Dolomite Perspectives on a Perplexing Mineral

Journal of Sedimentary Research 43, no. 4 
(December 1973): 965–984. 
>
Hydrothermal dolomitization. Fluids from deep 
within a basin can rapidly move up fault planes to 
dolomitize carbonates at shallower depths.
MattV_ORAUT09_Fig_10
Dolomite
Limestone
Flu
ids
flo
w 
up
fa
ult
p
la
ne
30. For more on this type of dolomite: Folk RL and
Siedlecka A: “The “Schizohaline” Environment: Its 
Sedimentary and Diagenetic Fabrics as Exemplified by 
Late Paleozoic Rocks of Bear Island, Svalbard,”
Sedimentary Geology 11, no. 1 (May 1974): 1–15. 
31. Kohout FA: “Ground-Water Flow and the Geothermal 
Regime of the Floridian Plateau,” 
Transactions, Gulf 
Coast Association of Geological Societies 17 
(1967): 339–354.
32. von der Borch CC and Lock D: “Geological Significance 
of Coorong Dolomites,” 
Sedimentology 26, no. 6 
(December 1979): 813–824.
33. Allan and Wiggins, reference 19. 
34. Warren, reference 2.
35. Warren, reference 2.
36. Allan and Wiggins, reference 19.
37. Allan and Wiggins, reference 19.
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40
Oilfield Review
A prime example of an ancient fault-related 
dolomite is found in the Ordovician Trenton–
Black River limestones of Michigan, USA, and 
southwestern Ontario, Canada.
38
There, dolomite 
defines zones of faulting and fracturing within 
the surrounding limestone. 
Microbial Mediation Model—Present-day 
low-temperature dolomite most often forms in 
restricted-marine or hypersaline coastal environ-
ments; however, these modern settings produce 
only a small fraction of the total dolomite found 
in the rock record. Although dolomite is abun-
dant in rocks of the Paleozoic era (250 to 540 Ma), 
it becomes increasingly scarce in younger rock or 
sediment—particularly in recent (Holocene) 
settings. By contrast, ancient massive dolomites 
are believed to have formed in a wide variety of 
settings, described previously. This disparity leads 
some researchers to question whether present-
day conditions actually reflect those that allowed 
the formation of massive ancient dolomites.
To understand the rarity of dolomite in the 
recent rock record, researchers sought to dis-
cover how dolomite forms. Until recently, they 
have struggled to synthesize the mineral in their 
laboratories. Reasoning that seawater contained 
the right ingredients needed for the creation of 
dolomite, geochemists used brine concentrations 
and pressure-temperature conditions thought to 
exist in nature during the formation of dolomite.
39
The inability to produce dolomite in the labora-
tory goes to the very heart of the problem that has 
plagued geoscientists for years (see “The 
Dolomite Problem,” page 1). Although magne-
sium, calcium and carbonate ions are common in 
seawater, the conditions necessary to arrange 
them in the neatly ordered, alternating layers 
that formed stoichiometric dolomite have appar-
ently changed. Once geoscientists understand 
how dolomite forms in a controlled environment, 
they may come closer to learning how it forms in 
nature and why it was once so prevalent and yet 
is so uncommon today.
The dolomite problem is tied to a number of 
interrelated processes involving thermodynam-
ics, chemical kinetics, hydrology, host-rock
texture and mineralogy. Discoveries in the
1990s revealed that another process—microbial 
action—should be factored into the equation 
(above right)
. Microbes became the focus of 
attention in the sulfate-rich sludges of shallow 
isolated lagoons, when it was discovered that 
calcium-rich dolomite precipitates under anoxic, 
hypersaline conditions. 
Sulfate-reducing bacteria in the Brazilian 
Lagoa Vermelha play an important role in the for-
mation of primary dolomite in lagoons along the 
coast east of Rio de Janeiro.
40
There, lagoonal 
hydrological cycles vary with alternating wet and 
dry seasons. During the wet season, precipitation 
and continental groundwater raise water levels; 
during the dry season, seawater recharges the 
lagoon, which becomes increasingly saline as 
evaporation intensifies. This dynamic system 
helps supply the ions needed for dolomite pre-
cipitation and anaerobic microbial activity. 
