Calcite-cemented Sandstone

Description

Calcite in sandstone refers to a sedimentary rock where the primary detrital grains (typically quartz, feldspar, and rock fragments) are bound together by a matrix of calcite (calcium carbonate). The calcite acts as a cement, filling the interstitial pore spaces between the sand grains. The proportion of calcite can vary significantly, from a minor component merely binding grains to a dominant matrix that can exceed the volume of the detrital grains. The calcite cement can be microcrystalline, sparry (coarsely crystalline), or poikilitic (large crystals enclosing multiple detrital grains).

How to Identify

Color

Variable, depending on the sand grains and impurities in the calcite. Often light grey, tan, buff, or reddish-brown. Pure calcite cement is typically white or colorless.

Luster

Dull to earthy for the sandstone matrix; vitreous to pearly for visible calcite crystals.

Texture

Clastic, granular, with sand-sized grains (0.0625 mm to 2 mm) cemented together. The calcite cement itself can be microcrystalline to coarsely crystalline, sometimes appearing as shiny cleavage faces.

Crystal Form

Calcite cement typically forms anhedral (irregular) masses filling pore spaces, but can also occur as euhedral to subhedral rhombohedra or scalenohedra in larger voids or vugs. The sand grains retain their original detrital forms.

Cleavage

Calcite exhibits perfect rhombohedral cleavage in three directions, visible in larger calcite crystals within the cement. The overall rock may not show distinct cleavage due to the detrital grains.

Geological Environment

Common in shallow marine, fluvial, deltaic, and eolian (desert) sedimentary environments where sand deposition occurs, followed by diagenetic processes involving calcium-rich groundwater. Often associated with carbonate platforms or areas with abundant biogenic carbonate sources.

Key Facts

• Hardness: Overall rock hardness varies, but calcite cement is 3 on the Mohs scale. Quartz grains are 7.
• Specific Gravity: 2.65 - 2.72 (for calcite component); overall rock density will vary based on porosity and grain composition.
• Crystal System: Trigonal (for calcite).
• Color: Variable, often light colors, but can be influenced by impurities and detrital grains.
• Luster: Vitreous to pearly for calcite, dull to earthy for the sandstone.
• Transparency: Translucent to opaque (for the rock); calcite crystals can be transparent to translucent.
• Fracture: Conchoidal to uneven (for calcite); overall rock fracture is granular.
• Cleavage: Perfect rhombohedral (3 directions at 74° and 106°) for calcite.
• Composition: Primarily quartz (SiO2) and other detrital minerals, cemented by calcium carbonate (CaCO3).

Quick Check

• Color: Variable (light grey, tan, buff, reddish-brown), often with white or colorless cement.
• Luster: Dull to earthy for the rock, vitreous to pearly for visible calcite.
• Streak: White (for calcite component).

Physical Characteristics

• Crystal Habit: Calcite cement typically forms anhedral to subhedral masses filling pore spaces, sometimes poikilitic or sparry. Detrital grains retain their original shapes.
• Cleavage Type: Perfect rhombohedral (for calcite).
• Fracture Type: Conchoidal to uneven (for calcite); granular for the overall rock.
• Tenacity: Brittle (for calcite).
• Luster Type: Vitreous to pearly (for calcite); dull to earthy (for the sandstone).

Formation

Calcite in sandstone forms when calcium carbonate (CaCO3) precipitates from circulating groundwater within the pore spaces of a pre-existing sandstone. This precipitation acts as a cement, binding the detrital sand grains together. The source of calcium and carbonate ions can be diverse, including dissolution of pre-existing carbonate rocks (limestone, shells), volcanic activity, or biogenic processes. The process is often diagenetic, occurring after initial deposition and burial, and can be influenced by temperature, pressure, pH, and the presence of organic matter.

Usage

Calcite-cemented sandstones are used as building stones, dimension stones, and aggregates, similar to other sandstones. The presence of calcite cement can affect the rock's durability and workability. In some cases, the calcite cement can be dissolved by acidic solutions, which can be a factor in weathering or in industrial processes like acidizing oil wells to improve permeability. It can also serve as a reservoir rock for hydrocarbons if porosity and permeability are sufficient.

Age Distribution

Found throughout the geological record, from Precambrian to Cenozoic, wherever conditions for sandstone deposition and subsequent calcite precipitation occurred.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Carbonates (for calcite)

Group

Calcite Group (for calcite)

Crystal System

Trigonal (for calcite)

Chemical Formula

CaCO3 (for calcite cement)

Composition

Calcium carbonate (calcite) as cement, with detrital silicate minerals (e.g., quartz, feldspar) as grains.

Explore Calcite-cemented Sandstone