THEORETICAL GROWTH FORMS OF NATURAL GARNETS

F forms of natural garnets are (112), (110), (123}, (001), (120) and 1332). For Mg3Al2(SiO4)3, the surface configurations of the elementary growth layers d220 and d240 are bounded by Al ions. For d220, a different boundary configuration exists as well, that of Si and Mg ions, just as the configuration of d040. For d112 two boundary configurations are possible, one with Mg and Al, and another with Al and Si. The attachment energies of the F faces have been calculated in different electrostatic point charge models, in which the point charges of the ions forming the Si-O tetrahedral bonds vary from the completely ionic model with formal charges, [Si4+ O42-], to the covalent one with reduced charges, [Si0O4-]. In all models the formal point charges of Al and Mg have been used. Because these energies are supposed to be directly proportional to their growth rates for F faces, theoretical growth forms have been constructed. For models with q(O) = - 2 Absolute value of e, the growth form consists only of (112). However, for the more covalent like models with lower q(O), {100) and (1 10) appear, while at the same time, (21 1) disappears slowly. If q(O) = - Absolute value of e, the growth form is merely bounded by (110). Ordering of the boundary ions changes fundamentally the growth form only at q(O) almost-equal-to - 1.75 Absolute value of e producing a growth form with equally important (211}, (110) and 1100). If the mineral growth of garnets proceeds with isolated silicate tetrahedra as building units, the theoretical growth form shows only (112).