CALCIUM OXALATE IN COAL

Calcium oxalate (CO) is a salt of calcium and oxalic acid (OA). There are two kinds of such minerals which are commonly found in coal: whewellite (CaC₂O₄·H₂O) and weddellite (CaC₂O₄·2H₂O). Also, small crystals of CO are found in the cell structures of modern-day plant tissues. In addition, CO is the main component of urinary stones in the bodies of mammals. But they are considered extremely rare in the geological environment. Since the 1960s, CO has been found in the seabed sediments, peat/lignite, low-rank coal, organic-rich soil, street sediments, limestone and so on.

Recently, small amounts of CO (weddellite) have been found in low-rank coal in China. For example, the occurrence of CO in Ge-bearing lignite from the Shengli coal field, Inner Mongolia has been detected. In this study, CO was discovered in a super large Ga ore deposit associated with coal seam from Heidaigou, Jungar. The shapes of the CO are spherical, ellipsoid and irregular. Based on the OA soaking experiment of different samples (granite, plagioclase, kaolinite, montmorillonite, the coal sample and parting), we found that Ga was significantly activated by OA: (1) the release content of Ga from these samples increased with the concentration of OA; (2) the Ga release ability of samples increased by coal, parting, rock (mineral) sequence, and Ga in clay minerals (kaolinite and montmorillonite) was easier to release into OA than in granite and plagioclase. We also found that OA and Ca²⁺ were easily formed as whewellite with the existence of humic acid, but humic acid could inhibit the growth of its crystal plane. The inhibiting effect was more obvious on the 020 crystal plane than on 010 plane and increased with the concentration of sodium humate. This is the reason why the CO in low-rank coal shows spherical, ellipsoid and other irregular shapes. We believe that the leaching of OA and other micromolecular organic acid during coal-forming process is the reason why dispersed elements like Ga and Ge are released from bedrock and are migrated into peat bog, which leads to the enrichment and mineralization of Ga and Ge in coal. The research may have implications on the mineralization of dispersed elements and the mechanism of forming urinary stone.