Calcite crystallization, phosphate

Phosphate Distribution During Calcite Crystallization. The crystallization rate data Illustrated in Figure 4 follow a rate equation... 

Figure 6. Calcite crystal groioth in the presence and absence of phosphate ion, as expressed by the rate function N -No vs.time.The symbols and the numbers beside the curves indicate the phosphate concentration multiplied by 10. Adapted...

A detailed examination of phosphate distribution between solution and solid phase during calcite crystallization in a simulated natural water shows that phosphorus adsorbs as a mono-layer, causing slight changes in the solution phosphorus concentration. It appears that under the conditions examined in this study, calcite- mediated phosphorus mineralization has a role in the movement of phosphorus from the water column to bed sediments, although the extent and rates of the process in natural systems remain to be determined. 

About 10% of the mass of an egg is the shell, the yolk is about 30%, and the remaining 60% is the white (Fig. 2.39). The shell is mostly made up of minerals calcium carbonate, or more precisely, calcite dominates with small amounts of magnesium carbonate and various phosphates. The shell also contains about 3.3% proteins in a form called mucopolysaccharide complex, which is a network of globular and fibrous protein bits which hosts a large number of calcite crystals (about... 

An important application of soluble oligopolyphosphates is in the threshold treatment of hard water to prevent blockages of pipes arising from the growth of calcite crystals. About one part per million, when added to hard water, will inhibit the precipitation of calcite. This is believed to be due to the absorption of phosphate anions on to the surface of the calcite nuclei, which then prevents further crystal growth. The flexibility and dimensions of the polyphosphate chain allow a good fit to be made on the pattern of ions on the exposed calcite face (12.31) [3]. ... 

The shell consists of calcite crystals embedded in an organic matrix or framework of interwoven protein fibers and spherical masses (protein-mucopolysaccharide complex) in a proportion of 50 1. There are also small amounts of magnesium carbonate and phosphates. 

The bones and teeth of humans and other vertebrate animals, for example, consist mainly of a composite material made up of an organic substance, collagen, and a biomineral, calcium carbonate phosphate (see Textboxes 32 and 61). The latter, which makes up about two-thirds of the total dry weight of bone, is composed of calcium phosphate containing between 4-6% calcite (composed of calcium carbonate) as well as small amounts of sodium, magnesium, fluorine, and other trace elements. The formula Ca HPChXPChMCChXOH) approximately represents its composition its crystal structure is akin to that... 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

While CaC03 crystals (calcite and aragonite) predominantly appear in egg shells and in biomineralisates from invertebrates, calcium phosphates are predominantly involved in processes which play an inportant role in medicine. They will be described here in detail the knowledge of their structures are most relevant for the understanding of the cellular and molecular processes in bones and teeth. 

A third problem with the mitochondrial theory of biomineralization is that many mineralized tissues contain carbonate rather than phosphate. Since bicarbonate ions do not pass across mitochondrial membranes with any ease, it has now been shown that in phosphate-free buffers, calcium will enter mitochondria if dissolved carbon dioxide is available. It appears that some mitochondria possess carbonic an-hydrase activity on the inner membrane or in the mitochondrial matrix and are thus able to synthesize bicarbonate within the organelle. In such cases, inhibitors of carbonic anhydrase block the accumulation of calcium and carbonate ions622) since crystals of calcite have been identified in the mitochondria of earthworms calci-ferous glands623. These cells freqently showed spherical granules in the cytoplasm and lumen of the glands during phases of mineral secretion and it was suggested that they were aspects of cellular breakdown which occurred at these times. 

Extensive seeded calclte growth experiments in the presence of phosphate ion indicate that the phosphate ion adsorbs onto the crystal surface as a monolayer. At a concentration of 10" M, phosphate ion can strongly inhibit calcite formation however, short term experiments show that this monolayer adsorption removes insignificant amounts of phosphorus from solution. In experiments lasting several days a further decrease in solution phosphate concentration occurs, presumably caused by nucleation of a surface calcium phosphate phase on the calcite seed. 

The phenomenon of polymorphism was noted by Martin Heinrich Klaproth °i in 1798, when he proposed that the minerals calcite and aragonite must have the same chemical composition, CaCOa. Calcite forms a rhombohedral uniaxial crystal and is the stable form under normal conditions, with a density of 2.71 g/ml. Its metastable polymorph, aragonite, is an orthorhombic biaxial crystal with a density of 2.94 g/ml. This work was continued by Louis Jacques Thernard, Jean Baptiste Biot, and Eilhard Mitscherlich. Mitscherlich, for example, reported on it in his studies of phosphates and arsenates. The transition from calcite to aragonite has been studied at different pressures. ... 

Beilstein Handbook Reference) Amine D BRN 3084620 Caswell No. 276 Dehydro-abietylamine EPA Pesticide Chemical Code 004206 HSDB 5665 13-lsopropylpodocarpa-8,11,13-trien-15-amine Podooarpa-8,11,13-trien-15-amine, 13-isopropyl-. Used as asphalt additive, as cationic collectors for calcite, sylrite, mica, feldspar, vermioilulite and phosphate rock concentration operations. Registered by EPA as an antimicrobial and fungicide (cancelled). Crystals [a]8 = - 56.10 (c=2.4, pyridine). Fluka Hercules Inc. 

Replacement. CFA may also crystallize through replacement of other minerals, particularly calcite or gypsum. This was first shown in a series of experiments in which powdered calcite was exposed to a phosphatic solution and converted to CFA (Ames 1959). This process has been widely reported in nature with phosphatization of foraminiferal and other shell material. Initially, this process was thought to involve P04 ions replacing COs ions within the structure, a realignment of the Ca, and addition of F. This method is unlikely because of the size differences of the phosphate and carbonate, the coordination difference of Ca between the two minerals, and completely different crystal structures. It has since been shown instead that the calcite or gypsum dissolves first before apatite crystallization (Nathan and Lucas 1972). Dissolution of either of these minerals would increase the Ca/Mg ratio and encourage crystallization of apatite. 

Neither group included phosphate. It is possible that the same calcite (or aragonite) exposed to different amounts of sea water, or the same amount of sea waters with different trace contaminant levels could yield different apparent solubilities due to different states induced into the outermost several molecular layers of the crystal surface. It must be noted that the crystal/sea water volume ratio employed in Berner s experiments is considerably smaller than that employed by Ingle et al. (1973) and for the saturometer experiments. This points out the need for experiments involving greatly different ratios of sea water volume to calcite siurface area and experiments comparing real and artificial sea... 

---