How to identify Banded Sandstone

To correctly identify Banded Sandstone (Sedimentary rock, likely sandstone with quartz veins or banding), 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 accessory minerals. Common colors include shades of tan, brown, red, yellow, gray, white, and sometimes green or black. The banding itself is defined by contrasting colors.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the matrix, but quartz grains can exhibit a vitreous luster. Quartz veins will have a distinct vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, ranging from fine-grained to coarse-grained. Grains are typically sand-sized (0.0625 mm to 2 mm). The texture within bands may vary, with some bands being finer or coarser than others. Quartz veins will have a crystalline texture, often interlocking.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual sand grains are typically anhedral to subhedral, often rounded to sub-rounded. Quartz veins will exhibit euhedral to subhedral quartz crystals, often prismatic, filling fractures or voids.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage in the sandstone matrix, but it may break along bedding planes. Quartz veins will exhibit conchoidal fracture, not cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Deposited in various sedimentary environments including fluvial (river), eolian (desert dunes), deltaic, shallow marine, and deep marine settings. The banding can be primary (depositional) or secondary (diagenetic/hydrothermal). Quartz veins indicate a history of fluid flow and mineralization, often associated with tectonic activity, burial, or igneous intrusions.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains and silica cement; softer if cemented by calcite or clay. Quartz veins are 7.

Specific Gravity

2.65-2.70 g/cm³ (average for quartz-rich sandstone).

Crystal System

Quartz (the primary mineral) is trigonal. The rock itself does not have a crystal system.

Color

Highly variable, characterized by distinct bands of contrasting colors.

Luster

Dull to earthy in the matrix; vitreous in quartz grains and veins.

Transparency

Opaque to translucent in the rock mass; individual quartz grains can be translucent to transparent.

Fracture

Irregular to conchoidal (for quartz grains and veins).

Cleavage

None in the rock mass; quartz has no cleavage.

Composition

Predominantly quartz (SiO2), with varying amounts of feldspar, lithic fragments, and accessory minerals. Cementing agents include silica, calcite, iron oxides, and clay minerals. Quartz veins are nearly pure SiO2.

Physical characteristics

• Crystal Habit: Sand grains are typically anhedral to subhedral, often rounded. Quartz veins exhibit euhedral to subhedral prismatic crystals.
• Cleavage Type: None (quartz has no cleavage).
• Fracture Type: Irregular to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy (matrix), vitreous (quartz grains and veins).

Explore Banded Sandstone