The document discusses the dolomite problem - why there is so little modern dolomite formation compared to extensive ancient dolomite. It outlines factors that limit modern dolomite precipitation from seawater including mineral kinetics, crystallization rates, hydration, carbonate activity, and sulfate concentration. Overcoming these constraints through processes like evaporation, dilution, or microbial sulfate reduction could have allowed for more dolomite formation in the past. Extensive ancient dolomite requires a source of magnesium, fluid flow to transport magnesium, and suitable physicochemical conditions for dolomitization over long time periods using seawater as the magnesium source.

1. Omar A. Radwan
o.a.radwan@gmail.com
PhD Student – Geosciences Dept. - KFUPM
GEOL533 - Carbonates and Evaporites
Assignment 3
The Dolomite Problem
I. Why there is so little modern dolomite?
II. Why there is so much ancient dolomite?

2. I. Why there is so little modern dolomite? - Outline
• Introduction
• What limits dolomite formation?
o Mineral kinetics
o Crystallization rates
o Hydration
o CO3 activity
o Sulfate (SO4
2-)
• Concluding remarks
Al-Awadi, 2009
2

3. Introduction
• CaMg (CO3)2
• Globally, there are many large:
hydrocarbon reservoirs (oil and gas) and
mineral deposits (mainly lead–zinc)
that are hosted in porous and permeable
dolostones.
• It is common in ancient platform carbonates,
yet it is rare in Holocene sediments.
Déodat de Dolomieu (1750 - 1801)
3
Warren, 2000

4. Introduction
• How can we explain the origin of
thick, pervasively dolomitized
successions of rocks that cover
thousands of square kilometers?
• At the core of this problem are the
facts that:
(1) dolomite has never been
synthesized in the laboratory
under low-temperature low-
pressure abiogenic conditions
(2) dolomite is rarely
precipitated from modern
seawater even though ocean
water is supersaturated with
respect to the mineral.
Land, 1998
4

5. Introduction
• At present there is a consensus that dolomite can
form:
• early in synsedimentary (authigenic) situations
oDolomite precipitation from waters of varying
composition can be expressed as either of the
following two equations:
Ca2+ + Mg2+ + 2CO3
2- ⇔ CaMg(CO3)2
Mg2+ + Ca2+ + 4HCO3
- ⇔ CaMg(CO3)2 + 2CO2 + 2H2O
• during early diagenesis in the relatively shallow
subsurface, or during late diagenesis under deep
burial conditions.
oDolomite replacement of calcite is generally
expressed as:
2CaCO3 + Mg2+ ⇔ CaMg(CO3)2 + Ca2+
5
Scholle & Ulmer-Scholle, 2003

6. What limits dolomite formation?
The fact that dolomite is not being precipitated from modern seawater
indicates that crystallization is inhibited by a variety of factors:
• Mineral kinetics
• Crystallization rates
• Hydration
• CO3 activity
• Sulfate (SO4
2-)
The abundance of dolostones throughout the geological record implies
that these constraints could have been suppressed at various times in
the past. 6

7. Mineral kinetics
Dolomite is a highly ordered mineral and
therefore requires more thermodynamic
energy to form than is necessary for calcite
or aragonite precipitation.
Modern seawater, with a Mg:Ca ratio of ~5:1
James & Jones, 2015
7

8. Mineral kinetics
How to overcome?
• This barrier to dolomite formation
can be overcome if the Mg:Ca ratio of
the fluid is raised to about 10:1 so
that more Mg ions become available.
This can be achieved through;
• evaporation that leads to the
preferential removal of Ca as LMC,
HMC, aragonite, gypsum, or
anhydrite, are precipitated.
• Conversely, dilution of seawater
with fresh water will also
overcome this kinetic problem. As
seawater is diluted, decreased
ion–ion interaction increases their
availability for crystallization.
James & Jones, 2015
8

9. Crystallization rates
Rapid precipitation from supersaturated solutions favors the formation of
calcium-rich dolomite (HCD) because there is insufficient time to allow
segregation of the Ca and Mg ions into distinct layers.
How to overcome?
o Dilution of the fluid by fresh water decreases the precipitation rate and
generally leads to the growth of more stoichiometric dolomite (LCD).
9
Scholle & Ulmer-Scholle, 2003

10. Hydration
The strength of the electrostatic attraction
of Mg to the OH− radical is about 20%
greater than that for Ca and much higher
than that for the CO3 ion.
It takes less energy to break the Ca–OH
bond than it does to break the Mg–OH bond.
How to overcome?
• From a theoretical perspective,
dolomitization should be easiest in hot,
saline fluids because high water
temperature and high salinity can help
overcome the hydration barrier.
10
James & Jones, 2015

