Although the processes affecting the global carbon cycle that occur deep inside Earth proceed far more slowly than ones occurring elsewhere, they strongly influence the amount of carbon in the atmosphere and global temperatures. Yet the nature of deeply buried carbon—the compounds it forms and their structures—remains controversial. So Eglantine Boulard and Wendy L. Mao of Stanford University; Giulia Galli of the University of Chicago; and coworkers applied synchrotron spectroscopy methods and quantum calculations to study Mg0.25Fe0.75CO3, a member of the ferromagnesite family of minerals believed to be deep-Earth carbon storage compounds. The team subjected a sample of the mineral to extreme pressures and temperatures found in the mantle, a region of Earth’s interior between its core and crust. They found that at pressures greater than 80 gigapascals, the carbonate transforms from a trigonal sp2-bonded CO3 phase to a phase featuring tetrahedral sp3-bonded CO4 groups (Nat. Commun. 2015, DOI: 10.1038/ncomms7311). The phases likely differ in terms of chemical reactivity. And the high-pressure phase is likely to be more viscous, which would inhibit mobility of carbonate melts in the mantle and lead to deep carbon reservoirs, thereby affecting the global carbon cycle, the team says.