Original paper
Dehydration of blödite, Na2Mg(SO4)2(H2O)4, and leonite, K2 Mg(SO4)2(H2O)4
Balić-Žunić, Tonči; Birkedal, Renie; Katerinopoulou, Anna; Comodi, Paola
European Journal of Mineralogy Volume 28 Number 1 (2016), p. 33 - 42
published: Mar 23, 2016
Abstract
The high-temperature behaviour of blödite (Na2 Mg(SO4)2 (H2O)4) and leonite (K2 Mg(SO4)2 (H2O)4) was studied by X-ray diffraction on powder samples in open capillaries and by thermo-gravimetry/calorimetry. The stability of blödite and leonite below 380 K and 420 K, respectively, shows that these minerals are limited to low-temperature assemblages of hydrous sulphates and are expected mostly as evaporite products on the surfaces of the Earth and other water-saturated planets. Their high-temperature transformations, however, link them to the fumarolic minerals löweite (Na12 Mg7 (SO4)13 (H2O)15), α-Na2 Mg2 (SO4)3 with still unknown crystal structure, β-Na2 Mg2 (SO4)3 isomorphous with langbeinite, and langbeinite (K2 Mg2 (SO4)3). Blödite dehydrates with two steps, at approximately 380 K and 500 K. During the first dehydration it transforms into löweite. At the second dehydration step, löweite transforms into α-Na2 Mg2 (SO4)3 and vanthoffite (Na6 Mg(SO4)4) or thenardite (Na2 SO4). The β-Na2 Mg2 (SO4)3 phase transforms at around 850 K into β-Na2 Mg2 (SO4)3 which is stable to over 980 K (all at atmospheric pressure). The two dehydrations have a strong endothermic character, whereas the 850 K phase transition gives a weaker endothermic signal. It is reversible and β Na2 Mg2 (SO4)3 cannot be quenched to room temperature. The thermal expansion of blödite increases on approaching the dehydration temperature. On the contrary, the expansion of löweite ceases on approaching the dehydratio. Unlike blödite, which shows relatively isotropic thermal expansion, leonite has a pronounced anisotropic expansion with shortening of the a crystal-lattice period with increasing temperature. The volume expansion of leonite does not show any significant departure from linearity. On dehydration at approximately 410–420 K it transforms into langbeinite and arcanite (K2SO4).
Keywords
phase • dehydration • leonite • langbeinite • arcanite • thermal expansion • transition • blödite • high temperature • sulphate • x-ray diffraction • calorimetry • thermogravimetry