Habitus

Homocystinuria Usually a failure of cystathionine synthase (Fig. 40-2 reaction 6). Rarely associated with aberrant vitamin B12 metabolism (Fig. 40-2) Thromboembolic diathesis, marfanoid habitus, ectopia lentis. Mental retardation is frequent. Diet low in methionine Vitamin B6 in pyridoxine-responsive syndromes Vitamin B12 in responsive syndromes Anticlotting agents... 

Endocrine system Cushing s habitus, hirsutism, retardation of growth, suppression of hypothalamopituitary-adrenal axis. 

As well as the Habitus variation described above, another term, Tracht, is applied to describe the variation of forms due to the combination and degree of development of faces within the same category of Habitus. There is no distinction in English between Habitus and Tracht, both being called crystal habit, but in German Tracht is distinguished from Habitus. 

Figure 2.3. Relation between crystal habit (Habitus) and the forms of unit cells. The figures in the upper two quadrants indicate the different Habitus expected for different unit cells, and those in the lower two quadrants indicate the different Habitus that the same unit cell can take.

The first is the prediction of the Habitus made from the characteristics of the crystal structure, entirely neglecting the effect of growth conditions. We will call this the structural form or abstract form. The second logical approach is to predict the Habitus thermodynamically when the crystal reaches the equilibrium state. This may be called the equilibrium form. The third is a method of analyzing the factors that may have an effect by correlating the Habitus and Tracht shown... 

Let us start by examining the origin of malformed Habitus that deviates greatly from the structural or equilibrium forms. 

In natural crystals, whose growth processes cannot be directly observed, the difference in R is recorded as the difference in separation in growth banding (see Chapter 6). Based on these observations, several papers were reported in which the direction of flow of ore-forming fluid was evaluated in pegmatite and hydrothermal veins. In many cases in which the natural mineral crystals exhibited extensively malformed Habitus from that predicted by the structural form, the malformation could be considered to be due to the remarkable anisotropy involved in the environmental conditions, such as the directional flow of the solution, which is similar to the situation of growth of NaCl from solution in between two glass plates, as discussed above. 

Figure 4.10. Kuroda s model explaining the repeated Habitus change of snow crystals [23]. Shaded areas are crystals dotted areas are QLLs (quasi-liquid layers).

The thickness of the QLLs is different on different faces, and becomes thinner as the temperature is decreased. Thus, the temperature at which a QLL disappears depends on the face. When the surface is covered by Q.LLs, crystal growth is regarded as a solution or melt phase growth, whereas on a naked surface it is due to vapor growth. Thus, the repeated Habitus change may be explained. The presence of QLLs was confirmed by ellipsometry. 

Tracht change differs from change in Habitus in that it describes a change in the form of a crystal grown in an isotropic environmental phase through the combination of different faces and the relative sizes of respective faces. Therefore, Trachts are forms determined by the relative ratio of the normal growth rate R of the different crystal faces present on the surface of a growing crystal. The... 

The structural form of brookite (TiO ) is expected to be bounded by 210 and 111, both being F faces by PBC analysis, but the actual growth form observed is platy Habitus bounded by largely developed 100, which is an S face. The misfit ratio between the PBC on the (0110) face of quartz and that on (100) ofbrookite is the smallest among any misfit ratios between the two crystal species. From this, it was found that the platy Habitus ofbrookite arose because quartz adsorbed in an epitaxial relation on 100 ofbrookite, thus diminishing thegrowthratekof(lOO) [30]. 

It is generally observed that crystals from the same species exhibit widely different Tracht and Habitus depending on whether they grow from the vapor or solution phases. 

The above examples describe the Habitus changes due to the difference between the vapor and solution phases, and between solvent components in solution... 

Even if crystals grow from the same aqueous solution, there are differences in Habitus. NaC103 crystals, for example, grow easily as polyhedral crystals, whereas NH Cl crystals always grow as dendrites, and NaCl crystals appear as hopper crystals. If Pb or Mn ions are added, cubic crystals of NaCl bounded by flat 100 faces may be obtained quite easily, but if NaCl is grown in pure solution all crystals take a hopper form, unless great care is taken to keep the supersaturation very low. These differences occur because the solute-solvent interaction energies, and, as a result, the values of Ap,/kT and A/x/kT, are different for different crystals. 

The reasons why we have Habitus variation for vapor and solution phases, and for different solvents when the crystals grow from the solution phase, can therefore be understood in terms of the factor of steps because of the different solute-solvent interaction energies. 

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