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)2Ankerite 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+ + 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 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 non-carbonate minerals that accumulate as carbonate minerals dissolve.