Selection of a mineralizer from organic salt solutions for hydrothermal growth of calcite single crystals

Abstract

Calcite single crystals with high purity and transparency, called Iceland spar, have been widely used for polarized devices. The artificial growth of calcite single crystals for these devices has been carried out by the hydrothermal method [1–12], TSSG (top-seeded solution growth) [13] and TSZM (travelling solvent zonemelting) technique [14, 15], using seed crystals. Both of the TSSG and TSZM techniques are based on the flux growth using Li2CO3 as a flux, which results in contamination of the grown crystals by Li. The hydrothermal method is the most suitable method to grow calcite crystals, because the dissociation of CO2 is suppressed under hydrothermal conditions and crystals with less thermal strains are grown at relatively low temperatures. The hydrothermal growth of calcite crystals has been carried out in the H2O-CO2 system [5, 6], or using inorganic salt solutions as a mineralizer, such as LiCl [1], NaCl [1, 9, 10], KCl [9], CaCl2 [1], NH4Cl [1], KNO3 [11], NaNO3 [11], Ca(NO3)2 [12], NH4NO3 [2, 3, 12], and K2CO3 [4]. However, the limited growth rate of calcite single crystals of up to 50 μm/day even under high temperature and high pressure hydrothermal conditions [4], obstructs the industrial production of calcite single crystals. The authors recently found that calcite crystals could be grown at high growth rate under mild hydrothermal conditions in an ammonium acetate solution, i.e. an organic salt solution [7, 8]. This result suggests the possibility of using other organic salt solutions as mineralizers for calcite crystal growth. In this paper, the solubility measurement and hydrothermal growth of calcite crystals were carried out in various organic salt solutions to find a suitable mineralizer for the hydrothermal growth of calcite crystals. The solubilities of calcite in various organic ammonium salt solutions were measured as described previously [16] and are shown in Table I. An autoclave lined with Hatelloy C with an inner volume of 12 cm3 was used for the measurements. The autoclave was equipped with a platinum mesh in the chamber to separate the solute from the solvent under hydrothermal conditions. A few pieces of cleaved natural single crystals of calcite (Iceland spar from Creel, Chihuahua, Mexico) were placed on the platinum mesh and one of the 0.1 M or 0.08 M organic ammonium salt solutions (60% fill ratio) was poured into the bottom of the autoclave after its pH was adjusted to 7.3 using aqueous ammonia. The contents of the autoclave were agitated at the desired temperature for 16 h by rotating the autoclave in an electric oven. After establishing the equilibrium, the rotation was stopped to separate the crystals from the solution under hydrothermal conditions, and then the autoclave was quenched into cold water. The solubility was determined from the weight difference of the crystals before and after the hydrothermal treatment. The results of the solubility measurements of calcite at 200 ◦C in various organic salt solutions (0.1 M), are summarized in Table I. After the hydrothermal treatment, a few solutions became black in color, which suggested that these organic salts decomposed under hydrothermal conditions. In general, calcite dissolves in ammonium mono-carboxylate solutions, although the solubility is lower than in the ammonium nitrate solution, an inorganic salt solution. The ammonium lactate and glutamate solutions gave high solubilities of calcite. The solubilities of calcite in ammonium monocarboxylate solutions (0.08 M) were measured up to