Yu. O. Punin

On the origin of anomalous birefringence in grandite garnets

Abstract The origin of anomalous birefringence of grossular-andradite (grandite) garnets from skarns in Mali and Russia was considered. The crystals had complex optical patterns which can be induced by superposition of two phenomena: mismatch compositional strain (stress birefringence) and growth ordering of atoms (growth dissymmetrization). Study of the crystals using several experimental techniques (optical microscopy, microprobe analysis, X-ray diffraction topography and X-ray single crystal diffraction) as well as calculations of anomalous birefringence has confirmed this hypothesis. Depending on the crystal composition and growth conditions, the relative magnitude of each phenomenon controls the various optical effects. As a result one can see two groups of crystals which are found to have fundamentally different anomalous optical properties: crystals with low (<0.001) and high (0.001-0.015) values of birefringence. The spatial distribution of birefringence within each group is different and this fact is related to different mechanisms causing optical anomalies: stress birefringence and growth dissymmetrization for these two groups, respectively.

Growth ordering and anomalous birefringence in ugrandite garnets

Abstract Intermediate members of the grossular−andradite and grossular−uvarovite series are known to display anomalous birefringence, which is inconsistent with the ideal cubic space group of garnet Ia3̅d. To determine the reason for such birefringence, the crystal structures of at least 15 samples were refined by different authors including the present ones. The crystals with the value of anomalous birefringence of ∆n > 0.001 are normally characterized by partial ordering of the octahedral cations (Al/Fe and Al/Cr for grossular−andradite and grossular−uvarovite series, respectively). This reduces the symmetry to the orthorhombic space group Fddd or even to the triclinic space group I1̅ . As is seen from the distribution of occupancies over octahedral sites in the triclinic space group, eight occupancies are grouped into two quartets of similar occupancies, leading to pseudo-orthorhombic crystal structures. The variety of structures with different degrees of pseudo-orthorhombicity is due to the action of the growth dissymmetrization mechanism. Simulations of the optical indicatrix in the point-dipole approximation confirm cation ordering as the main cause of the anomalous birefringence.

Effect of crystal composition and growth rate on sector zoning in solid solutions grown from aqueous solutions

Abstract Sector zoning in solid solutions of Tutton′s salts (NH4)2M(SO4)2−6H2O (M = Co, Ni, Zn, Mg) and nitrates of divalent metals Me(NO3)2 (Me = Sr, Pb, Ba) grown from low-T aqueous solutions under controlled conditions is studied. The effect of crystal composition and growth rate (supersaturation) on the intersector difference in concentrations of isomorphic components is examined experimentally and modelled theoretically based on the phenomenological approach to crystallization in a ‘solid solution−aqueous solution’ system. Calculations show a predominating contribution of the selective adsorption phenomena in the formation of a sector-zoned crystal with usually minor, but sometimes a major contribution of growth kinetics.

Effect of growth conditions on the birefringence of mixed crystals revealed in alum solid solutions

Abstract An experimental study of optical anomalies in solid solutions of alums in relation to their composition, growth temperatures and rates, and hydrodynamic regime was carried out. Theoretical analysis of the data showed that they can be understood in terms of a mechanism of kinetic ordering of the isomorphous atoms (the phenomenon of so-called ‘growth dissymmetrization’). A theoretical model is presented which is supported by data for a number of minerals and synthetic compounds, which are known for their optical anomalies due to growth dissymmetrization.

An application of the point-dipole model to the problem of optical anomalies in grandite garnets

Abstract The origin of anomalous birefringence in grossular-andradite (grandite) garnets is considered. Using the point-dipole model we have calculated the anomalous optical indicatrix for four crystals (three from the paper by Takéuchi et al. (1982) and one studied previously by the present authors). The calculated indicatrix is in good agreement with the observed one. This suggests that the origin of anomalous birefringence in grandite garnets is due to Al/Fe ordering. Interrelations between the observed optical indicatrix, the structure of the growing face, the site occupancy in the crystal structure and the calculated optical indicatrix suggest a growth origin of Fe/Al ordering in grandite garnets and confirm the growth dissymmetrization hypothesis.

On the complex zonality in grandite garnets and implications

Abstract The available data on compositional zoning in grossular-andradite (grandite) garnets occurring at different scales, mainly due to the variations of their Fe3+/Al ratios, and the hypotheses on the origin of this zoning are reviewed. Four zoned crystals of grandites showing different optical properties have been studied by means of X-ray diffraction. Optical and structural studies suggest three superimposed scales of Fe3+/Al zonality along [110]. The drastic change of the structural characteristics from sample to sample correlates with the change of their optical patterns. The superfine oscillatory zoning has been described in terms of irregular interstratified structures with a high degree of segregation. The self-affinity of zoning at different scales testify to the possible origin of the zonality due to the non-linear dynamics at the growth from. The wide variation of the layer compositions revealed by the modelling of the X-ray diffraction profiles seems to contradict the hypothesis of unmixing in grandite garnets.

Specific features of pathogenic mineral formation in the human body

Key relationships in pathogenic mineral formation within the human body were summarized. It was shown that pathogenic organomineral aggregates in human body are formed in complex multicomponent media containing organic and inorganic components. In the course of lithogenesis, the composition of biological fluid undergoes major changes. Phase formation in human urine and oral fluid in the initial stages leads to emergence of less thermodynamically stable phases, i.e., is kinetically controlled. Amino acids, as well as a number of inorganic impurities, are actively involved in crystallization of the main phases of renal calculi.

Curved-face growth of NaNO3 crystals

Curved-face growth of NaNO3 crystals under the action of the (NH4)2Mo2O7 impurity is studied experimentally and possible causes of face bending are analyzed. Selective impurity adsorption on the basal pinacoid results in growth of platelike crystals with spherically bent faces. Growth-rate measurements show that the impurity decelerates pinacoid-face growth by the Bliznakov kink-poisoning mechanism. Face bending can be explained by gradual deceleration of step growth by the impurities accumulated before step fronts. A model of step deceleration is suggested that is based on the assumption of slow impurity adsorption resulting in the instability of a step train. The model allows one to qualitatively interpret the experimental data.

Autodeformation bending of gypsum crystals grown under the conditions of counterdiffusion

The dynamics and kinetics of growth-induced bending of gypsum crystals grown from solution have been studied. Crystallization was performed by the method of chemical reaction under the conditions of component counterdiffusion. It is established that autodeformation bending occurs in the [001] direction at the growth front and is caused by cationic impurities. The crystal curvature depends on the anisotropy of growth rate and increases at lower supersaturations. The mechanism of growth-induced crystal bending is suggested which takes into account the heterometry stresses providing the appearance of a bending moment at the growth front.

Effect of Carbamide Adsorption on the Growth Kinetics of the NH4Cl Crystals

This work supplements the adsorption model of 2D-isomorphism proposed earlier for the NH 4 Cl-CO(NH 2 ) 2 system and reveals the correspondence between the growth rates of the NH 4 Cl crystals in the presence of carbamide and the co-crystallization of carbamide with ammonium chloride. It follows from the kinetic data that the NH 4 Cl . CO(NH 2 ) 2 impurity capture becomes significant when the carbamide concentration in solution is sufficient for almost monolayer adsorption of the impurity on the crystal surface and the onset of multilayer adsorption. The concentration of the guest phase intergrowths falls with the increase in the host crystal growth rate.