How to identify Argillite

To correctly identify Argillite (Argillite), 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, commonly dark gray, black, greenish-gray, reddish-brown, or purple, depending on the original sediment composition and presence of organic matter or iron oxides.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes slightly waxy or sub-vitreous on fresh surfaces, but generally not as lustrous as slate or phyllite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained (aphanitic), smooth to slightly gritty. Individual mineral grains are typically not visible to the naked eye. May exhibit relict bedding or faint fissility.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microscopic, platy clay minerals (e.g., illite, chlorite) and fine quartz grains. No macroscopic crystal forms are typically observed.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Generally lacks true slaty cleavage. May exhibit a blocky fracture or a poorly developed parting along original bedding planes (fissility), but this is less pronounced and less planar than slaty cleavage. It breaks into irregular, angular fragments.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in sedimentary basins that have undergone burial and subsequent low-grade regional or contact metamorphism. Often associated with sequences of shales, siltstones, and sandstones in tectonically active regions or areas of deep burial.

Key Facts

Hardness

3-4 on Mohs scale (harder than shale, softer than many slates)

Specific Gravity

2.6-2.8 g/cm³

Crystal System

Not applicable (rock composed of microscopic minerals)

Color

Highly variable (dark gray, black, greenish-gray, reddish-brown, purple)

Luster

Dull to earthy, sometimes sub-vitreous or waxy

Transparency

Opaque

Fracture

Subconchoidal to irregular, blocky, or splintery. May show poor fissility.

Cleavage

Absent or very poorly developed parting along relict bedding planes; not true slaty cleavage.

Composition

Predominantly clay minerals (illite, chlorite, kaolinite), fine-grained quartz, feldspar, and micas. May contain organic matter, iron oxides, and other accessory minerals.

Physical characteristics

• Crystal Habit: Microcrystalline to cryptocrystalline aggregates of platy and granular minerals.
• Cleavage Type: None (rock); individual mineral grains may have cleavage, but the rock as a whole does not exhibit a macroscopic cleavage plane like slate.
• Fracture Type: Subconchoidal, irregular, blocky, or splintery.
• Tenacity: Brittle
• Luster Type: Dull, earthy, sub-vitreous, or waxy.

Explore Argillite