Cristalización de gemas en solución de alta temperatura: técnica del flujo

• Autores: María Victoria López-Acevedo Cornejo
• Localización: Boletín de la Real Sociedad Española de Historia Natural. Sección geológica, ISSN 0583-7510, Tomo 96, Nº 3-4, 2001, págs. 5-16
• Idioma: español
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• Resumen
  • español

    En el presente trabajo se analizan las posibilidades de una variante del crecimiento en solución, a alta temperatura, conocida como técnica del flujo, en relación con la cristalización de gemas. Esta técnica se considera como una de las que mejor simula las condiciones naturales de formación de estos materiales. Se caracteriza porque el sistema parte de la disolución de un compuesto de alto punto de fusión, en una sal u óxido inorgánico fundido que se denomina flujo. La cristalización se produce por descenso de temperatura de esta disolución y además, en el caso de que el sistema no esté cerrado, por evaporación del flujo. El proceso se puede realizar en un horno capaz de alcanzar temperaturas de hasta 1400 ºC, equipado con un sistema de programación para conseguir la velocidad de enfriamiento más apropiada. La imposibilidad de observar directamente dicho proceso constituye el principal inconveniente que plantea la técnica. Este problema se trata desde un punto de vista metodológico, como un ejemplo de "caja negra" y se definen los métodos que más información pueden proporcionar para su conocimiento. Desde esta perspectiva se analiza el proceso de cristalización del rubí, considerado históricamente como la más significativa de todas las gemas sintéticas, en un flujo ce criolita.

  • English

    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.