Ulexite from the Rio Grande deposit, Uyuni (Bolivia): mineralogical and thermal evolution
García-Vallès, M.; Alfonso, P.; Tomasa, O.; Martínez, S.; Arancibia, J.R.H.; Parcerisa, D.
Type of activity
Presentation of work at congresses
Name of edition
12th Mediterranean Conference on Calorimetry and Thermal Analysis
Date of publication
Book of congress proceedings
Thermal Analysis and calorimetry: Recent advances and application
Borate minerals constitute the main source of boron, which covers a multitude of industrial applications. The industrial uses of borate minerals greatly depend upon its thermal properties. Thus, it is important to know them to recommend the optimal application. In this study a characterization of the mineralogy and thermal properties of borates from the Rio Grande borate deposit is presented. Large volumes of borate resources occur in Bolivia, being the most important are those from the playa-la...
Borate minerals constitute the main source of boron, which covers a multitude of industrial applications. The industrial uses of borate minerals greatly depend upon its thermal properties. Thus, it is important to know them to recommend the optimal application. In this study a characterization of the mineralogy and thermal properties of borates from the Rio Grande borate deposit is presented. Large volumes of borate resources occur in Bolivia, being the most important are those from the playa-lake type deposit of Rio Grande This deposit comprises an area about 50 km2 located close to the southern part of the Salar of Uyuni in the contact between fluvio-deltaic and lacustrine sediments in the delta of the Grande de Lípez ríver. Here Borates occur in beds and lenses of variable thickness, from 0.5 m to 2 m. Chemical composition was determined by inductively coupled plasma mass spectrometry (ICP-MS). The mineralogy of natural and thermal treated samples at different temperatures was determined by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Original borates textures were observed by scanning electron microscopy (SEM). Thermal evolution of each mineral phase was determined by differential thermal analysis and thermogravimetry (DTA-TG) at a temperature range between 25 and 1200 ºC with linear rate of temperature gradient set to 10 ºC/min. According to DTA-TG results, in order to determine the mineral evolution with temperature, a treatment temperature, from 550 to 1050 ºC, with setting time of half an hour, and subsequent analysis by XRD was performed. XRD patterns show that ulexite (NaCaB5O6(OH)6•5H2O) is the main mineral phase in the borate deposit, which can be the only mineral present. However, in most outcrops other evaporite minerals also occur, mainly halite (NaCl) and gypsum (CaSO4•2H2O). Ulexite occurs as crystalline aggregates with elongated fibres morphology oriented parallel each other along  with a length higher than 100 microns. Infrared spectroscopy, FTIR, confirmed the mineral phases determined by XRD. Thermal decomposition of borates is a complex mechanism which involves dehydration, polymorphic transition, melting and solid phase transformation. The decomposition of ulexite begins at about 70ºC and proceeds up to ~550ºC; changes at these temperatures are attributed to dehydration and dehydroxylation processes . TG curves indicate that these processes led to a loss of weight in three steps: 5-7% of mass is lost at around 100 °C; 14 - 20% of loss occurs at 150-300ºC, attributed to removal of the crystal water, and 6-11% is lost in the interval of 300-550ºC and corresponds to dehydroxylation of ulexite. A last weight loss of 1-5%, at 800ºC due to the removal of Cl2 from the decomposition of the sodium chloride present in the samples. These transformations were controlled by XRD analysis at temperature intervals. In the DTA analysis four endothermal events are observed. The three first are related to the removal of the water in NaCaB5O6(OH)6•5H2O to form NaCaB5O6(OH)6•3H2O, at 113 ºC, NaCaB5O6(OH)6, at 183 ºC and NaCaB5O9 (amorphous) at 550 °C. The exothermic peak at 658 ºC is related to the crystallization process of NaCaB5O9. A small endothermic peak occasionally occurs at around 780-804 ºC due to the halite melting. The next endothermal event, at 822-877 °C, is related to the decomposition of NaCaB5O9 in a crystalline phase, CaB2O4 and amorphous NaB3O5. Finally at 1050 °C XRD pattern evidence that CaB2O4 still remains as a crystalline phase. The melting temperature, temperature of borate descomposition and amorphization of the solium borate depends on the halite content.