Crushed Gravel

Description

Crushed gravel consists of angular to sub-angular rock fragments, typically ranging in size from fine sand to coarse pebbles or cobbles (e.g., 2 mm to 64 mm, though specific gradations vary widely). The composition is highly variable, reflecting the geology of the source quarry. It can be monomineralic if derived from a very pure rock type (e.g., crushed limestone), but more commonly it is polymictic, containing fragments of various sedimentary, metamorphic, and sometimes igneous rocks. The fragments exhibit fresh fracture surfaces due to the crushing process. Color, density, and other physical properties are entirely dependent on the parent rock material.

How to Identify

Color

Extremely variable, dependent on source rock. Can be grey, white, black, brown, red, green, or mixed.

Luster

Dull to vitreous, dependent on the mineralogy of the constituent rock fragments. Freshly broken surfaces may appear dull to earthy.

Texture

Angular to sub-angular fragments with rough, freshly fractured surfaces. Grain size is typically well-sorted by screening, but overall texture is clastic and fragmental.

Crystal Form

Not applicable to the aggregate as a whole. Individual mineral grains within the rock fragments may exhibit their characteristic crystal forms, but the fragments themselves are irregular.

Cleavage

Not applicable to the aggregate. Cleavage may be present within individual mineral grains or rock fragments (e.g., mica in schist fragments, calcite in limestone fragments).

Geological Environment

Anthropogenic. Found in quarries, construction sites, roadbeds, and stockpiles. The source rocks originate from various natural geological environments (e.g., marine basins for limestone, mountain belts for metamorphic rocks, volcanic regions for igneous rocks).

Key Facts

• Hardness: Variable, dependent on the hardness of the constituent rock fragments (e.g., quartz-rich fragments are harder than calcite-rich fragments). Mohs hardness can range from 3 (limestone) to 7 (quartzite).
• Specific Gravity: Variable, typically 2.5 to 2.9 g/cm³ for common rock types, but can range from 2.0 to 3.3 g/cm³ depending on the density of the source rock.
• Crystal System: Not applicable to the aggregate. Individual mineral grains within the fragments will have their own crystal systems.
• Color: Highly variable, reflecting the source rock geology.
• Luster: Dull to vitreous, depending on the mineralogy and freshness of fracture surfaces.
• Transparency: Opaque to translucent, depending on the individual rock fragments.
• Fracture: Conchoidal, irregular, or splintery, depending on the constituent rock type and mineralogy.
• Cleavage: Not applicable to the aggregate. Cleavage may be present in individual mineral grains or rock fragments.
• Composition: Mixed lithic fragments (e.g., quartz, feldspar, calcite, dolomite, mica, amphibole, pyroxene, clay minerals) derived from various igneous, sedimentary, and metamorphic rocks.

Quick Check

• Color: Variable (grey, brown, white, black, red, green, mixed)
• Luster: Dull to vitreous (depending on constituent minerals)
• Streak: Not applicable for a mixed rock aggregate; individual minerals would have their own streak.

Physical Characteristics

• Crystal Habit: Not applicable to the aggregate; fragments are irregular.
• Cleavage Type: Not applicable to the aggregate; present in constituent minerals/rocks.
• Fracture Type: Variable (conchoidal, irregular, splintery) depending on the rock type.
• Tenacity: Brittle (fragments break easily under impact).
• Luster Type: Variable (dull, earthy, vitreous, greasy) depending on the constituent minerals.

Formation

Crushed gravel is an anthropogenic material produced by mechanically crushing larger rocks (quarry rock, boulders, cobbles) into smaller, angular fragments. The source rocks can be of any geological age and type, including igneous (e.g., granite, basalt), sedimentary (e.g., limestone, sandstone, dolomite), and metamorphic (e.g., gneiss, schist, quartzite). The crushing process typically involves jaw crushers, cone crushers, and impact crushers, followed by screening to achieve specific size gradations.

Usage

Crushed gravel is a fundamental construction material. Its primary uses include: aggregate in concrete and asphalt mixes; road base and sub-base material; railway ballast; drainage material; erosion control; landscaping; and fill material. The angularity of crushed gravel provides better interlocking and compaction properties compared to naturally rounded gravel, making it superior for many engineering applications.

Age Distribution

Variable, dependent on source rock age; processing is modern.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a mineral; it is an aggregate of rock fragments.

Group

Anthropogenic Aggregate / Clastic Sediment (processed)

Crystal System

Not applicable to the aggregate.

Chemical Formula

Not applicable; it is a physical mixture of various rock types.

Composition

Heterogeneous mixture of silicate minerals (e.g., quartz, feldspar, micas, amphiboles, pyroxenes), carbonate minerals (e.g., calcite, dolomite), and other rock-forming minerals, depending on the source geology.

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