Mechanisms of carbonate mineral formation – Experimental approaches

Almost 20% of Phanerozoic sedimentary rocks are comprised of carbonate minerals. Most of our knowledge of past climatic conditions stems from the analyses of trace elements and stable isotopes in carbonate minerals. The main focus of the group of Mineralogy and Hydrogeochemistry of TU Graz is to mechanistically describe the physicochemical parameters controlling

  • the chemical distribution of traces/impurities in carbonates and
  • the stable isotope composition of major and trace elements in carbonates.

Specific tasks can be seen in the following sections.

Incorporation of traces (Mg, Sr, Ba) during CaCO3 precipitation

Contact: Martin Dietzel

Stable isotope distribution of major elements during the formation of CaCO3, SrCO3 and BaCO3

Contact: Vassilis Mavromatis

Transformation of amorphous calcium carbonate (ACC) in the presence of organic and inorganic ligands
(Subproject of CHARON II – DFG Forschergruppe 1644)

Carbonates are widely distributed in marine and terrestrial environments and have been extensively studied in regard to reconstruct Earth’s system evolution. The occurrence and formation of calcium carbonate minerals is in most cases related to precipitation from aqueous solution where mineral growth proceeds along different pathways. The formation of ACC and its transformation to crystalline calcium carbonate (e.g. (Mg-)calcite, aragonite) is a key process in the formation of many biominerals.

To reconstruct formation conditions and crystallization pathways of those minerals the application of elemental, isotopic and structural proxies is a powerful tool. In particular, this requires detailed understanding on the distribution behavior of elements and isotopes during the formation and transformation of ACC to the crystalline phase. Therefore, ACC transformation experiments were carried out under precisely defined physicochemical conditions and in the presence or absence of organic and inorganic molecules.

Contact: Katja Götschl

ACC act as a precursor of Mg-calcite that forms at a later stage in the experimental run. © TU Graz/IAG

Formation Mechanisms of Carbonates via Amorphous Precursors

Austrian Science Fund (FWF) - Hertha-Firnberg Project: T 920-N29

Recent studies focused on mineral growth from solutions supersaturated with respect to carbonates have shown that mineral formation does not always follow classic crystallization pathways, but proceeds via the formation of an intermediate amorphous phase. However, the mechanisms involved in this (trans-)formation are insufficiently understood and the chemical and isotopic evolution of fluids and solid counterparts during crystallization of amorphous precursors is, to date not explored at all. This project aims at investigating the reaction mechanisms controlling the formation of hydrous and anhydrous crystalline carbonates via amorphous precursors using well-established experimental set-ups.

Main focus of this project will be given on the formation behavior of calcium magnesium carbonates (CaxMg(1-x)CO3.nH2O) and calcium strontium carbonates (CaxSr(1-x)CO3). The innovative character of this project is that mineralogical observations, together with chemical and isotopic results, will be used to identify and describe formation mechanisms of crystalline carbonates via amorphous precursors. In this context, the fate of primary chemical and isotopic composition after the transformation of amorphous precursors to crystalline carbonate minerals will be assessed at high temporal resolution.

Contact: Bettina Purgstaller

Figure: 3D Surface Plot of in situ Raman spectra showing the transformation behavior of amorphous Ca0.9Mg0.1CO3 0.49H2O dispersed in a MgCl2-NaHCO3 solution.

© B. Purgstaller

ACC formation in the scope of tailored transformation

Amorphous calcium carbonate is an important precursor for biomineralization, but highly metastable with transformation to crystalline polymorphs of CaCO3 between minutes and few days, depending on synthesis and storage conditions. Therefore characterising the amorphous precursor phase and gathering information about parameters governing crystallization is challenging with respect to monitor this highly metastable phase respectively the fast transformation process.

The aim of this study is to find synthesis routes on how to obtain long-time metastable amorphous calcium carbonate and subsequently be able to decipher crystallization pathways leading to the respected calcium carbonate polymorphs.

Contact: Martin Dietzel

The role of organics on the trace element uptake during CaCO3 precipitation (Calibration of elemental and isotope proxies by inorganic precipitation experiment)

The main task of this study is to develop proper proxy tools for the reconstruction of paleo-environmental conditions during the formation of biogenically induced carbonates. On that account the study focuses on the element partitioning and isotope fractionation processes in inorganic calcium carbonates precipitated under controlled laboratory conditions with and/or without the presence of selected organic substances.

The carbonate formation will be analyzed by using mineralogical and microstructural methods as well as elemental ratios, traditional and non-traditional stable isotopes, clumped isotopes and redox-sensitive elements. (Brachiopods as sensitive tracers of global marine Environment: Insights from alkaline, alkaline Earth metal, and metalloid trace element ratios and isotope systems)

Link:  BASE-LiNE Earth
Contact: Anja Füger

Oxygen and clumped  isotope fractionation during carbonate mineral formation

Contact: Martin Dietzel

Experimental formation of dolomite via Ca-Mg carbonate intermediates: Controls of temperature, reactant calcium carbonate phase and alkalinity

Modern dolomite, CaMg(CO3)2, formation is slow, scarce and restricted to a limited number of specific marine, evaporitic, lagoonal and lacustrine environments, despite being the most oversaturated mineral phase of the modern ocean. In contrast dolomite is highly abundant throughout ancient carbonate platforms; the paucity of modern sedimentary dolomite formation vs. the abundance of ancient sedimentary dolomites is commonly referred to as “the dolomite problem”. Resolution to the dolomite problem has been sought for decades owing to its importance for understanding carbonate diagenesis and associations of dolomite bodies with economic hydrocarbon and mineral deposits as well as freshwater aquifers.

Resolution to the dolomite problem requires a strong understanding of how ancient dolomites were formed. The vast majority of ancient dolomites have been interpreted to have formed during early to late, burial diagenesis by the replacement of calcium carbonates (e.g. limestone), however there is a lack of knowledge regarding the physiochemical conditions under which they formed. Interpreting the physiochemical conditions under which these ancient dolomites formed requires knowledge of how changes in the formation environment (e.g. temperature, pH, alkalinity, fluid Ca & Mg concentrations, etc) effect the fundamental dolomite properties of stoichiometry and cation ordering as well as the partitioning of elements and fractionation of stable isotopes.

This project therefore aims to determine the controls of parameters such as temperature, alkalinity, pH and reactant carbonate phase on the development of stoichiometry and cation ordering as well as the partitioning of elements and fractionation of stable isotopes in dolomite precipitated under controlled laboratory conditions and to assess their use as proxies for interpreting the formation conditions surrounding dolomite.

Isaac Kell Duivestein

Crystallization of carbonates and sulfides from aqueous solutions using CO2 diffusion technique

Contact: Martin Dietzel

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