Results on alunite and basaluminite (felsobanyaite) dissolution kinetics presented at the 2015 Goldschmidt Conference

Our work on alunite and basaluminite (IMA-accepted mineral name: felsobanyaite) dissolution has been recently presented at the 2015 Goldschmidt Conference, held in Prague, Czech Republic, between the 16th and the 21st of August 2015. Our contribution, entitled "Alunite and basaluminite dissolution: Comparison and insights from batch experiments and atomistic computer simulations" described many of the results obtained during the EC-funded RASMIM project and compared the dissolution rates of alunite and basaluminite (felsobanyaite) under similar pH and temperature conditions. 

One of the main findings of the study is the higher stability of alunite, with dissolution rates noticeably slower than the ones for felsobanyaite under similar conditions. Even though the dissolution rates for both minerals seems to increase with both the increase of pH and temperature, such dependence is much weaker in the case of alunite than in the case of felsobanyaite.

Atomistic computer simulations for alunite suggest that most mineral surfaces expose K atoms and/or hydroxyl groups. Thus, these components should be the easiest to detach from dissolving alunite, whereas Al and sulphate tetrahedra seem to be much less accessible to the solution and should be the ones limiting alunite dissolution.

The conference contribution is available in the form of conference abstract at the Goldschmidt 2015 website.

RASMIM Project results presented at the EGU 2015

Some of the results of the RASMIM Project have recently been presented at the European Geosciences Union General Assembly 2015, held 12-17 April, 2015 in Vienna, Austria, as a part of the session entitled "The nature of the mineral-fluid interface: relevance to dissolution and growth processes".

Our contribution, entitled "Influence of pH and temperature on alunite dissolution rates and products", by Patricia Acero and Karen Hudson-Edwards, presents some of our data on the dissolution rates of alunite under similar conditions to the ones in many sulphate natural waters (pH between 3 and 8 and temperature between 6 and 40 degrees celsius). The solution and mineral data are interpreted together with the assistance of X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy and geochemical modelling, among other techniques.

The abstract of the contribution with further information can be accessed via the Copernicus webpage.

Atomistic computer simulations to understand alunite dissolution

Atomistic computer simulations are being carried out within the framework of the EC-funded RASMIM project and in collaboration with Dr. Julian Gale and Dr. Kate Wright, from the Department of Chemistry of the University of Curtin (Perth, Western Australia) . 

The bulk and surface structure of alunite have been modelled and the effect of solvation (attachment of water molecules) on different types of alunite surfaces have been explored. This has allowed observing and quantifying some of the initial effects that dissolution may have on this mineral, such as surface relaxation and reordering of molecules at the interface solid-solution.

The ongoing work will help understand the results obtained in alunite dissolution experiments, shedding light on the behaviour of this key mineral in mining environments.

Morphology obtained for non solvated (top) and solvated (bottom) alunite crystal by means of atomistic computer simulations (visualization using GDIS). The more preferential development of the (003) surface in the solvated specimen can be clearly observed.

Preliminary alunite dissolution results presented at SEM Conference 2014

Some of the preliminary results obtained about the dissolution rates and mechanisms of alunite dissolution have been presented in the 34th Annual Meeting of the Spanish Mineralogical Society, held in Granada (Spain) between the 1st and the 4th of July 2014.

Dr Karen Hudson-Edwards and Dr Patricia Acero awarded NERC grant

Dr Karen Hudson Edwards and Dr Patricia Acero have received a NERC grant for the project Characterisation of Nanometre-Sized Aluminium Sulphates: Implications for Mobility of Aluminium from Mine Wastes. The grant will allow accessing the Facility for Environmental  Nanoparticle Analysis and Characterisation (FENAC) at the University of Birmingham and will contribute to better achieve the objectives of the RASMIM project.

For further information, read this related post in the News section from Birkbeck College.

Alunite and Basaluminite kinetic dissolution experiments

The dissolution experiments aimed at studying the kinetics of alunite and basaluminite dissolution are ongoing. With these experiments, the dissolution rates of these two important aluminium sulphates will be obtained and their dissolution reactions and mechanisms will be better understood. This will allow improving predictive calculations and modelling of Al release and behaviour in mine environments and in the treatments designed for their remediation.

