How to identify Calcite on Sandstone

To correctly identify Calcite on Sandstone (Calcite (CaCO3) on 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

   Sandstone matrix typically ranges from white, gray, yellow, brown, to red, depending on mineralogy and iron oxide content. Calcite can be colorless, white, yellow, brown, or gray. The contrast between the calcite and the sandstone can be distinct.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Sandstone grains typically have a dull to vitreous luster. Calcite crystals will exhibit a vitreous to sub-vitreous luster, sometimes pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Sandstone has a clastic, granular texture, often gritty to the touch. Calcite can appear as fine-grained cement filling pore spaces, or as distinct, often euhedral to subhedral crystals growing on or within the sandstone, giving a crystalline or sugary texture to the calcite-rich areas.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Calcite can form a wide variety of crystal habits, including rhombohedra, scalenohedra, prismatic, tabular, or massive aggregates. On sandstone, it often forms drusy coatings of small, intergrown crystals or larger, well-formed crystals in vugs or open spaces.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Calcite exhibits perfect rhombohedral cleavage in three directions, producing characteristic rhomb-shaped fragments. Sandstone grains typically do not show macroscopic cleavage, though individual mineral grains (e.g., feldspar) may have cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in sedimentary basins where sandstones are present and have been subjected to diagenetic processes involving calcium-rich fluids. This includes shallow marine, fluvial, deltaic, and eolian environments. Also found in areas with hydrothermal activity or where groundwater has circulated through carbonate-rich strata.

Key Facts

Hardness

Sandstone (quartz grains): 7 on Mohs scale; Calcite: 3 on Mohs scale. The overall hardness of the rock will depend on the proportion and distribution of calcite cement.

Specific Gravity

Sandstone: 2.6-2.7 g/cm³ (depending on mineralogy); Calcite: 2.71 g/cm³.

Crystal System

Calcite: Trigonal; Sandstone: Constituent minerals vary (e.g., quartz is trigonal).

Color

Variable, as described above.

Luster

Variable, as described above.

Transparency

Calcite: Transparent to translucent; Sandstone: Opaque to translucent.

Fracture

Calcite: Conchoidal to uneven; Sandstone: Uneven to granular.

Cleavage

Calcite: Perfect rhombohedral in three directions at 74°55'; Sandstone: None (for the rock mass), but constituent minerals may have cleavage.

Composition

Sandstone: Predominantly SiO2 (quartz) with varying amounts of feldspar, lithic fragments, and other minerals; Calcite: CaCO3 (calcium carbonate).

Physical characteristics

• Crystal Habit: Calcite: Rhombohedral, scalenohedral, prismatic, tabular, massive, granular, fibrous, stalactitic, botryoidal. On sandstone, often drusy or as discrete crystals in voids. Sandstone: Clastic, granular.
• Cleavage Type: Calcite: Perfect rhombohedral {1011}; Sandstone: Not applicable for the rock mass.
• Fracture Type: Calcite: Conchoidal to uneven; Sandstone: Uneven to granular.
• Tenacity: Calcite: Brittle; Sandstone: Brittle.
• Luster Type: Calcite: Vitreous to sub-vitreous, sometimes pearly; Sandstone: Dull to vitreous.

Explore Calcite on Sandstone