Control of the Crystal Growth of Calcium Oxalates

Spherulites

“…Today, biomineral forming substances grown under exotic conditions with unusual additives frequently populate the spherulite literature. Calcite (CaCO 3 ) holds the pride of place. − Calcium oxalates − and apatite (Ca 5 (PO 4 ) 3 (OH,F)) ,, have also been studied intensively in relation to biomineralization. Other examples of spherulites from solution include several fluorides, rare earth carbonates, hematite (Fe 2 O 3 ), , β-FeO(OH), rare earth tartrates, , zinc oxide, several copper iodates, and scheelite (CaWO 4 ). , …”

“…(b) Calcium oxalate monohydrate. From ref with permission by A. Thomas. (c) Calcium oxalate dihydrate.…”

“…Double-leaf core morphologies have been studied in detail (see section 6.3) ,,, Their evolution is comparable for polymers ,, and molecular crystals ,, grown from the melt, as well as for solution grown crystals. ,, The only difference relates to the contents of the so-called eyes (Figure ) that flank the initial nuclear needle. Because of a lack of material supply in solution-grown spherulites the eyes can be voids (Figure a). ,,, Spherulites from the melt may contain filled eyes composed of ordered fibers with gradually changing orientations (Figure c,h), chaotically misoriented fibers (Figure b,d), one single crystal (Figure e), or even one flat leaf covering the whole region (Figure f). Fiber growth directions in the eyes and the rest of spherulite may be different (Figure g) .…”

“…Section 3 shows that it is easy to get spherulites by crystallization of slightly soluble salts from highly supersaturated solution by precipitation or by growth in a gel. In both cases, thermodynamic supersaturation divided by the number of ions in a neutral complex, Δμ/ RTv , is extremely high exceeding 2.3 for calcium carbonate polymorphs, 2.5 for calcium oxalate dihydrate, or 9 for scheelite, CaWO 4 . Highly soluble salts that grow at low supersaturations (Δμ/ RTv < 0.3) rarely form spherulites.…”

“…Electron micrographs ,,− show that crystallites in spherulites are polygonal with flat faces. This observation, along with the constancy of growth rate, ,, argues for interface controlled growth, at least in part.…”