The decomposition of konyaite: importance in CO_2 fixation in mine tailings

摘要

The sodium-magnesium hydrated double salt konyaite, Na_2Mg(SO_4)_2·5H_2O, has been studied by singlecrystal X-ray diffraction and thermogravimetry on synthetic samples and by quantitative X-ray diffraction utilizing the Rietveld method on natural samples from the Mount Keith mine, Western Australia. Konyaite crystallizes in space group P2_1/c, with the cell parameters: a = 5.7594(10), b = 23.914(4), c = 8.0250(13) A, β = 95.288(9)°, V = 1100.6(3) A~3 and Z = 4. The crystal structure has been refined to R_1 = 3.41% for 2155 reflections [F_o>4σ(F_o)] and 6.44% for all 3061 reflections, with all atoms located. Quantitative phase analysis utilizing the Rietveld method was undertaken on five samples of konyaite-bearing mine tailings from the Mount Keith Nickel Mine, Western Australia. Konyaite was found to decompose over time and after 22 months had transformed to other sulphate and amorphous phases. Blodite did not increase in any ofthe samples indicating that konyaite may not always transform to blodite. Over the same time frame, synthetic konyaite completely decomposed to a mixture of thenardite (Na_2SO_4), hexahydrite (MgSO_4·6H_2O), blodite (Na_2Mg(SO_4)_2·4H_2O) and loweite (Na_(12)Mg_7(SO_4)_(13)). Detection ofkonyaite and other Mg-rich sulphates is important in terms of CO_2 fixation. Magnesium bound to sulphate mineral phases reduces the overall potential of tailings piles to lock up atmospheric carbon in Mg carbonates, such as hydromagnesite. Amorphous sulphates are also highly reactive and may contribute to acid mine drainage ifpresent in large quantities, and may dissolve carbonate phases which have already sequestered carbon.