Microbial Dolomite in Abu Dhabi
Friday, October 8, 2010 6:06:53 PM
Formation of dolomite may sound simple. You take some limestone, which consists of calcite (CaCO3) and add some Magnesium (Mg) to get dolomite CaMg(CO3)2), a process used to be called dolomitization. A problem is that this doesn't seem to work in the lab. The assumed chemical reaction will not take place at low temperature and atmospheric pressure. Calcium carbonate does not react with magnesium cations in solution at room temperature: no conversion of limestone into dolomite is therefore possible under conditions typical of the earth's surface. (Instead you may possibly end up with a magnesium rich limestone - “magnesium limestone”). No reaction can be measured to take place at room temperature between calcium carbonate and magnesium sulphate or magnesium chloride in solution. A working trick is to add the right bacteria to your mixture.
If the water is slightly alkaline, dolomite formation might take place via a reaction a bit like this:
Mg2+ + HCO3 + CaCO3 <--> CaMg(CO3)2 + H+
with the necessary bicarbonate (HCO3) supplied by bacterial sulphate reduction.
Past studies have shown that microbially-mediated dolomite formation may be associated with activity of sulphate reducing bacteria. This bacterial sulphate reduction, combined with oxidation of organic matter, results in increasing alkalinity, promoting dolomite precipitation. Sadooni and co-authors show that dolomite in recent sabkha sediments from Abu Dhabi appears to be initiated in what they call “micro-niches” or small isolated nucleation sites. The dolomite occurred in nucleation sites such as pores in foraminiferal tests (popularly known as shells) and micro-depressions between clay mineral plates. Local anoxia (oxygen depletion) at these sites may permit microbial sulphate reduction and dolomite formation. Combination of neighbouring micro-niches in porous carbonate sediments would lead to dolomitization of an entire section.
The study suggests that “microbial” dolomite may be more widespread than has been previously recognised.
I wonder whether dolomite formation is at all possible without some sort of microbial mediation, but have no answer ready for that.
I understand that the exact role of microbes in dolomite formation is not fully understood, but it is suggested that during bacterial sulphate reduction, organic matter is oxidised, and the alkalinity increases promoting dolomite formation. As sulphate is consumed MgSO4 (magnesium sulphate) becomes disassociated and Mg++ comes available for further dolomite formation.
Sadoon et al.
Microbial Dolomites from Carbonate-Evaporite Sediments of the Coastal Sabkha of Abu Dhabi and their Implications
Journal of Petroleum Geology, Vol. 33(4), October 2010, pp 289-298