Crystal growth of calcium oxalate monohydrate and calcium carbonate from dilute solutions

By controlling evaporation, calcium carbonate (calcite) is precipitated together with calcium oxalate monohydrate from equimolar (1 x 10(-5) M) solutions of calcium chloride and sodium oxalate under decreasing (37.5 degrees - 32.7 degrees C) and increasing temperature (22 degrees-42 degrees, 22 degrees-45 degrees, 22 degrees-50 degrees C), and initial pH's of 6.6 and 6, respectively. If, however, the pH of the solutions is, respectively, 8 and 8.5 and the temperature is 32 degrees C, oxalate breaks down to carbonate and calcium precipitates solely as calcite. This process materializes as the pH of both solutions initially adjusts to about 7.5. Both the quality and the size of the crystals of calcite and calcium oxalate monohydrate varied markedly. Euhedral crystals of calcite, measuring in excess of 200 micrometers in cross-section, and grown at 37.5 degrees C were remarkably ordered as shown by the lack of twinning, streaks and/or diffuseness around the diffracta, which is evidenced by single-crystal x-ray diffraction analysis. In contrast, calcium oxalate monohydrate was almost always disordered due to twinning along the (101) plane and never exceeded 100-150 micrometers along a coordinate axis.