On the morphological changes and twinning of ZnS (sphalerite) crystallitesunder hydrothermal conditions

Morphological characteristics and twinning mechanism of ZnS crystals under hydrothermal conditions have been investigated in this paper. It was shown that under hydrothermal conditions the morphology of ZnS crystallites chang es along the four-fold axis directions, and the crystals are observed in a positive or negative tetrahedron, or in a combination of positive and negat ive tetrahedra depending on the growth conditions. The positive tetrahedral areas on the crystallites get larger with increase of the concentrations o f OH- and S2- in solutions, whereas the twinned crystallites of ZnS taking an elliptic shape with(lll) as composition plane are easily formed in weak basic solutions. It can be found that the morphologies of ZnS crystals are in accordance with the crystallization orientations of positive or negative coordination tetrahedra ([S-Zn-4](6+), [Zn-S-4](6-)) in the crystal althou gh, in some cases, the practical morphology could be greatly affected by gr owth conditions, and the twinning mechansim can be suggested based on the l inkage of growth units of positive and negative coordination tetrahedra, wh ich were formed in the solution. The present investigations further indicat ed that the crystal chemistry approach based on the linkage/incorporation o f growth units previously proposed by us can be sucessfully applied to inte rpret the growth mechanisms of the crystals and to control a desirable morp hology.