GEO 403 Section 6 Part II

What factors control the stability of calcite?

CO₂ pressure, temperature, and pH.

How does higher CO₂ pressure affect calcite?

It causes calcite to dissolve.

What happens to calcite at lower temperatures?

Calcite dissolves.

What is the effect of low pH on calcite?

It leads to dissolution of calcite.

Which seas produce more carbonate, tropical or polar?

Tropical seas produce more carbonate than polar seas.

What chemical reaction occurs when CO₂ dissolves in water?

CO₂ + H₂O → H₂CO₃ (carbonic acid).

What happens to carbonic acid in water?

It can release protons (H⁺) and become bicarbonate (HCO₃⁻).

What is ocean acidification?

The process where increased CO₂ leads to lower pH in ocean waters.

How do speleothems form in caves?

From CO₂-rich groundwater contacting low pCO₂ air, leading to calcite precipitation.

What is the first step in speleothem formation?

CO₂-rich groundwater absorbs CO₂ and forms carbonic acid.

What happens when groundwater enters a cave?

CO₂ escapes, raising pH and favoring calcite precipitation.

Why do coral reefs grow faster on the side exposed to waves?

More CO₂ degassing occurs, enhancing carbonate precipitation and nutrient availability.

What is the Carbonate Compensation Depth (CCD)?

The depth where the rate of calcite supply equals the rate of calcite dissolution.

What is the lysocline?

A transition zone in the ocean where calcite dissolution increases rapidly.

What defines the Carbonate Snowline?

The depth at which calcium carbonate stops being deposited as marine snow.

What are the two main types of carbonate sedimentary rocks?

Limestones (mainly calcite) and dolostones (mainly dolomite).

What are the crystal systems of calcite and aragonite?

Calcite is hexagonal (rhombohedral), while aragonite is orthorhombic.

What happens to aragonite during diagenesis?

It spontaneously converts to calcite.

What is High-Mg calcite?

Calcite that contains magnesium ions, either primary or secondary.

How does temperature affect Mg²⁺ incorporation in calcite?

Higher temperatures lead to more Mg²⁺ incorporation.

What is Ferroan Calcite?

Calcite with Fe²⁺ substituting for Ca²⁺, indicating reducing conditions.

What is the significance of siderite (FeCO₃)?

It is a pure iron carbonate common in Precambrian iron formations.

How are carbonate rocks classified?

Based on grain composition, texture, and depositional environment.

Where is Ferroan Calcite commonly found?

In evaporitic sequences and hydrothermal settings, rarely outside Iron Formations.

What are the two major classification systems for carbonate rocks?

Folk's Classification (1962) and Dunham's Classification (1962).

What are allochemical constituents?

Materials precipitated from solution but later moved as solids into the basin.

What are orthochemical constituents?

Chemical precipitates that show no evidence for significant transportation.

What is micrite?

Microcrystalline calcite representing low-energy environments like lagoons.

What is sparite?

Coarse crystalline cement indicating high-energy environments where grains are cemented.

What are the four major classes of carbonate rocks in Folk's classification?

1. Sparry rocks with allochems, 2. Micritic rocks with allochems, 3. Micritic rocks without allochems, 4. Reef rocks.

What is the primary focus of Dunham's classification?

Relative abundance of allochems and micrite, without differentiating types of carbonate grains.

What does carbonate diagenesis refer to?

The processes that carbonate rocks undergo after formation, including dissolution, cementation, replacement, and compaction.

What is dissolution in carbonate diagenesis?

The process where components, especially aragonite, dissolve, forming secondary porosity.

What is drusy cement?

Sparry calcite crystals lining the pore walls of limestone, with crystal size increasing towards the center of the pore.

What is blocky cement?

Large, square to rectangular sparry calcite crystals that fill pores and intraskeletal chambers.

What is rim cement?

A fringe of equal thickness around grains or pores, consisting of fibrous aragonite or high-Mg calcite.

What is the most common replacement process in carbonate diagenesis?

Calcite replacing aragonite during early diagenesis.

What occurs during compaction in carbonate rocks?

Carbonates undergo burial, which affects their texture and porosity.

What indicates low-energy conditions in carbonate environments?

The presence of micrite (microcrystalline calcite).

What indicates high-energy environments in carbonate settings?

The presence of sparry calcite cement.

What are peloids?

Micritic carbonate pellets found in carbonate rocks.

What are lithoclasts?

Reworked carbonate fragments that are part of allochemical constituents.

What is the significance of stylolites in carbonate rocks?

They result from pressure solution, leaving behind insoluble residues from dissolved carbonate.

What is the role of mineralogy in the classification of carbonate rocks?

Mineralogy plays a small role since carbonates are essentially monominerallic.

What does the grain/micrite ratio indicate in carbonate classification?

It helps determine the type of carbonate rock based on the relative abundance of grains to micrite.

What is the primary constituent used in the classification of carbonate rocks?

The type of carbonate grain or allochem.