Chapter 6: Carbonate Sedimentary Rocks
There are two main categories of carbonate rocks:
•
Calcite (CaCO3)
•
Dolomite (CaMg(CO3)2)
Both Calcite and Dolomite will contain varying amounts of Calcium
and Magnesium. Other elements make up only trace amounts.
Calcite
Dolomite
Calcite Group
Iceland Spar CaCO3
Rhodochrosite
MnCO3
Magnesite MgCO3
Siderite FeCO3
Smithsonite
ZnCO3
Dolomite Group
Dolomite CaMg(CO3)2
Ankerite Ca(Mg,Fe,Mn)(CO3)2
Aragonite Group
Aragonite CaCO3
Strontianite SrCO3
Cerussite PbCO3
Witherite BaCO3
Limestone (composed of primarily CaCO3) textures
Carbonate Grains
• Carbonate clasts (extraclasts & intraclasts--Lithoclasts)
•Extraclast: derived from older limestone located outside the depositional
environment.
•Intraclast: derived from seafloor, adjacent tidal flats or a carbonate beach
•Lithclast: a nonspecific term used when the distinction between extra &
intraclast cannot be made.
• Skeletal particles
• Ooids
• Peloids
• Aggregate Grains
Matrix cements: either sparry calcite or micrite
A: Rounded
clasts cemented
by sparry calcite.
B: Angular clasts
in micrite.
C: Fossiliferous
limestone with
sparry cement.
D: Normal ooids
cemented with
sparry.
E: Radial ooids
cemented with
sparry &
micrite.
F: Pellets
cemented with
sparry.
Ooid
Aggregate grain (Grapestone)
Microcrystalline calcite (Micrite) versus Sparry calcite
Classification of Carbonate Rocks
Limestone classification based on textures
Origin of Carbonate Rocks
Limestone:
CO2 + H2O ↔ H2CO3 (carbonic acid)
H2CO3 ↔ H+ + HCO3- (bicarbonate ion)
HCO3- ↔ H+ + CO32- (carbonate ion)
__________
H2O + CO2 + CaCO3 ↔ Ca2+ + 2HCO3(where the CaCO3 can be either Calcite or Aragonite)
Principle factors that affect inorganic precipitation of
CaCO3 in water (Table 6.4 pg. 175)
Water
condition
Direction of
change
Directed effect
Effect on
CaCO3
solubility
Kind of
CaCO3
precipitated
Temperature
Increase
Loss of CO2,
increase in pH
More likely to
precipitate
Micrite or
ooids
Pressure
Decrease
Loss of CO2,
increase in pH
More likely to
precipitate
Micrite or
ooids
Salinity
Decrease
Decrease in
More likely to
activity of
precipitate
“foreign cations”
Micrite or
ooids
Organic activity and CaCO3 precipitation
•Extraction of CaCO3 from water
Growth of shells and tests
•Photosynthesis
Removes CO2 from water, thereby increasing pH.
•Decay of soft tissue
Increases pH of water
•Feeding, sediment ingestion
Reshapes sediment
•Bacterial activity
Promotes CaCO3 precipitation
Era
Period
Dominate Carbonate
Mineral
Ceno-zoic
Neogene-Quaternary
A + HMC
(Aragonite Sea)
Paleogene
Mesozoic
Cretaceous
Low-magnesian
Calcite (LMC)
(Calcite Sea)
Jurassic
Triassic
Permian
Pennsylvanian
Aragonite (A) + Highmagnesian Calcite
(HMC)
(Aragonite Sea)
Paleozoic
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Low-magnesian
Calcite (LMC)
(Calcite Sea)
Calcite versus Aragonite
Dolomite Classification and Variation
Dolomite formation:
The Dolomite problem….
Scientists have not yet been successful in the laboratory in
precipitation perfectly ordered Dolomite (50% Calcium and 50%
Magnesium) at the normal temperatures and pressures of the Earth’s
surface.
Ca2+(aq) + Mg2+(aq) + 2CO32-(aq) = CaMg(CO3)2(solid)
2CaCO3(solid) + Mg2+(aq) = CaMg(CO3)2(solid) + Ca2+(aq)
Sabkha Environment
Carbonate Diagenesis
Carbonate Diagenesis continued…
Stylolites: a pressure-solution
feature common in carbonate
rocks. These features are often
associated with clay minerals
and other fine-size noncarbonate minerals that
accumulate as carbonate
minerals dissolve.