SYNTHETIC BERYL GROUP MINERALS: CONNECTIONS BETWEEN SYMMETRY, STRUCTURAL ORDERING, CHEMICAL COMPOSITION AND ANATOMY OF THEIR CRYSTALS

摘要

Problem of the ordering-disordering in minerals and synthesixed substance of the beryl structural type has appeared due to considered polymorphism of the Mg_2Al_4Si_5O_(18) compound (Myashiro, 1957). In the present paper the symmetry of beryl crystals was estimated by the value of distortion index A There has been revealed three factors affecting symmetry of beryl group crystals: chemical, crystal-chemical and mechanical. The chemical factor acts through the ratio of species-forming components occupying the tetrahedral positions. In the basic cordierite the hexagonal pyramid (DELTA = 0.00 deg) of the pinacoid growth is characterized by the sum (Si + Al_(IV)) = 8.86 f. u. (formula unites) and the orthorhombic one (DELTA = 0.20 deg) of the prism growth is marked by the sum (Si + Al_(IV)) = 8.94 f. u. The admixtures of Mn, Cr, Ti, V, Li, Rb and Cs compounds, giving the (Si + Al_(IV)) value from 8.89 to 8.96 f. u. in the cordierite varieties, provide their orthorhombic syngony (DELTA = 0.10--0.27 deg), while the addition of B and Cu oxides results in (Si + Al_(IV)) = 8.83--8.85 f. u., so in their hexagonal pattern. The hexagonal solid solutions of the whole cordierite -- beryllian indialite -- beryl range go down to 7.92 f. u. of (Si + Al_(IV)), so as orthorhombic solid solutions (DELTA = 0.06--0.26 deg) have no less than 8.08 f. u. The hexagonal (8.04 f. u.) compound (Mg_(1.79)Al_(0.21)) (Be_(0.91) Al_(2.24)Si_(5.80))O_(18), produced by solid phase synthesis, remains with invariable distortion index (DELTA) value (0.00 deg); when (Si + Al_(IV)) = 7.89--7.92 f. u.) at the deficit of SiO_2 or BeO. But the Al_2O_3 addition decreases the solid solutions symmetry down to DELTA + 0. 10--0.24 deg with (Si + Al_(IV)) = 8.08-8.26 f. u. That selective introduction of the species-forming components and admixtures (Si, Al, Mg, Ti, Cr, V И Fe) into growth pyramids of pinacoid and prism of cordierite, beryllian indialite and beryl has been revealed for the first time. The effect of this selection on the symmetry (crystal-chemical factor) means the more orthorhombic pattern of growth pyramids of the prism, relatively to pinacoid, in some cordierites and solid solutions of cordierite -- beryllian indialite, up to forming a dissymmetric individual of the basic cordierite-indialite. The mechanical factor, connected with tension of the matrix, becomes superimposed upon the chemical and crystal-chemical factors.