The Crystal Chemistry of Boussingaultite, (NH4)2Mg(SO4)2·6H2O, and Its Derivatives in a Wide Temperature Range

Abstract

The crystal structure, thermal behavior, and vibrational spectra of the anthropogenic analogue of boussingaultite, (NH4)2Mg(SO4)2·6H2O, and its dehydrated counterpart efremovite, (NH4)2Mg2(SO4)3, were studied in detail. The sample from the Chelyabinsk burning coal dumps has the composition of (NH4)1.92(Mg1.02Mn0.01Fe0.01)∑1.04(SO4)2·6H2O and crystallizes in the space group P21/a, with a = 9.3183(4), b = 12.6070(4), c = 6.2054(3) Å, β = 107.115(5)°, V = 696.70(5) Å3 (at 20 °C), Z = 2. The thermal evolution steps are as follows: boussingaultite (NH4)2Mg(SO4)2·6H2O (25–90 °C) → X-ray amorphous phase (100–150 °C) → efremovite (NH4)2Mg2(SO4)3 (160–340 °C) → MgSO4 Cmcm + Pbnm (340–580 °C) → MgSO4 Pbnm (580–700 °C). Thermal expansion is anisotropic, with the coefficients (×106 °C−1) α11 = 52(2), α22 = 68(2), α33 = –89(3), and αv = 31(3) at T = –123 °C; and α11 = 53(2), α22 = 67(2), α33 = 15(1), and αv = 136(3) at T = 60 °C. The maximal thermal expansion is along the b-axis and is due to straightening of corrugated pseudolayers (within the ab plane) of Mg(H2O)6 octahedra and SO4 tetrahedra with NH4 groups in the interlayer space. Vibrational spectroscopy outlines the general trend of dehydration and deammonization as the difference in the temperature intervals of these transformation steps allows separation of O–H and N–H vibrations in the process of dehydration by infrared and Raman spectroscopy. The intermediate partially dehydrated modification of boussingaultite was detected by in situ Raman spectroscopy at 110 °C that may correspond to ammonium leonite.