NATURAL KIND CLUSTERING, PLANETARY EVOLUTION, AND THE CLASSIFICATION OF CARBON-BEARING MINERALS

Minerals preserve a vivid narrative of the physical, chemical, and biological histories of their origins and subsequent alteration, and thus provide our clearest window to the evolution of Earth and other worlds through billions of years of Cosmic history. Mineral properties, including trace and minor elements, ratios of isotopes, structural defects, inclusions, external morphologies, and other idiosyncratic attributes, represent information that points to specific modes of formation and subsequent environmental histories—information essential to understanding the co-evolving geosphere and biosphere. This perspective presents an opportunity to characterize the diverse stages of planetary evolution by amplifying the existing system of mineral classification, which is based on idealized end-member chemical compositions and crystal structures—fictive “species” that are rarely observed in nature.

We propose an alternative and complementary “evolutionary system” of mineralogy—a classification scheme that emphasizes the diverse chemical and physical attributes of minerals that emerge from the rich and varied constellation of dynamic planetary processes. This system is particularly applicable to common carbon-bearing minerals, including diamond, moissanite, and calcite, each of which occurs in multiple natural kinds with diverse modes of origins. Thus, for example, we distinguish different natural kinds of diamond, including those forming in low-pressure stellar environments, at static high pressure in Earth’s mantle, and through impact processes. Similarly, several kinds of moissanite (SiC) are distinguished by their unique isotopic signatures—a consequence of differing stellar hosts. And calcite displays numerous natural kinds that arise from varied physical, chemical, and biological processes.