Abstract
In zircon (ZrSiO4), four different Periodic Bond Chains (PBCs) can be described, i.e. [001], (100), (1/2 1/2 1/2 ) and (1/2, 1/2, 11/2). F faces which grow slowly according to a layer mechanism, are {100} and {011}. Calculation of attachment energies, which are supposed to be directly proportional to the growth rates, is performed in electrostatic point charge models, model I: Zr 4+ Si 4+ O 2-, model II: Zr 4+ Si 2+ O~ 5- and model III Zr 4+ Si~ . The theoretical growth form is short prismatic following {100} and is terminated by the dipyramid {011}. The lower the oxygen charge the more elongated is the crystal parallel to the c axis. The slice d011 can be defined either bounded by zirconium (db~01) or by silicate tetrahedra (doBll. As these slice boundaries differ in height of one half slice with thickness do22, and as these half slices d0A22 and dB22 are F faces, the growth of {011} may also take place by elementary growth layers of thickness do=. In that case the growth rates of COl 1} increase and the growth models are even more prismatic. The S form {031} could only be present on the theoretical growth form with formal charges, provided that the growth of {011} takes place with half slices dog2 and the adsorption of impurities establishes additional strong bonds parallel to its slice boundaries.
