How to identify Sandstone

To correctly identify Sandstone (Sedimentary rock, clastic), 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, ranging from white, gray, yellow, brown, red, pink, to black, depending on the mineral composition of the grains and cement (e.g., iron oxides impart red/brown hues, organic matter can make it gray/black).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, depending on the cement and porosity. Individual quartz grains may exhibit a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular. Grains are sand-sized (0.0625 mm to 2 mm), visible to the naked eye, and feel gritty when rubbed. Can be well-sorted (uniform grain size) or poorly sorted (wide range of grain sizes).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual grains are typically anhedral to subhedral, often rounded or sub-rounded due to transport. The rock itself does not exhibit a macroscopic crystal form.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No rock cleavage. Individual mineral grains (e.g., feldspar) may exhibit cleavage, but the rock as a whole fractures irregularly across grain boundaries or through cement.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in a wide variety of sedimentary environments where sand accumulates, including fluvial (river channels, floodplains), deltaic, beach, shallow marine, deep marine (turbidites), and eolian (desert dunes) settings.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains, but the overall rock hardness depends heavily on the cement. Calcite-cemented sandstone is softer (3-4), while silica-cemented sandstone is harder (7).

Specific Gravity

2.2 - 2.8 g/cm³ (variable, depending on mineral composition and porosity)

Crystal System

Not applicable to the rock as a whole; individual mineral grains (e.g., quartz) are trigonal.

Color

Highly variable (white, gray, yellow, brown, red, pink, black)

Luster

Dull to earthy (individual grains vitreous)

Transparency

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

Fracture

Irregular to conchoidal (if silica-cemented and fine-grained), often breaking across grains and cement.

Cleavage

None (as a rock); individual mineral grains may exhibit cleavage.

Composition

Predominantly quartz (SiO₂), with varying amounts of feldspar, mica, rock fragments, and accessory minerals. Cementing materials include silica (SiO₂), calcite (CaCO₃), iron oxides, and clay minerals.

Physical characteristics

• Crystal Habit: Not applicable to the rock; individual grains are typically anhedral to subhedral, often rounded.
• Cleavage Type: None (as a rock).
• Fracture Type: Irregular to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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