Hydrotalcites As a Secondary Carbon Sink in Serpentinites

Hydrotalcite group minerals are commonly found within ultramafic rocks such as serpentinites, which may be used as feedstocks for mineral carbonation programs. Particular members of the hydrotalcite mineral group have the capacity to trap and store CO₂ via anion exchange. Hydrotalcite group minerals, which are also known as layered double hydroxides, undergo anion exchange by replacement of interlayer anions such as Cl^- and OH^- for aqueous CO₃^2-. This makes hydrotalcite group minerals a novel secondary sink for carbon within serpentinites.

To better understand the role hydrotalcites may play in CO₂ sequestration, a wide reaching investigation must be undertaken to determine (1) the range of abundance of hydrotalcite minerals in serpentinites, (2) how readily these minerals take up carbonate into their crystal structures and (3) how to fingerprint uptake of atmospheric or industrial CO₂ into these minerals.

We are investigating hydrotalcite mineral abundance and carbonation at a number of serpentinite-hosted mineral deposits. Multiple sites were selected in order to investigate a diverse selection of serpentinites and hydrotalcite minerals: the abandoned Woodsreef Chrysotile Mine (New South Wales, Australia), the Lord Brassey Nickel Mine and several small workings within the Dundas Mineral Field (Tasmania, Australia). Our X-ray diffraction results indicate that samples from these sites are dominated by the presence of serpentine minerals, and many also include magnetite, chromite and brucite. Several of the samples also contain carbonate minerals, including magnesite and dolomite. Pyroaurite [Mg₆Fe³⁺₂(CO₃)(OH)₁₆·4H₂O], the carbonate-bearing magnesium-iron end-member of the hydrotalcite group is found as a minor phase at Woodsreef with between 0 and 7% abundance, which is consistent with the findings of Oskierski et al. (2013). Pyroaurite appears to have developed on weathering surfaces and may have formed at low temperatures at Woodsreef (Oskierski et al. 2013) and Lord Brassey. Stichtite [Mg₆Cr³⁺₂(CO₃)(OH)₁₆·4H₂O] the magnesium-chromium analogue of pyroaurite, is common from multiple localities within the Dundas mineral field. Stichtite appears to have formed from replacement of chromite by high temperature fluids. Both of these hydrotalcite minerals are carbonate bearing and represent an existing store for CO₂. However, little is known about the isotopic signatures of anion exchange in hydrotalcite minerals; thus it is difficult to recognise the source of CO₂ in these minerals and whether or not they represent a stable sink for this greenhouse gas.

Anion exchange experiments were conducted to study the rate and isotopic signature of carbonate uptake into a chlorine-bearing hydrotalcite mineral, iowaite [Mg₆Fe³⁺₂(Cl₂)(OH)₁₆·4H₂O]. Aliquots of iowaite were placed in stirred tank reactors containing deionised water, which was allowed to equilibrate with atmospheric CO₂ over 46 hours. This resulted in complete exchange of the interlayer Cl^- for atmosphere-derived dissolved inorganic carbon (DIC) to form pyroaurite. Stable carbon isotopic analysis of the synthetic pyroaurite shows a marked depletion in ¹³C relative to atmospheric CO₂, indicative of diffusion fractionation during anion exchange. This apparent preference for light carbon will need to be further investigated as an indicator of CO₂ uptake in natural settings.

Keywords: carbon mineralisation, X-ray diffraction, anion exchange, hydrotalcites

References:

Oskierski H. C., Dlugogorski B. Z. & Jacobsen G. 2013. Sequestration of atmospheric CO₂ in chrysotile mine tailings of the Woodsreef Asbestos Mine, Australia: Quantitative mineralogy, isotopic fingerprinting and carbonation rates. Chemical Geology 358, 156-169.