11. CO3 activity
Mg and Ca cations bond most easily with
CO3. Normal seawater, however, has a
pH of 7–8 with most carbonate in the
form of HCO3 and therefore not readily
available for bonding.
Naturally occurring alkaline fluids
include:
(1) continental groundwaters that
have been involved with the
weathering of siliceous rocks;
(2) waters containing products
derived by the dissolution of
continental alkaline minerals; or
(3) continental waters that have
undergone anaerobic bacterial
sulfate reduction.
11
James & Jones, 2015

12. Sulfate (SO4
2-)
The abundant sulfate in seawater acts as an inhibitor to dolomite precipitation.
How to overcome?
The sulfate in solution can be reduced by:
(1) the action of sulfate-reducing bacteria;
(2) the precipitation of sulfate minerals such as gypsum and anhydrite;
(3) dilution with fresh water in coastal aquifers; or
(4) microbial reduction of organic-rich seafloor sediments.
12

13. Sulfate (SO4
2-)
13

14. Concluding remarks
James & Jones, 2015
14

15. Concluding remarks
15
• The details of the formation of dolomite remain obscure, but recent
experimental studies plus geologic observations have furnished
fairly convincing evidence that most dolomite is not a primary
precipitate but forms rather as a product of slow reaction altering
the originally deposited calcium carbonate.
• Solutions that accomplish the alterations are most effective if they
have a fairly high salinity and pH, a low Ca2+/Mg2+ ratio, and
somewhat elevated temperature.

16. Next
II. Why there is so much ancient dolomite?
16
The Sella Platform
James & Jones, 2015

17. II. Why there is so much ancient dolomite? - Outline
• How to form extensive dolomite?
oThe source of magnesium
oFluid flow
oDolomitization Models
• Concluding remarks
Al-Awadi, 2009
17

18. How to form extensive dolomite?
Given enough time, dolomite
should form as long as some of
the above constraints can be
overcome. But that is not enough;
there must also be:
(1) a source of abundant Mg;
(2) hydrologic conditions
that allow for the delivery of
the Mg to the zone of
dolomitization; and
(3) physicochemical
conditions that are suitable
for the formation of
dolomite.
18
The Sella Platform
James & Jones, 2015

19. The source of magnesium
Modern seawater
contains ~1250 ppm Mg2+ and ~411 ppm Ca2+ with a Mg:Ca ratio of ~5:1,
River water
has ~4 ppm Mg2+ and ~15 ppm Ca2+ and a Mg:Ca of 0.25:1.
Formation waters
are highly variable, but generally have Mg:Ca ratios from 1.8:1 to 0.4:1.
∴ In short, seawater must be involved in any processes that create large volumes
of dolomite.
As temperature rises, the amount of Mg needed for dolomitization falls.
At 25°C, ~650 m3 of normal seawater are needed to dolomitize 1 m3 of limestone.
At 50°C ~450 m3 of fluid are needed to dolomitize 1 m3 of limestone.
19

20. Fluid flow
Dolomitization is , in many respects,
an issue of hydrodynamics.
Fluid flow can be driven by:
(1) storms that drive ocean
waters onto nearly tidal
flats;
(2) elevation in the form of
a hydraulic head of
meteoric or marine waters;
(3) gradients in fluid density
that are brought about by
differences in water
temperature or salinity; and
(4) pressure due to burial or
tectonic compaction.
20
Tucker, 2009

21. The different modes of dolomite formation
21
James & Jones, 2015

22. Tucker, 2009
Thedifferentmodesofdolomiteformation

23. References
• Al-Awadi, M., Clark, W. J., Moore, W. R., Herron, M., Zhang, T., Zhao, W., ... &
Sadooni, F. (2009). Dolomite. Perspectives on a Perplexing Mineral, Oilfield
Review, Autumn, 21(3).
• James, N. P., & Jones, B. (2015). Origin of Carbonate Rocks. John Wiley & Sons.
• Land, L. S. (1998). Failure to Precipitate Dolomite at 25° C fromDilute Solution
Despite 1000-Fold Oversaturation after32 Years. Aquatic Geochemistry, 4(3),
361-368.
• Morrow, D. W. (1982). Diagenesis 1. Dolomite-Part 1: The chemistry of
dolomitization and dolomite precipitation. Geoscience Canada, 9(1).
• Scholle, P. A., & Ulmer-Scholle, D. S. (2003). A Color Guide to the Petrography of
Carbonate Rocks: Grains, Textures, Porosity, Diagenesis, AAPG Memoir 77 (Vol.
77). AAPG.
• Tucker, M. E. (2009). Sedimentary petrology: an introduction to the origin of
sedimentary rocks. John Wiley & Sons.
• Warren, J. (2000). Dolomite: occurrence, evolution and economically important
associations. Earth-Science Reviews, 52(1), 1-81.
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Thank You