The Sheared Mechanism of Spiral and Vortex Dislocation on Beryl Crystal

Qi Li-jian (1) C.G. Zeng (2) Sun Da-lian (3)

1. Gemmological Institute, China University of Geosiences , Wuhan, 430074 , China;
2. Nan Yang Gemological Institute , Singapore;
3. State Key Laboratory of Crystal Material, Shandong University, Jilan, 250100,China; 

Abstract: Natural Beryl from Sichuan,China formed in under pneumatogenic hydrothermal condition has a characteristic low iron high alkali chemical composition; w ( K 2O Na 2O) 1.40%, w( FeO T) 0.25%. The amount of w ( H 2O)varies between 2.70% and 3.30%. Other impurities amounting to less than 0.1%.

Most of these beryl crystals grow in hexagonal thick slab along the c 0001 with occasionally short hexagonal prism. Under normal growth condition, certain crystal faces such as the hexagonal prism faces or hexagonal bipyramid faces may be missing and the crystal adopt a unique lateral growth direction. Beryl crystal is usually made up of pinacoid c 0001 hexagonal bipyramid p 10 1 s 11 1 , hexagonal prism m 10 0 .

Surface microtopography growth defects and growth mechanism of natural beryl are studied the testing methods of atomic force microscope AFM differential interference microscope DIM scanning electron microscope SEM and STEM are employed in the research. The results show Fig1-2 that there is well developed . hexagonal spiral growth dislocation on the largest face, pinacoid c{0001}. Partly adopting the sinistral spiral dislocation while the majority adopting the enclosed spiral dislocation, this demonstrates the symmetrical characteristic of a hexagonal crystal system. Within the spiral dislocation, the steps have about equal distance in spacing (0.4 0.8 m) and height 2.5 5nm ; Sinistral spiral dislocation has more perfect growth in the smaller m{10 0} face. This spiral shows a rhombus shape due to the changes in the density and symmetry at the twist of the spiral dislocation. The longer axis is almost parallel to the pinacoid c 0001 , and with the outcrop of the dislocation as axis center point, it continues to grow outward in a spiral step-like manner. The steps have about same distances and heights of 4.5 m and 3.5nm respectively. On the larger s{11 1} faces, a rare type of vortex growth dislocation is observed. This is different from the usual vacant core found in previous spiral dislocation. It has characteristic sinistral and dextral vacant cores at the outcrop of the vortex dislocation. The vacant core measures at 5 8 m in diameter, and part

of the inner boundary on the steps of the inner rings may appear zigzag and partially broken. Thickness of the spiral steps increase as it nears the vacant core reaching between 42 50nm while decreasing dramatically outward. Due to the existence of other similar vortex dislocation, the growth rate of the s{11 1} face reduces resulting in the crystal faces being well developed. The changes of lattice-binding energy and dislocation stress field caused by the liquid vortex movement, the distortion of the atomic structure at the center of the dislocation and the Sheared Mechanism of crystal spiral and vortex dislocation need further investigation.

Fig1 Vortex and spiral dislocation on beryl crystal

The growth process of this type of beryl crystal belongs to an unbalanced and very complex process of phase change. This growth process is governed by a combination of various conditions in thermodynamics and dynamics. It is the result of physics, chemistry and geochemistry in combination. The growth mechanism of

Fig2 Vortex dislocation on {22 3}face of beryl crystal

crystals under different growing dynamics can be demonstrated through their microscale growth defects found on them. This is especially true to the spiral and vortex dislocations that are very sensitive to the changes in growing conditions. Besides showing the essential combination of physical, chemical and geochemical conditions, they also demonstrate the importance of the existence and movement of certain element in the metallogenic dynamics of the Earth.

Key words: beryl crystal atomic force microscopy microscale vortex dislocation crystalline phase vortex nucleation

*Published in 29th International Gemmological Conference, 2004 (page 63 - 66)