In 1976, Hazen joined the Carnegie Institutions Geophysical Laboratory as a research associate. After a brief stint measuring optical properties of lunar minerals with Peter Bell and David Mao, he started to do X-ray crystallography with Larry Finger. He later recalled, "It was a match made in mineralogical heaven: Larry loved to write code, build machines, and analyze data; I loved to mount crystals, run the diffractometers, and write papers." They collaborated for two decades and determined about a thousand crystal structures at variable pressures and temperatures, work summarized in their 1982 book Comparative Crystal Chemistry'.
Much of the work that Hazen was doing could be classified as mineral physics, a cross between geophysics and mineralogy. Although the field had pioneering contributions from the Nobel Prize winner Percy Bridgman and a student of his, Francis Birch, in the early- to mid-20th century, it did not have a name until the 1960s, and in the 1970s some scientists were concerned that a more interdisciplinary approach was needed to understand the relationship between interatomic forces and mineral properties. Hazen and Prewitt co-convened the first mineral physics conference; it was held on October 17–19, 1977 at the Airlie House in Warrenton, Virginia.