Definition

Noun: 1. A fine-grained sedimentary rock: Argillite is a type of sedimentary rock composed primarily of clay-sized particles (clay minerals). It is more indurated (hardened) than shale due to being bound by silica (a cementing agent), but it lacks the distinct foliation (slaty cleavage) characteristic of slate, which is formed by metamorphism.

Usage Examples

• Noun:
  • The cliffs along the coast are composed of a dark gray argillite.
  • Geologists identified the layer as argillite because it was harder than shale but showed no signs of metamorphic foliation.
  • This argillite sample is bound by silica, giving it its characteristic durability.

Advanced Usage

• In geological classification: Argillite occupies an intermediate position between shale (a softer, fissile sedimentary rock) and slate (a low-grade metamorphic rock with cleavage). It is often described as a "mudrock" that has undergone slight compaction and cementation without significant metamorphic alteration.
  • The sequence shows a transition from shale to argillite, indicating increased lithification over time.

Variants and Related Words

• Argillaceous (adj): Of, relating to, or containing clay; clayey.
  • The argillaceous sediments eventually hardened into rock.

Synonyms

• Mudstone (Note: Mudstone is a closely related fine-grained sedimentary rock; the terms are sometimes used interchangeably, though "argillite" often implies a harder, more silica-cemented rock.)
• Claystone (Note: Another closely related rock type composed mainly of clay minerals.)

Different Meanings

• This word has a single, specific meaning within the field of geology and petrology (the study of rocks). It does not have common idiomatic or figurative uses.

Noun

1. a sedimentary rock differing from shale in being bound by silica and from slate in having no slate cleavages