How to identify Calcite-cemented Sandstone

To correctly identify Calcite-cemented Sandstone (Calcite (CaCO3) in Sandstone), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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).

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