New rock types emerge during the history of the Earth.  For example, the silica-rich felsic rocks such as granite that characterize continental crust, accumulated during the course of Earth history.  Granite only forms in certain specific tectonic settings, such as above subduction zones and when lower crustal rocks melt in mountain belts.  But what about the minerals themselves?  Have they been around since the Earth formed, or did they too only appear on the scene later as a result of some geological process?
The question of how and when the minerals evolved is a relatively new subject, and was, and continues to be, pioneered by this episode's guest.  Bob Hazen is Senior Staff Scientist at the Earth and Planets Laboratory of the Carnegie Institution for Science and Professor of Earth Sciences at George Mason University.  At a Christmas party in 2006, a well-known biophysicist asked him the question: “Were there clay minerals in the Archean?”  Apparently, nobody had given this much thought prior to 2006.  The topic quickly became the focus of his research, rapidly blossoming into a whole new branch of mineralogy.