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 (H 2 O) 0 is first discussed, as most oxalate minerals incorporate (H 2 O) 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 Fe 2+ ion is exemplified in humboldtine [Fe 2+ (C 2 O 4 )•2H 2 O]. The Fe 2+ 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 (C 24 H 12 ) and idrialite (C 22 H 14 ) 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.Les minéraux organiques sont des composés organiques naturels ayant à la fois une composition chimique et des propriétés cristallographiques bien définies. Leurs présence témoigne de la concentration élevée de certains composés organiques dans les milieux naturels. Donc, l'origine et les processus de formation des minéraux organiques nous permettront de comprendre le sort et le comportement de molécules organiques dans la lithosphère. Dans le but de contribuer à l'essor de nouveaux développements en minéralogie, nous divisons les minéraux organiques en deux groupes: (1) minéraux organiques ioniques, dans lesquels les anions organiques et divers cations sont liés par des liaisons ioniques, et (2) minéraux organiques moléculaires, dans lesquels §

The crystal structure of whewellite [Ca(C2O4)·H2O] and the dehydration mechanism of caoxite [Ca(C2O4)·3H2O] have been studied by means of differential thermal analysis, X-ray diffraction (powder and single-crystal) analysis and infrared analysis. The first and second analyses confirmed the direct transformation of caoxite into whewellite without an intermediate weddellite [Ca(C2O4)·2H2O] stage. Infrared spectra obtained from caoxite, weddellite and whewellite emphasize the similarity of the O–H-stretching band and O–C–O-stretching band in whewellite and caoxite and the unique bands of weddellite. The structure refinement at low temperature (123 K) reveals that all the hydrogen atoms of whewellite form hydrogen bonds and the two water molecules prop up the crystal structure by the hydrogen bonds that cause a strong anisotropy of the displacement parameter.Comparing the structural features of whewellite with those of weddellite and caoxite suggests that caoxite and whewellite have a sheet structure consisting of Ca2+ ions and oxalate ions although weddellite does not. It is additionally confirmed that the sheets of caoxite are corrugated by hydrogen bonds but whewellite has flat sheets. The corrugated sheets of caoxite would be flattened by dehydration so the direct transformation of caoxite into whewellite would not occur via weddellite. Essential for this transformation is the dehydration of interlayered water molecules in caoxite leading to the building of the crystal structure of whewellite on its intralayered water molecules. The difference in conformation of water molecules between those two crystal structures may explain the more common occurrence of whewellite than of caoxite in nature.

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