The batch dissolution experiments are being carried out in the Wolfson Laboratory of Environmental Geochemistry (University College of London, London, UK) and in the Laboratory of Environmental Geochemistry at Birkbeck College (London, UK). In these batch experiments, 100 mg of synthetic pure alunite or basaluminite are placed in glass beakers, stirred to 400 rpm and kept in contact with 200 mL of different types of solutions (pH 2, 3 and 4 H₂SO_4, pH 5.5 MES-buffered, pH 5.5 unbuffered deionized water, pH 8 TRIS-buffered, etc) and under controlled temperature conditions (4, 20ºC and 40ºC) during reaction times between 24 hours and 2 weeks. These conditions are intended to be similar to the ones found in environments affected by acid drainage after contact with sulphide minerals.

During the experiments, 6 ml aliquots are being removed at regular intervals, filtered using 0.22 mm filters and acidified to 1% HNO₃ for ICP analyses of the dissolved concentrations of Al, S and K, used to monitor the progress of the dissolution process and to obtain the dissolution rates. Solution pH and temperature are also monitored during the experiments. All the experiments are run at least in triplicate. 

Mineral samples are also being collected at the beginning and end of the experiments and studied by several mineralogical techniques (X-ray Diffraction, Raman spectroscopy, Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy, etc) to characterize the changes induced by the dissolution process on the surface of the reacting solids.

Basaluminite is now included in wikipedia

A new article in wikipedia has been created for basaluminite, including the information relative to its descreditation in favour of felsobanyaite.

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Basaluminite is a hydrated aluminium sulphate, with ideal formula of Al₄(SO₄)(OH)₁₀·5H₂O. The mineral name has been discredited (Burke, 2006) by the Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association (IMA) in favour of the name of Felsobanyaite.

It is a white, yellow, orange or brown mineral of low crystallinity. This mineral was first described by Bannister & Hollingsworth (1948) and it seems to form after the dehydration, even on air-drying, of the highly hydrated Hydrobasaluminite (Al₄(OH)₁₀SO₄·12-36H₂O). Further dehydration at 150ºCwould lead to the formation of Metabasaluminite (Al₄(OH)₁₀SO₄; Hollingsworth & Bannister, 1950).

Typically it appears as a weathering product of clays, as the result of acid released from the oxidation of pyrite; probably always a dehydration product of hydrobasaluminite; as coatings on joint surfaces and veinlets in ironstone (Irchester, England); in chalk (Clifton Hill, England); a reaction rim surrounding carbonate concretions (Chickerell, England); on fractures in garnet–sillimanite laterite (Kanogami, Japan) (http://www.handbookofmineralogy.com/pdfs/basaluminite.pdf).

 Basaluminite has been described as the dominant Al mineral formed when acid mine drainage is mixed with circumneutral buffered pH waters (Clayton, 1980; Bigham and Nordstrom, 2000).

Reference list

• http://www.handbookofmineralogy.com/pdfs/basaluminite.pdf
• Bannister, F. A., & Hollingworth, S. E. (1948). Two new British minerals. Nature, 162, 565.
• Bigham J. M. and Nordstrom D. K. (2000) Iron and aluminium hydroxysulfates from acid sulfate waters. In Sulfate Minerals – Crystallography, Geochemistry and Environmental Significance. Mineralogical Society of America, Washington DC, USA, pp.351–403.
• Brydon J. E.; Singh S. Shah, 1969: The nature of the synthetic crystalline basic aluminum sulphates as compared with basaluminite. The Canadian Mineralogist 9, Part 5(Pages 644-654.
• Burke, E. A. (2006). A mass discreditation of GQN minerals. The Canadian Mineralogist, 44(6), 1557-1560.
• Clayton T. (1980) Hydrobasaluminite and basaluminite from Chickerell, Dorset. Mineral. Mag. 43, 931–937.
• Hollingworth, S. E. & Bannister, F. A. (1950): Basaluminite and hydrobasaluminite, two new minerals from Northamptonshire. Mineralogical Magazine, 29,1-17.