CRYSTAL CHEMISTRY AND GENESIS OF ORGANIC MINERALS: A REVIEW OF OXALATE AND POLYCYCLIC AROMATIC HYDROCARBON MINERALS

Organic minerals are natural organic compounds with both a well-defined chemical composition and crystallographic properties; their occurrences reveal traces of the high concentration of certain organic compounds in natural environments. Thus the origin and process of formation of organic minerals will lead us to understand the fate and behavior of the organic molecules in the lithosphere. With the aim of their contribution to new developments in mineralogy, we subdivide organic minerals into two groups: (1) ionic organic minerals, in which organic anions and various cations are held together by ionic bonds, and (2) molecular organic minerals, in which electroneutral organic molecules are bonded by weak intermolecular interactions. This review is composed of four sections. The first section is concerned with the definition of both organic minerals and the above two groups. The second deals with crystal chemistry and geochemistry of oxalate minerals, which are the most typical ionic organic minerals. In this section, the role of (H2O)(0) is first discussed, as most oxalate minerals incorporate (H2O) 0 into their crystal structures. Then the phase relationships among hydrous and anhydrous calcium oxalate minerals, namely their structural hierarchy, are described, owing to the fact that they are the most abundant ionic organic minerals. In addition, the weak Jahn-Teller effect of the Fe2+ ion is exemplified in humboldtine [Fe2+(C2O4)center dot 2H(2)O]. The Fe2+ ion causes distortions of octahedra in this organic mineral, though the effect has hardly been observed in inorganic minerals. In the third section, we describe the crystal chemistry and process of formation of polycyclic aromatic hydrocarbon (PAH) minerals, which are the most typical molecular organic minerals. Those of karpatite (C24H12) and idrialite (C22H14) are particularly considered in detail. In the fourth section, we summarize the characteristics of organic minerals and discuss their contribution to Earth and planetary sciences.