GROWTH DISSYMMETRIZATION OF MINERALS' CRYSTAL STRUCTURES
O.Frank-Kamenetskaya, A.Shtukenberg, Yu.Punin, I.Rozhdestvenskaya
Saint Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia, E-mail: frank@cryst.geol.pu.ru
Keywords: mineral structures, inorganic solid solutions, dissymmetrization, abnormal birefringence
Lowering of optical symmetry of crystals in the course of their growth is typical for many natural and artificial minerals. One of the reasons of growth dissymmetrization is the kinetic ordering of structural units (atoms, ions, vacancies) on crystal face and subsequent burying of this ordered distribution within the crystal volume. This phenomenon was studied for several inorganic solid solutions with optical and X-ray diffraction single crystal methods. All the investigated crystals display pronounced optical anomalies with sectorial and subsectorial distribution of birefringence, which usually increases while going from extreme members of isomorphous series.
The lowering of crystal symmetry affects negligibly on the linear and angular lattice parameters but leads to significant difference in the intensities of the symmetrically equivalent reflections of the ideal cubic diffraction class. The degree of the crystal dissymmetrization evaluated as the difference in reflections' intensities correlates with the birefringence value.
a-Alums isomorphous series A+M3+(SO4)2*12H2O, A+ - K, NH4, Rb; M3+ - Al, Fe, Cr, Ga. Lowering of symmetry from Pa 3- to P1 or P 1- is realized due to the partial ordering of A+ or M3+ cations in water octahedra. The degree of dissymmetrization is substantially higher at the substitution in A-site. The adoption of Cr admixture in K(Al0.95Cr0.05)(SO4)2*12H2O structure leads to the slight deformations of M-octahedra and to increase in the orientation disorder of SO4-tetrahedra.
Analcime-pollucite system NaAlSi2O6*H2O-CsAlSi2O6. Lowering of the symmetry from Ia3-d to I41/acd (analcime [1]) and to I212121 (sodium pollucite) is caused by the partial ordering of Al and Si cations in tetrahedra and vacancies in sodium octahedra (analcimes) and in additional water molecules in Cs-cuboctahedra (pollucites). The direct relation between the fractions of Al atoms in tetrahedra and Na and/or Cs atoms in the adjacent polyhedra indicates the interrelation between those types of cation distributions.
Grandite garnets Ca3(Al,Fe)2(SiO4)3. Lowering of the symmetry from Ia3-d to P1 or P1- is caused by partially ordered distribution of Al and Fe cations in octahedra [2]. Investigation of the effect of this phenomenon on the structural geometry is now in progress.
The lower symmetry state of the crystals is metastable. The degree of dissymmetrization decreases gradually with a rate increasing exponentially with temperature. Process of kinetic ordering of isomorphous components in the structure of growing crystal is described by competitional adsorption equation, which is in a good agreement with the experimental data for the growth conditions' influence on the abnormal birefringence of crystals.
This work was supported by RFFI (projects NN 96-05-65579 and 98-05-64151).
1. Mazzi F. , Galli E. Amer. Miner. 63
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2. Takeuchi Y. et al. Z.Krist. 158 (1981) 53-99