How to identify Weathered Sandstone

To correctly identify Weathered Sandstone (Sandstone (with biological growth)), 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, often muted or stained. Original sandstone colors (tan, brown, red, grey, white) are typically obscured or altered by weathering products (e.g., iron oxides producing reds/yellows) and biological growth (e.g., green from algae/moss, black/grey from lichens/fungi, orange from certain lichens or iron oxides).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy on weathered surfaces; biological growth often has a matte or slightly waxy appearance.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough, friable, granular, or powdery on weathered surfaces. Biological growth can create a crusty, velvety, or fibrous texture. Individual sand grains may be visible but loosely bound. Pitting, flaking, and spalling are common.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual quartz grains are typically anhedral to subhedral. Biological growth does not exhibit crystal form but rather amorphous or colonial structures.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None in the rock mass. Individual mineral grains (e.g., feldspar) may exhibit cleavage, but the rock as a whole fractures irregularly. Biological growth does not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Exposed outcrops, cliffs, rock shelters, arid and humid environments, coastal areas, riverbeds, and any terrestrial environment where sandstone is exposed to atmospheric conditions and can support microbial life. Common in areas with moderate to high humidity and temperature fluctuations.

Key Facts

Hardness

Variable, typically 6-7 for individual quartz grains, but the bulk rock hardness is significantly reduced by weathering, often becoming friable (Mohs hardness <3).

Specific Gravity

2.2 - 2.7 g/cm³ (lower than fresh sandstone due to porosity and biological material).

Crystal System

Not applicable for the rock as a whole; constituent quartz is trigonal.

Color

Highly variable, often discolored by weathering and biological growth.

Luster

Dull to earthy.

Transparency

Opaque in bulk; individual quartz grains can be translucent.

Fracture

Irregular to granular, often friable.

Cleavage

None in the rock mass.

Composition

Predominantly quartz (SiO2), with varying amounts of feldspar, lithic fragments, and accessory minerals. Cementing agents (silica, calcite, iron oxides, clay) are often altered. Organic matter from biological growth is present on the surface.

Physical characteristics

• Crystal Habit: Not applicable for the rock; constituent quartz grains are typically anhedral to subhedral.
• Cleavage Type: None.
• Fracture Type: Irregular to granular, often friable or crumbly.
• Tenacity: Brittle, but often friable or crumbly due to weathering.
• Luster Type: Dull to earthy.

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