Formation of Vortex Dendrite Crystal on Yellow Sapphire
under Hydrothermal Condition

Qi Li-jian (1) C. G. Zeng (2) Zhang Chang-long (3)
1. Gemmological Institute , China University of Geosiences , Wuhan , 430074, China;
2. Nan Yang Gemological Institute, Singapore;
3. Guangxi Institute of Geology for Mineral Resources , Guilin, 541004, China;

Abstract: An efficient technique of growing synthetic yellow sapphire is developed by integration of hydrothermal method, oxidation-reduction buffer and slow released technique of coloring ion Ni. The growing experiment of the crystal are performed dissolved zone temperature at 550 to 580 ¡ã C, growing zone temperature at 505-515 ¡ã C, difference of temperature from 45 to 65 ¡ã C, growing pressure at 150 to 180 MPa, solution used is a mixture of KHCO 3 and Na 2CO 3, seed plate orientation parallel to n{223}, coloring elements are Ni 2+and Cr 3+.

The crystal habits of these synthetic yellow sapphire crystals are usually thick slab. Hexagonal dipyramid n{223} and rhombohedron r{10 1} faces and occasionally hexagonal prism a{110} faces are commonly encountered. The pinacoid c {0001} faces are very rare. The growing characteristic of crystal shows that, under normal hydrothermal condition, synthetic yellow sapphires primarily follow a two dimension nucleation growth mechanism . The characteristics of surface microtopography are ligulate or papilla pattern growth marks, straight stratiform, and irregular oblique growth steps. However, when the growth environment is adjusted, the growth mechanism of synthetic yellow sapphire crystals will change from a typical two dimension stratiform growth mechanism , to a mixture of vortex growth dislocation - two dimension stratiform , finally to two dimension stratiform ¨C vortex growth dislocation mechanism ( Fig1). The characteristics of surface microtopography on yellow sapphire crystals will also be changed to microscale vortex dendrite crystal.

The testing results of atomic force microscopy (AFM)¡¢differential interference microscope (DIM) and SEM show that the dendrite crystal developed along n{223} faces on synthetic yellow sapphire exhibits a very rare three-dimensional vortex growth dislocation. In three-dimensional space, the vortex growth dislocation is consisted of four parts from interior to exterior: vacant core, interior layer, exterior layer and end layer. The vacant core has a sinistral loudspeaker shape, maximum diameter is 18.5 ¦Ì m, height of vertex wall is less than one half of cell scale (about 0.35 nm). Interior layer is primarily consisted of fine dense vortex rhombus prisms that show complex reversing flow lines, and flow lines diverge outward.

Fig1 Surface microtopography on {223} face of synthetic yellow sapphires

The surface of vortex rhombus prisms in this layer is undulant. Due to rapidly growth rates, the height of vortex rhombus prisms perpendicular to n{223} face is about 1.2nm. As growth rate reduced obviously in exterior layer, the surface of flow line shape vortex rhombus prisms become gentle undulant and smooth, but the width and height of prism increase prominently to 4.6 ¦Ì m and 15 ¡« 20nm respectively, resulting in some prisms distorted and broken. While from end layer outward, the surface and edges of vortex rhombus prisms gradually become similar branching shape, with typical characteristic of fractal texture ( Fig2)

The growing experiment results of the crystal show that, depending on the growing conditions, there are two different types of particle transport modes during crystal growing: diffusion and convection or vortex. The former rely on the movement of molecules and atoms, while the latter was carried by the particles dissolved in fluid to migrate with eddy current in the process of fluid movement. During the process of crystallization, convention takes place with the difference in temperature, and vortex caused by temperature and concentration gradient moving from the hotter region to slightly cooler sites, the over-saturated flux fluid is formed in the autoclave. The transmission and saturation of vortex between dissolved and growth sites are controlled by difference in temperature between the growth and dissolved regions. Crystal growth is an unbalanced process; the density difference between fluid phase and crystal phase far away from the boundary of vortex is very slight. Furthermore, atomic distance of fluid structure is of little difference with that of crystal phase. On the whole, the vortex structure is also significant to free energy of boundary.

Fig2 Vortex Dislocation on {223} face of synthetic yellow sapphires

Through the tracing of the microscale vortex dislocation in hydrothermal synthetic yellow sapphires, it shows that, besides the essential components of raw materials, heat and gas-water solutions, the microscale vortex movement and vortex structure of metallogenic fluid are equally important to the interfacial free energy. Hence, microscale vortex migration of metallogenic fluid and crystal phase vortex nucleation could be another important form of particle movement and existence of metallogenic dynamics system in the Earth.

Key words: hydrothermal synthetic synthetic yellow sapphire £» dendrite crystal £» microscale vortex dislocation £» atomic force microscopy £» crystal growth mechanism

*Published in 29th International Gemmological Conference, 2004 (page 66 - 68)