How to identify Sandstone

To correctly identify Sandstone (Sedimentary rock, predominantly quartz arenite), 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

   Highly variable, depending on the cementing agent and minor constituents. Common colors include white, gray, tan, yellow, brown, red (due to iron oxides), and sometimes green or purple.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, reflecting the granular nature and often porous surface. Individual quartz grains may exhibit a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular, and gritty to the touch. Individual sand grains are visible to the naked eye or with a hand lens. Grains are typically sub-angular to well-rounded. Quartz arenite specifically tends to have well-sorted, well-rounded grains.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock as a whole, as it is composed of detrital grains. Individual quartz grains are anhedral to subhedral, often rounded.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No rock cleavage. Individual quartz grains exhibit poor to indistinct cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in a wide range of sedimentary environments where sand accumulates, including fluvial (riverbeds, floodplains), eolian (deserts, dunes), deltaic, beach, shallow marine, and deep marine settings (turbidites). Quartz arenites are characteristic of stable tectonic settings with extensive weathering and long transport distances, such as cratonic interiors or stable continental margins.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for the quartz grains, but the overall rock hardness depends on the cementation. Poorly cemented sandstone can be friable (easily crumbled), while well-cemented quartz arenite can be very hard.

Specific Gravity

2.65-2.70 for quartz grains; bulk rock density typically 2.0-2.6 g/cm³ depending on porosity and cementation.

Crystal System

Not applicable for the rock. Individual quartz grains are trigonal.

Color

White, gray, tan, yellow, brown, red, green, purple.

Luster

Dull to earthy; individual quartz grains are vitreous.

Transparency

Opaque to translucent (for the rock); individual quartz grains can be transparent to translucent.

Fracture

Conchoidal to irregular for individual quartz grains; the rock typically exhibits granular fracture, breaking around the grains.

Cleavage

No rock cleavage. Individual quartz grains exhibit poor to indistinct cleavage.

Composition

Predominantly quartz (SiO₂), typically >90-95% for quartz arenite. Minor amounts of feldspar, rock fragments, and heavy minerals may be present. Cementing materials include silica (quartz overgrowths), calcite (CaCO₃), iron oxides (e.g., hematite, goethite), or clay minerals.

Physical characteristics

• Crystal Habit: Not applicable for the rock. Individual quartz grains are anhedral to subhedral, often rounded.
• Cleavage Type: Not applicable for the rock. Individual quartz grains have poor/indistinct cleavage.
• Fracture Type: Granular (for the rock); conchoidal to irregular (for individual quartz grains).
• Tenacity: Brittle (for the rock, especially well-cemented varieties).
• Luster Type: Dull to earthy (rock); vitreous (individual quartz grains).

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