atmosphere and collides with the ground.
microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns).
moon The natural satellite of any planet.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
pressure Force applied uniformly over a surface, measured as force per unit of area.
quartz A type of mineral made from silicon dioxide. The most common mineral on Earth, it can occur in any rock type: igneous, metamorphic or sedimentary.
Queensland One of the states that makes up the northeast corner of the country of Australia.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
Meeting: R.M. Hazen. Natural kind cluster analysis of mineral chemistry data: A new approach to mineral classification. American Geophysical Union Meeting 2018. December 11, 2018. Washington, D.C. V21A-03.
Journal: R.M. Hazen. An evolutionary system of mineralogy: Proposal for a classification of planetary materials based on natural kind clustering. American Mineralogist. Vol. 104, published online November 2, 2018. doi: 10.2138/am-2019-6709.