How to identify River Rock (Argillite/Slate with Quartz Veins)

To correctly identify River Rock (Argillite/Slate with Quartz Veins) (Sedimentary or Metamorphic Rock (likely a type of argillite or slate with quartz veins)), 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, typically shades of grey, black, green, or reddish-brown for the argillite/slate matrix. Quartz veins are usually white, translucent, or milky.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to sub-vitreous for the argillite/slate matrix; vitreous (glassy) for the quartz veins.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and rounded due to fluvial abrasion. The argillite/slate matrix is very fine-grained, often appearing homogeneous. Quartz veins will feel harder and may protrude slightly due to differential weathering.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the overall river rock clast. Individual mineral grains within the argillite/slate are microscopic. Quartz in veins will be anhedral (lacking well-formed crystal faces) due to infilling fractures.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Argillite may show incipient cleavage or none. Slate exhibits strong slaty cleavage (the ability to split into thin, parallel sheets). Quartz veins show no cleavage but conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments (riverbeds, floodplains, terraces), alluvial fans, glacial outwash plains, and coastal areas where rivers discharge.

Key Facts

Hardness

Matrix: 3-5 (Mohs); Quartz veins: 7 (Mohs)

Specific Gravity

2.6-2.8 g/cm³ (for argillite/slate); 2.65 g/cm³ (for quartz)

Crystal System

Not applicable for the rock as a whole. Quartz is trigonal.

Color

Variable, typically dark grey, black, green, or reddish-brown with white quartz veins.

Luster

Dull to sub-vitreous for the matrix; vitreous for quartz veins.

Transparency

Opaque for the matrix; translucent to transparent for quartz veins.

Fracture

Conchoidal to sub-conchoidal for quartz; splintery to irregular for argillite/slate.

Cleavage

Argillite: poor to absent; Slate: excellent slaty cleavage; Quartz: none.

Composition

Predominantly clay minerals (illite, chlorite, kaolinite) and fine-grained quartz, feldspar, and micas in the argillite/slate matrix. Quartz veins are composed almost entirely of SiO₂.

Physical characteristics

• Crystal Habit: Not applicable for the rock clast. Microscopic platy minerals (micas, clay minerals) in the matrix; anhedral quartz in veins.
• Cleavage Type: Slaty cleavage (if slate); poor to absent (if argillite); no cleavage (quartz).
• Fracture Type: Splintery to irregular (argillite/slate); conchoidal (quartz).
• Tenacity: Brittle.
• Luster Type: Dull to sub-vitreous (matrix); vitreous (quartz veins).

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