Etch pits on beryl as indicators of dissolution behaviour

Beryl crystals from Minas Gerais (Brazil), Lumaki (Finland), Sichuan (China), and the Himalayas have been investigated by scanning electron microscopy in order to characterize the shape of etch pits on different crystal faces. The objective was to find out whether etch pits on natural crystals can be used as tools to study provenances and the dissolution process. The crystals were characterized by electron microprobe analysis, infrared spectroscopy, and powder X-ray diffraction. Most of the beryl crystals are normal beryl in the sense that they only slightly deviate from the ideal chemical composition; two crystals, which are also classified as tetrahedrally substituted, show minor substitution of alkalis with Fe (super 2+) for Al. Etch pits are classified as F-type (flat), P-type (pointed), S-type (stepped), H-type (appearing hollow, equal to extremely steep P-type), and C-type (canoe-shaped with curved edges). Different types of etch pits are explained by different defects; F-types occur on point defects, S-types on overlying point defects, P (super -) , H (super -) , and C-types on line defects (dominantly screw dislocations). Different shapes are explained by different dissolution rates in different directions of the crystal. Etch pits were created experimentally at 600 degrees C, 200 MPa and fluid compositions of 0.5 N NaOH, 0.5 N HCl, 1 N NaCl, and 2 N KCl solutions. Combined results of experiments and the natural crystals show that the dissolution starts by leaching of cations from the channel, followed by the removal of cations from the AlO (sub 6) octahedron. The factors that control dissolution of beryl are orientation of the crystal structure (maximum dissolution is perpendicular and parallel to the c axis), crystal defects (inclusions, dislocations), substitution of cations, symmetry of the faces, and chemical composition of the solvents. The morphology of etch pits - rectangular, square, canoe-shaped, rhombic - on the first-order prisms of samples from different localities is significantly different and, if more data will be available in the future, opens the possibility to distinguish their provenance.