Hans Oskierski

Mineral carbonation offers permanent and safe disposal of anthropogenic CO2. Well distributed and abundant resources of serpentine minerals and natural weathering of these mineral to stable and environmentally benign carbonates favour the exploitation of these minerals as the most suitable raw material for mineral carbonation. However, slow dissolution kinetics are impeding the large scale implementation of mineral carbonation. Heat treatment of serpentine minerals results in enhanced reactivity for subsequent carbonation processes at the expense of an additional energy penalty4. Heat treatment of these minerals results in the removal of structurally bound hydroxyl groups which leads to partial amorphisation of the structure and enhanced reactivity. Therefore, understanding the role of the mineralogical changes during dehydroxylation and determination of activation energy (Ea) is crucial for providing an energy efficient solution for commercialisation of mineral carbonation...

Natural examples of mineral carbonation, the conversion of Mg-silicate to Mg-carbonate and silica, are common in ultramafic rocks throughout the world and reflect the thermodynamic instability of Mg-silicate minerals in the presence of CO2. However, the industrial implementation of mineral carbonation as a means of safely storing CO2 in the form of carbonate minerals is hampered by slow kinetics and the cost associated with heat-activation and carbonation reactors at high pressures and temperatures.

In the Great Serpentinite Belt, New South Wales, Australia, natural carbonation occurs in the form of weathering derived magnesite deposits, carbonate crust on ultramafic mine tailings and hydrothermal silica-carbonate alteration. At Attunga, low temperature (10 to 50 °C) meteoric waters have altered serpentinite to typical cauliflower-like magnesite nodules and veins, usually accompanied by late stage amorphous silica. Consistently low δ13C and small radiocarbon contents point to overlying soil as the source of carbon in the magnesite. Textural observations suggest carbonation progressed via fractures and porosity created by weathering of the host-rock, producing intermediate phases with decreased Mg/Si ratios in the process. For the mine tailings of the Woodsreef Asbestos Mine a relationship between textures, mineral content and isotopic fingerprint indicates that carbonate crusts covering the tailings formed from evaporating meteoric fluids, which absorbed CO2 directly from the atmosphere. Rate estimates based on the carbonate content and time since closure of the mine indicate that carbonation of the mine tailings proceeds at much higher rates than background uptake of CO2 by chemical weathering. Lensoid masses of silica-carbonate rock and magnesite veins at the Piedmont magnesite deposit formed by hypogene replacement of serpentinite at temperatures between 165 and 225 °C. The magnesite is usually Fe-rich, indicating reducing conditions during formation, and often accompanied by dolomite and quartz with alteration fluids ascribed to hydrothermal and magmatic sources.

Each of the above processes created a distinct set of textures, minerals and isotope-geochemical signatures which reflect conditions and mechanisms favourable for carbonation, but also the associated limitations that need to be overcome for industrial implementation. A better understanding of natural analogues to mineral carbonation informs the development of accelerated carbonation processes for large scale industrial storage of CO2 in carbonate minerals.

The Woodsreef Asbestos Deposit, New South Wales, Australia, is a chrysotile mineralisation hosted in the ultramafic rocks of the Great Serpentinite Belt, predominantly consisting of schistose and massive serpentinite, as well as partially serpentinised harzburgite. Chrysotile has been extracted from the deposit intermittently between 1906 and 1983, producing 24.2 Mt of ultramafic tailings. The tailings result from dry-grinding of chrysotile ore and are stored above ground on an area covering about 0.5 km2. Extensive carbonate crusts have formed on the tailings pile since the closure of the mine. Natural weathering dissolves Mg-silicate minerals present in the tailings and precipitates Mg-carbonate minerals in the form of crusts and cements. Isotopic signatures of the carbonate minerals (δ13C, δ18O, F14C) indicate that carbonate crusts consisting of hydromagnesite predominantly incorporate CO2 of atmospheric origin. Estimation of the carbonate content has shown that large amounts of CO2 have been sequestered in the tailings at Woodsreef at rates significantly elevated above the background CO2 uptake rate by chemical weathering of coherent silicate rocks. There is potential to further enhance the rates of CO2 sequestration by optimizing the tailings storage for this purpose. Natural weathering of ultramafic tailings thus represents a viable option for low-energy, low-cost sequestration of CO2, directly from the atmosphere. Since the carbonation of mine tailings during weathering occurs in the aqueous phase additional information on the process can be unlocked by investigating the chemistry and isotopic composition of waters interacting with the tailings. The isotopic composition of these waters also represents an intermediate step in the pathway of the sequestered carbon and can thus serves to better constrain isotopic fractionation during formation of hydrated Mg-carbonates in these settings. In this contribution we consider the chemistry and isotopic signatures of natural waters that are associated with the carbonation of the tailings of the Woodsreef Asbestos Mine. Measurements of pH, T, conductivity, cation content, δ2H, δ13CDIC, δ18O and F14C of water samples are presented and used to discuss the interaction of these waters with the tailings material

Introduction Carbon dioxide sequestration or disposal is an essential component of the international effort to stabilise CO2 emissions to the atmosphere. Of the proposed sequestration schemes, mineral sequestration represents the most geologically stable and environmentally benign method for carbon disposal (Lackner et al. 1995). Mineral carbonation mimics natural silicate weathering processes that bind CO2 in stable carbonate minerals. Ultramafic rocks from ophiolite belts, containing high abundances of magnesia as serpentine and olivine, represent the best potential feedstock for mineral carbonation (Metz et al. 2005). At present, research efforts focus on the development of economically viable and energy efficient processes for large-scale industrial implementation of mineral carbonation. These efforts could be assisted by gaining enhanced understandingand characterisation of the natural carbonation of ultramafic rocks. Earlier studies have shown the outstanding potential of serpentinites of the Great Serpentinite Belt for CO2 sequestration. Based on an RCO2 of 2.46 (the number of tonnes of rock required to sequester one tonne of CO2) and geophysical modelling of part of Great Serpentinite Belt in the northern New South Wales, Davis (2008) concluded that 24 × 10^9 t CO2 could be sequestered, equivalent to 308 y of the total stationary emissions for NSW at 2005 levels. Natural carbonation of the ultramafic rocks of the Great Serpentinite Belt is common and, among others, manifests itself in the development of magnesite deposits and silica carbonate alteration zones (Ashley 1995, 1997). The first step in the understanding of these analogues to mineral sequestration is to identify and trace the reactants. Commonly, cross plots of stable isotopes of carbon (delta13C) and oxygen (delta18O) are used to differentiate between magnesite occurrences and to deduce their sources (Kralik et al. 1989). The sources of carbon cannot always be unequivocally identified, since different processes and mixing can lead to similar, ambiguous carbon and oxygen fingerprints of the magnesite minerals. However, pathways and mechanisms of formation can be constrained if the isotopic fractionations associated with the reaction steps involved are known. In the case of vein deposits of magnesite associated with ultramafic rocks debate has focussed on plant respiration and decay in contrast to metamorphic exhalation as the source of carbon in these deposits (Abu-Jaber and Kimberly 1992). This study seeks to constrain the sources of carbon involved in the carbonation of the ultramafic rocks of the Great Serpentinite Belt that led to the formation of the Attunga magnesite deposit.