Cristalización de gemas en solución de alta temperaturatécnica del flujo

Laburpena

We here discussed the different aspects regarding gem crystallization by flux growth. This method is one of the best to simulate natural conditions of gem formation. A high-temperature fusion compound is solved in a molten salt or inorganic oxide (flux). Supersaturation can be reached by: 1) a decrease in temperature; or 2) evaporation of the flux in an open system. Dissolution of the initial solid (Al2O3) is done in a muffle that can reach temperatures of up to 1400 ºC. The muffle has a programme designed to obtain the most suitable cooling rates. A main problem of this method relates to the impossibility to directly observe the process (which occurs within the muffle). However, direct observations (input output data) coupled to theoretical considerations (black-box approach) offer some insights into the process. A series of experiments were designed to obtain crystallization of ruby from cryolite flux. The high solubility of Al2O3 in molten cryolite (Na3AlF6), coupled to the fact that the process takes place at temperatures well below the melting point of pure Al2O3, are the two main reasons to regard cryolite as an ideal flux to obtain corundum crystals. Cr2O3 is used as dopant agent to obtain the characteristic red colour of ruby. Three different mixtures of these reagents have been tested at different temperatures and cooling rates. The identificacion and characterization of the products was done by optical microscopy, scanning electron microscopy (SEM), X-ray dispersed energy chemical microanalysis, X-ray diffraction (DRX), and microRaman (mR). Red, pink, and colourless corundum, cryolite, and decomposition products (diaoyudaoite, NaAl7O11, villiaumite, NaF, etc.) were obtained as the result of our experimental work (Table 1). Ruby crystals formed simultaneously from a high-temperature solution and a vapour phase, within a heated platinum vessel. On the upper wall and on the lid of the crucible, the crystals grow from a vapour phase, whereas at the bottom the crystals grow from the liquid phase. The vapour phase is mainly formed by Al2O3(g), Na3AlF6(g), and to a lesser extent, by decomposition products (g). Formation of the vapour phase induces a variation in the rate [solution/flux], which in turn results in supersaturation. The crystals obtained from vapour are well developed. They form haxagonal plates and polyhedral morphologies (Lám. I, figs. 1 and 5). On the other hand, the crystals formed from the solution display morphologies including hopper-type crystals, aggregates, and glassy forms (Lám. I, figs. 1 and 5). The different morphologies suggest three types of growth mechanisms: 1) spirals; 2) two-dimensional nucleation; and 3) continuous growth. All of them may form in one single experiment, which suggests variable rates of supersaturation. Crystallinity and density of nucleation are improved when less pronounced time-temperature ramps are used.