DECOMPOSITION OF KONYAITE AND ITS IMPORTANCE IN CARBON SEQUESTRAION

Konyaite, Na₂Mg(SO₄)₂∙5H₂O, is an interesting sodium-magnesium salt. Described from the Great Konya Basin in Turkey, it has only been recorded from a handful of localities, although experimental observations suggest that it should be more common than its decomposition product blödite, Na₂Mg(SO₄)₂∙4H₂O. Little is known about konyaite and how it behaves in the geological environment.

Konyaite can be prepared easily at 35(2)°C and ~70% humidity, forming as intergrown large platy crystals. Konyaite crystallises in space group P2₁/c, with the unit-cell a = 5.7594(10), b = 23.914(4), c = 8.0250(13) Å, β = 95.288(9)°, V = 1100.6(3) and Z = 4. It’s crystal structure was solved by direct methods to R₁ = 3.41%. The crystal structure was then used to determine phase abundances in natural mine tailings by the Rietveld method at the Mount Keith Nickel Mine in Western Australia.

At Mount Keith, the mining operation produces approximately 350,000 tonnes of greenhouse gases and approximately 11 Mt of ultramafic tailings each year. Tailings are composed primarily of antigorite and lizardite with hydrotalcite-group minerals, whilst minor brucite, talc, magnetite, chromite, quartz, magnesite, dolomite, calcite and trace vermiculite are common. Efflorescent crusts of halide and sulphate minerals occur at the surface of tailings. Halite, hexahydrite, and blödite are the dominant phases in the efflorescences. Minor amounts of konyaite, löweite, epsomite, and gypsum are less common. Secondary hydromagnesite occurs within and just below the efflorescent crusts and is preserved at depth within the tailings upon burial.

Rietveld studies of natural konyaite in mine tailings show partial to complete decomposition after 1.5–2 years to either thenardite + hexahydrite, blödite, blödite and an amorphous sulphate or complete decomposition to an amorphous sulphate. In terms of carbon sequestration, konyaite and its decomposition products steal enough Mg to mineralise ~2 wt.% CO₂ as hydromagnesite.