Dolomite precipitation requires Mg
2+
, Ca
2+
and 
CO
3
2–
ions, whereas a continuous supply of SO
4
2– 
ions provides oxygen required to sustain the 
metabolic activity of the sulfate-reducing bacte-
ria. The most favorable time for dolomite precipi-
tation is the dry season, when the main source of 
groundwater recharge is seawater, which delivers 
the ions necessary for both dolomite precipita-
tion and sulfate reduction.
In some geochemical models, sulfate is thought 
to inhibit dolomite production. Experiments have 
shown that in a purely inorganic system without 
benefit of bacterial action, the sulfate does indeed 
inhibit dolomite precipitation. However, this is just 
the opposite of the Lagoa Vermelha case, in which 
sulfate is necessary to maintain the microbial 
activity required to produce dolomite. The hydro-
logic system furnishes sulfate ions to the zone of 
active sulfate reduction where sediments become 
enriched with dolomite, which, once nucleated, 
continue to grow with burial. The right strain of 
bacteria is also a key to dolomite precipitation,
as evidenced by the fact that dolomite is not pre-
cipitating in most other anoxic, organic-carbon-
rich marine sediments.
Laboratory experiments were able to simulate 
the chemistry of the dry-season anoxic hypersaline 
lagoonal waters. Bacteria taken from the lagoonal 
sludge were used to inoculate a cultural medium. 
They were incubated for one year in a refrigerator 
at 4°C [39°F]. After incubation, a dolomite pre-
cipitate was recovered. Scanning electron micro-
scope (SEM) and X-ray diffraction (XRD) analysis 
showed that a ferroan dolomite with a fairly high 
degree of cation order had been precipitated. 
Subsequent laboratory experiments using two 
aerobic bacteria cultures, Halomonas meridiana 
and Virgibacillus marismortui, were shown to 
precipitate dolomite in just 30 days at 25°C and 
35°C [77°F and 95°F], respectively.
41
These experi-
ments also showed that the time required for ini-
tiation and precipitation of dolomite decreased 
with increasing temperature, while the quantity of 
crystals increased with greater incubation time. 
Here, bacterial metabolic activity involves produc-
tion of ammonia [NH
3
], which creates an alkaline 
microenvironment around the bacteria cells. The 
bacteria also produce CO
2
, which dissolves and 
transforms into either HCO
–
3
or CO
–2
3
at higher pH. 
In the presence of Ca
2+
and Mg
2+
, the culture 
medium becomes supersaturated with respect to 
dolomite. These physiochemical changes influ-
ence the geochemical environment and promote 
>
Scanning electron microscope photomicrograph of rod-shaped microbial 
cells inhabiting the surface of a basalt sample. These microbes have 
precipitated dolomite after three months in anaerobic groundwater. 
Differences in crystal encrustation may be due to microbial residence time on 
the basalt surface or may simply reflect differences in metabolic activity. 
Each cell is approximately 1 µm long. (From Roberts et al, reference 43.)
MattV_ORAUT09_Fig_11
0.5 µm
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41
dolomite precipitation. Other related experiments 
are helping researchers develop oxygen isotope 
paleothermometers to evaluate conditions of 
ancient dolomite formation.
42
These analyses proved that microbial media-
tion of dolomite production can be achieved 
under low-temperature anoxic conditions, and in 
a relatively short time. When dolomite is associ-
ated with sediments that are rich in organic
carbon, biological influences should therefore
be investigated.
A different type of biomineralization was 
reported in 2004 when methanogens, rather than 
sulfate reducers, were found to be responsible for 
dolomite nucleation and precipitation. Rather 
than examining a hypersaline lagoon, groundwa-
ter researchers conducted a long-term evaluation 
of a petroleum-contaminated freshwater aquifer 
in Minnesota, USA. There they discovered dolo-
mite on the cells of methanogenic microbes that 
colonized a subsurface basalt layer in a highly 
reducing environment.
43
In this setting, dolomite formation is seen as