Effect on crystal habit

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

The nature of solvent has been found to have a profound effect on crystal habit of ibuprofen. Ibuprofen crystals precipitated from ethanol and acetone (solvents having high surface tension, dielectric constant, and less specific gravity) were thin, platy, and nearly circular-shaped, whereas those obtained from propylene... 

Impurities often have a profound effect on crystal habit. Adsorption of an impurity on faces of a crystal may retard the growth of certain crystal faces, and these will therefore become prominent in the final crystal, as described earlier (see Figure 2.7). For example, sodium chloride ... 

J. Wang, Z. Berkovitch-Yellin, and L. Leiserowitz, Location of tailor-made additives in the crystal and their effect on crystal habit. A study on the host-additive system L-asparagine-L-aspartic acid monohydrate, Acta Crystallogr., Sect. B, B41, 341-348 (1985). 

Sodium chloride and sodium cyanide are isomorphous and form an unintermpted series of mixed crystals. The ferrocyanide ion has a marked effect on the habit of sodium cyanide crystallized from aqueous solution (50). Sodium cyanide and sodium carbonate form a molten eutectic at approximately 53 wt % sodium carbonate and 465°C. The specific conductivity of molten 98% sodium cyanide is 1.17 S /cm (51). 

Garti, N. Karpuj, L. Sarig, S. The effect of solvents and crystallization conditions on crystal habit of cholesterol. Cryst. Res. Technol. 1981, 76(10), 1111-1115. 

Garti N, Leci CL, and Sarig S. The Effect of Solvents on Crystal Habit of 1,4-Di-Tert-Butylbenzene (DTB ). J Cryst Growth 1981 54 227-231. 

Changes in the solvent used or the presence of an impurity can also profoundly affect the crystal habit. Figure 2.17 shows the effect of aluminum fluoride on the habit of anhydrous calcium sulfate. The impurity transforms the needle-like habit to a cubic looking crystal. Figure 2.18 demonstrates the effect of urea on sodium chloride crystals. A large amount of qualitative information exists on the effect of impurities on crystal habit (Mullin 1993 ... 

A problematic area is how to ratify the effect of solvent or impurities. When analyzing the effect of solvent or impurities on crystal habit, two factors are essential to understanding the process. 

Whetstone, J. (1949b) The effect of crystal habit modification on the setting of inorganic... 

FIG. 2 Polymorphism in ammonium nitrate (top) and effect of additives on crystal habit modifications (bottom). 

Both supersaturation and temperature can have different effects on the growth rates of different faces of the same crystal. Such occurrences have implications with respect to crystal habit, and these are dealt with in a later section. 

Rubinsky, B. DeVries, A.L. (1989). Effect of ice crystal habit on the viability of red blood cells. Cryobiol. 26, 580 (abstract). 

The presence of a solvent, especially water, and/or other additives or impurities, often in nonstoichiometric proportions, may modify the physical properties of a solid, often through impurity defects, through changes in crystal habit (shape) or by lowering the glass transition temperature of an amorphous solid. The effects of water on the solid-state stability of proteins and peptides and the removal of water by lyophilization to produce materials of certain crystallinity are of great practical importance although still imperfectly understood. 

In the host/additive systems described above, such as (/ ,S)-ser/(/ ,S)-thr and glycine/(/ ,S)-a-amino acids, the change in crystal habit and the enantiomeric segregation of occluded additive indicates that only a subset of the possible symmetry-related sites may be occupied by an additive depending on through which faces the additives are occluded. Such an effect would lead to a reduction in the overall symmetry of the host/additive crystal. 

In a recent paper (70 Hartman studies the effect of surface relaxation on the habit of corundum and hematite. The habits observed on natural and synthetic crystals of these systems did not agree with calculated relaxed equilibrium or growth habits. Hartman concluded that these observations could be understood by invoking specific solvent adsorption on (111) faces. 

The work discussed in the previous paragraphs provides the framework for the prediction of crystal habit from internal structure. The challenge is to add realistic methods for the calculation of solvent and impurities effects on the attachment energies (hence the crystal habits) to allow this method to provide prediction of crystal habit. Initial attempts of including solvent effects have been recently described (71. 721. The combination of prediction of crystal habit from attachment energies (including solvent and impurity effects) and the development of tailor made additives (based on structural properties) hold promise that practical routine control and prediction of crystal habit in realistic industrial situations could eventually become a reality. 

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