2018 年 47 巻 1 号 p. 34-42
Organic minerals are natural organic compounds with both well-defined chemical composition and crystallographic properties; their occurrences show traces of the high concentration of certain organic compounds in natural environments. Thus the origin and formation process of organic minerals will lead us to understand the fate and behavior of the organic molecules in the lithosphere. All of each organic mineral can be classified into the one of following two groups: ionic organic minerals in which organic anions and various cations are held together by ionic bonds, and molecular organic minerals in which electroneutral organic molecules are bonded by weak intermolecular interactions. Karpatite, a natural crystal of coronene (C24H12), is the most typical molecular organic minerals and its crystal/molecular structures and carbon isotopic composition suggests that this mineral was crystallized from PAHs (polycyclic aromatic hydrocarbons)-rich hydrothermal petroleum by hydrothermal activity. In the process of formation of organic minerals, the formation of structural units, such as organic acid anion and PAH molecules, precedes their migration and concentration. The first stage includes the formation or cleavage of C-C bonds, but the latter stage does not. In addition, we have investigated the influence of size, morphology, surface structure, and aggregation state on the reductive dissolution of hematite with ascorbic acid using two types of nanoparticles with average diameters of 6.8 ± 0.8 nm and 30.5 ± 3.5 nm, referred to as Hem-7 and Hem-30, respectively, in this paper. TEM (transmission electron microscope) observation revealed that previous to dissolution, Hem-7 is present as both dispersed particles and as aggregates. Dispersed particles dissolve initially before aggregates, which influences its dissolution rate. The Hem-30 hematite has nanoscale surface steps and internal defects, and its dissolution initiates from the steps, defects, or sharp edges of the crystals. This study directly shows the importance of size, surface roughness, defects, crystal morphology and aggregation states on dissolution rates of nanoparticles.
