Chapter 6: Carbonate Sedimentary Rocks
Dec 20, 2015
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
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.
Origin of Carbonate Rocks
Limestone:
CO2 + H2O ↔ H2CO3 (carbonic acid)
H2CO3 ↔ H+ + HCO3- (bicarbonate ion)
HCO3- ↔ H+ + CO3
2- (carbonate ion)
__________
H2O + CO2 + CaCO3 ↔ Ca2+ + 2HCO3-
(where the CaCO3 can be either Calcite or Aragonite)
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 activity of “foreign cations”
More likely to precipitate
Micrite or ooids
Principle factors that affect inorganic precipitation of CaCO3 in water (Table 6.4 pg. 175)
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
Calcite versus AragoniteEra Period
Dominate Carbonate Mineral
Neogene-Quaternary
A + HMC (Aragonite Sea)
Cen
o-z
oic
Paleogene
Cretaceous
Low-magnesian Calcite (LMC)
(Calcite Sea)
Jurassic Me
sozo
ic
Triassic
Permian
Pennsylvanian
Aragonite (A) + High-magnesian Calcite
(HMC)
(Aragonite Sea)
Mississippian
Devonian
Silurian
Ordovician
Pal
eozo
ic
Cambrian
Low-magnesian Calcite (LMC)
(Calcite Sea)
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)