Thermal analysis, X-ray diffraction and infrared emission spectroscopy of the borate mineral meyerhofferite CaB3O3(OH)5·H2O

Abstract

Meyerhofferite is a calcium hydrated borate mineral with formula Ca2(H3B3O7)2·4H2O and occurs as white complex acicular to crude crystals in sedimentary or lake-bed borate deposits. Simultaneous thermogravimetric analysis (TG) and derivative thermal analysis were performed. The TG curve shows a main decomposition at 195 °C followed by a second decomposition centered at about 436 °C. The total loss of mass is 28.1 % upon heating up to 1000 °C. The decomposition of meyerhofferite is also followed by the infrared emission spectra. A very sharp peak is observed at 3614 cm−1 in the 100 °C spectrum, which is attributed to the stretching vibration of OH units. This band shows a red shift with increasing temperature. The intensity of this band is lost by 250 °C. X-ray powder diffraction of the product of heating at 200 and 650 °C shows an amorphous phase, and at temperature up to 1000 °C is observed a partial reordering of the crystal structure, including calcium hexaboride, boron oxide, calcium peroxide, as well as unidentified phases.