Crushed Gravel

Description

Crushed gravel consists of angular to sub-angular rock fragments, typically ranging in size from a few millimeters to several centimeters (e.g., 3/8 inch to 2 inches, or 10 mm to 50 mm). Unlike naturally occurring river gravels, which are typically rounded due to fluvial transport, crushed gravel exhibits sharp edges and fractured surfaces. Its composition is highly variable, reflecting the geology of the source quarry. It can be composed of granite, basalt, limestone, sandstone, quartzite, gneiss, schist, or any combination thereof. The color, density, and strength are direct functions of the parent rock type(s).

How to Identify

Color

Highly variable, depending on the source rock. Can be grey, black, white, red, brown, tan, or multicolored.

Luster

Dull to vitreous, depending on the mineralogy of the constituent fragments. Often appears dull due to fractured surfaces.

Texture

Angular to sub-angular, rough, and gritty due to fractured surfaces. Fragments are typically well-sorted by size during processing.

Crystal Form

Not applicable to the aggregate itself, but individual mineral grains within the rock fragments may exhibit their characteristic crystal forms (e.g., quartz, feldspar).

Cleavage

Not applicable to the aggregate as a whole. Individual mineral grains within the rock fragments may exhibit cleavage (e.g., feldspar, mica).

Geological Environment

Crushed gravel is an anthropogenic material, not naturally occurring in its crushed form. Its source rocks originate from various geological environments, including igneous intrusions (granite, gabbro), volcanic flows (basalt), sedimentary basins (limestone, sandstone), and metamorphic terrains (gneiss, quartzite).

Key Facts

• Hardness: Variable, depending on constituent rock types (e.g., quartz 7, calcite 3). Overall bulk hardness is a function of the weakest component and inter-particle friction.
• Specific Gravity: Variable, typically 2.5 to 2.9 g/cm³ (e.g., limestone ~2.7, granite ~2.65, basalt ~2.9).
• Crystal System: Not applicable (aggregate of various rock fragments).
• Color: Highly variable, reflecting source rock.
• Luster: Dull to vitreous, depending on mineralogy.
• Transparency: Opaque to translucent, depending on individual fragments.
• Fracture: Conchoidal, irregular, or splintery, depending on the mineralogy and texture of individual fragments.
• Cleavage: Not applicable to the aggregate; present in some constituent minerals.
• Composition: Mixed silicates (quartz, feldspar, mica, amphibole, pyroxene), carbonates (calcite, dolomite), oxides (iron oxides), etc., depending on the source rock types.

Quick Check

• Color: Variable (grey, black, white, red, brown, tan)
• Luster: Dull to vitreous
• Streak: Not applicable (aggregate, not a single mineral)

Physical Characteristics

• Crystal Habit: Not applicable (aggregate).
• Cleavage Type: Not applicable to the aggregate; present in some constituent minerals (e.g., perfect in mica, good in feldspar).
• Fracture Type: Irregular to conchoidal, depending on the rock type and mineral composition of individual fragments.
• Tenacity: Brittle (individual fragments).
• Luster Type: Variable (e.g., earthy, dull, vitreous).

Formation

Crushed gravel is an engineered material produced by mechanically crushing larger rocks (boulders, cobbles, bedrock) into smaller, angular fragments. The source rocks can be of any geological origin (igneous, sedimentary, metamorphic) and are selected based on local availability, desired physical properties, and economic factors. The crushing process typically involves primary and secondary crushers (e.g., jaw crushers, cone crushers, impact crushers) followed by screening to sort fragments by size.

Usage

Crushed gravel is one of the most widely used construction materials globally. Its primary applications include: aggregate in concrete and asphalt mixes; road base and sub-base material for highways, railways, and airfields; drainage material; erosion control; landscaping; railway ballast; and fill material for various construction projects. The specific application often dictates the required size, angularity, and strength of the crushed gravel.

Age Distribution

Variable, depending on source rock age; typically modern processing of geologically ancient rocks.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not applicable (rock aggregate).

Group

Not applicable (rock aggregate).

Crystal System

Not applicable (rock aggregate).

Chemical Formula

Variable (mixture of various mineral and rock compositions).

Composition

Heterogeneous mixture of rock-forming minerals (e.g., quartz, feldspar, calcite, mica, amphibole, pyroxene) and rock fragments (e.g., granite, basalt, limestone, sandstone, quartzite, gneiss).

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