Shale (Ferruginous)

Physical Properties

Hardness: 3-4 (Mohs scale); Color: Dark brown to black (iron-stained); Luster: Dull/Earthy; Crystal Structure: Clastic fine-grained; Cleavage: Fissile (splits into thin layers); Specific Gravity: 2.4-2.8

Formation & Geological History

Formed through the lithification of mud and clay particles deposited in slow-moving water environments like lake beds or deep ocean floors. The brownish hue indicates presence of iron oxides (hematite or goethite).

Uses & Applications

Used in the production of bricks, cement, and ceramics. Some oil-rich shales are processed for natural gas or petroleum (fracking).

Geological Facts

Shale is the most common sedimentary rock on Earth, making up an estimated 70% of the sedimentary rocks in the crust. Many shales preserve fossils incredibly well due to their fine-grained nature.

Field Identification & Locations

Identify by its ability to split into thin sheets (fissility) and its soft, fine-textured surface. Commonly found in river basins and near ancient seafloors. It usually produces a 'clink' sound when tapped with a hammer.

Identified on: 5/20/2026

Mode: Standard