University of Minnesota
Something was lacking in the rocks Cameron Meyers used in his mineral physics experiments at the University of Minnesota. He realized what it was with insight gained during his 2016 Department of Energy National Nuclear Security Administration Stewardship Science Graduate Fellowship (DOE NNSA SSGF) practicum.
While at Lawrence Livermore National Laboratory, Meyers worked with scientist Rebecca Dylla-Spears to perfect an additive manufacturing (3-D printing) process to make transparent glass objects. The method could help tailor lenses for optics and lasers. “The end goal was to be able to print variable titanium concentrations in a silica glass,” creating precise changes in the way the lens bends light, says Meyers, a rock and mineral physics doctoral candidate.
The team’s process, described in the journal Advanced Materials, starts with silica nanoparticles suspended in solvents. “That creates a goo-like suspension” that’s extruded through a nozzle in concentric circles to form a solid cylinder. The researchers dried the cylinder at around 100 C and then at 600 C to remove the solvent and other contaminants. Finally, the cylinder is briefly fired at 1,500 C to consolidate it into a dense, transparent glass.
During his practicum, the research team Meyers joined also was developing methods, using a similar direct-ink-write additive manufacturing process, to print transparent ceramic laser gain media, material that boosts a laser beam’s power.
The experience prompted Meyers to reconsider his research back in Minnesota, where he probes how rocks behave in Earth’s mantle, far below the surface. Myers and David Kohlstedt, an earth sciences professor, focus on olivine, a green, abundant mineral thought to control the mechanical properties of the Earth’s upper mantle. In experiments, the researchers squeeze, twist or pull olivine-rich rocks under high pressures and temperatures, replicating – on much smaller time and length scales – processes that occur deep underground. How subsurface rocks deform and flow at the atomic and grain scales under these extreme conditions can influence large-scale phenomena like plate tectonics and earthquakes. Meyers specifically examines physical processes at grain boundaries, the interface where neighboring crystals meet within a rock. Geophysicists extrapolate lab data like those Meyers produces to understand what’s happening far below ground.
But naturally exposed olivine-rich rock, called dunite and brought to the Earth’s surface from the mantle, won’t do for his tests. Meyers needs rocks with consistent microstructures and clean grain boundaries. Most naturally exposed dunite “no longer really looks like we expect it to look in the Earth’s mantle” because chemical reactions and brittle cracking heavily alter it as it’s exhumed.
Meyers and most other researchers in his field instead synthesize olivine rock samples for experiments. They pulverize gem-quality olivine crystals to a fine powder, with particles measured in millionths of a meter, then squeeze it under high pressures and temperatures to form a dense ceramic body – a synthetic rock.
Rocks that result from this hot-pressing process, used in research throughout Meyers’ field, are opaque and milky-green. But on his practicum Meyers saw Livermore researchers synthesize transparent ceramics. He began “thinking about the possibility that the rocks we were making were not fully dense because they were not transparent.” In other words, “if the gems you’re starting with are transparent and the rock you’re making is opaque, there has to be something scattering that light.”
Tests found he was right: Conventionally hot-pressed olivine contains tiny pores. The voids are invisible, but heating causes them to grow and “all of a sudden all this trapped contamination becomes visible” under a microscope. The voids can cause crystal grains to grow more slowly. “If that structure is interrupted, you’re no longer able to measure the physics that’s happening at those grain boundaries,” which is critical to understanding deformation processes in mantle rocks. In some cases, experiments using conventionally hot-pressed material “may measure physics related to the contamination. You’re no longer measuring what you’re interested in.”
The contamination, Meyers says, comes from the crystal powder’s exposure to air. With Kohlstedt and research associate Mark Zimmerman, he developed a new process, called evacuated hot pressing, that squeezes and heats the fine olivine crystals while under a vacuum so the contaminants and pore spaces are vented. The result: dense, green, transparent synthetic rocks that resemble the gem-quality starting material, in contrast with previous opaque specimens.
Meyers is continuing his deformation research using samples made with his new technique, trying to understand how their characteristics might change experimental outcomes. “It definitely could rewrite some significant data that people have been using for a long time.” Other researchers already are inquiring about collaborations even before the rock-synthesis method is published.
Meyers expects to graduate in spring or fall 2018. He plans to pursue postdoctoral fellowships or go into industry, perhaps in materials science.
Image caption: Olivine gems (the irregularly shaped rocks to the right and left) surround synthetic olivine samples. The two milky green, coin-shaped samples at center right were produced using a conventional hot-press process. The transparent coin-shaped samples and the column at center left were made with Cameron Meyers’ evacuated hot-press process. Image courtesy of Cameron Meyers.