How to identify Mixed Gravel

To correctly identify Mixed Gravel (Sedimentary rock fragments (various compositions)), 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, reflecting the colors of the constituent rock and mineral fragments. Can be shades of grey, brown, red, black, white, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, depending on the luster of individual clasts. Can range from dull to vitreous, earthy, or metallic.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular, and coarse-grained. Individual clasts are typically visible to the naked eye. The clasts can be angular to well-rounded. The overall texture is unconsolidated or poorly consolidated.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the aggregate as a whole. Individual mineral grains within the gravel may exhibit their characteristic crystal forms, but most rock fragments will be anhedral or subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the aggregate. Individual mineral grains within the gravel may exhibit cleavage (e.g., feldspar, mica).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial (riverbeds, floodplains, alluvial fans), glacial (moraines, outwash plains), coastal (beaches, nearshore marine), lacustrine (lake shores), and colluvial (talus slopes).

Key Facts

Hardness

Variable (depends on constituent minerals/rocks, e.g., quartz is 7 on Mohs scale)

Specific Gravity

Variable (typically 2.5 - 2.8 g/cm³ for common rock types)

Crystal System

Not applicable (aggregate of various rock and mineral fragments)

Color

Highly variable, often multi-colored

Luster

Variable, from dull to vitreous, depending on clast composition

Transparency

Opaque to translucent (for individual clasts)

Fracture

Variable (depends on constituent rocks/minerals)

Cleavage

Not applicable to the aggregate; present in some constituent minerals (e.g., feldspar, mica)

Composition

Heterogeneous mixture of rock fragments (igneous, metamorphic, sedimentary) and mineral grains (e.g., quartz, feldspar, mica, amphibole, pyroxene, calcite).

Physical characteristics

• Crystal Habit: Not applicable (aggregate)
• Cleavage Type: Not applicable to the aggregate; individual mineral grains may exhibit cleavage (e.g., perfect, good, poor)
• Fracture Type: Variable (e.g., conchoidal for quartz, irregular for many rock fragments)
• Tenacity: Brittle (for individual clasts)
• Luster Type: Variable (e.g., vitreous, dull, earthy, metallic)

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