How to identify Alluvial Gravel and Sand

To correctly identify Alluvial Gravel and Sand (Unconsolidated Sediments (Quartz, Feldspar, Lithic Fragments)), 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 source rocks and mineralogy. Commonly light grey, tan, brown, or yellowish, but can be reddish (iron oxides) or dark (mafic lithic fragments).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous, depending on the dominant mineral grains. Quartz grains are typically vitreous, while feldspar can be dull to pearly.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, unconsolidated. Ranges from coarse-grained (gravel) to medium-grained (sand). Grains are typically rounded to sub-rounded, but can be angular, especially in proximal deposits. Exhibits stratification (layering) and cross-bedding in many deposits.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains (quartz, feldspar) typically anhedral to subhedral, reflecting transport and abrasion. Lithic fragments are irregular.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the aggregate sediment. Individual mineral grains may exhibit cleavage (e.g., feldspar has two good cleavages at ~90 degrees; quartz has no cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments: river channels, floodplains, point bars, braided streams, alluvial fans, deltas. Also found in coastal environments where rivers meet the sea.

Key Facts

Hardness

Variable, depending on constituent minerals. Quartz (7 on Mohs scale), Feldspar (6-6.5).

Specific Gravity

Variable, typically 2.6-2.7 for quartz and feldspar, but can be higher if heavy minerals are present (e.g., magnetite ~5.2, gold ~19.3).

Crystal System

Not applicable to the aggregate. Individual minerals have their own crystal systems (e.g., Quartz: Trigonal; Feldspar: Monoclinic or Triclinic).

Color

Highly variable, reflecting source rocks and mineralogy.

Luster

Dull to vitreous.

Transparency

Individual grains can be transparent to opaque.

Fracture

Not applicable to the aggregate. Individual grains exhibit conchoidal (quartz) or uneven (feldspar, lithic fragments) fracture.

Cleavage

Not applicable to the aggregate. Individual minerals may exhibit cleavage (e.g., feldspar).

Composition

Primarily quartz (SiO2), feldspar (e.g., KAlSi3O8, NaAlSi3O8, CaAl2Si2O8), and various lithic fragments (e.g., granite, basalt, shale, schist fragments). May also contain mica, heavy minerals (magnetite, garnet, zircon, ilmenite, rutile), and organic matter.

Physical characteristics

• Crystal Habit: Not applicable to the aggregate. Individual grains are typically anhedral to subhedral, rounded by transport.
• Cleavage Type: Not applicable to the aggregate. Individual minerals may show cleavage (e.g., feldspar: perfect basal and prismatic).
• Fracture Type: Not applicable to the aggregate. Individual grains show conchoidal (quartz) or uneven fracture.
• Tenacity: Friable (unconsolidated).
• Luster Type: Dull to vitreous.

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