Argillite Identification Guide

What Argillite Looks Like

Argillite is a fine-grained sedimentary (weakly metamorphosed) rock made mostly of consolidated clay and silt particles. It is intermediate in induration between soft shale and true slate: harder and more compact than shale, but lacking the well-developed slaty cleavage and sheen of slate. It is typically dull and earthy with a smooth, very fine texture; individual grains are too small to see. Colors are commonly gray, greenish-gray, dark red-brown, or black, and it breaks into blocky, irregular pieces rather than thin sheets.

• Color: gray, greenish, reddish, brown, black
• Luster: dull, earthy (not glossy)
• Texture: very fine-grained, smooth, massive
• Breakage: blocky/conchoidal-to-irregular, NOT splitting into thin layers

Step-by-Step Field ID Checklist

1. Confirm fine grain: the rock should feel smooth with no visible grains (clay/silt size).
2. Test for fissility: try to split it; argillite does NOT split into thin sheets (unlike shale) and lacks the flat cleavage planes of slate.
3. Check hardness/induration: it is hard and compact, hard to scratch with a fingernail, harder than soft shale.
4. Look at luster: dull and earthy, without the silky sheen of slate or phyllite.
5. Tap test: argillite gives a duller sound than the clear ring of slate.

Key Diagnostic Tests

• Mohs hardness: roughly 3 to 4 overall (clay minerals are soft, but the rock is well-cemented); a knife scratches it but it resists crumbling
• Streak/scratch: does not produce a clay smear as readily as soft mudstone
• Cleavage/fissility: none to poor; this absence is the key field trait
• Fracture: subconchoidal to blocky
• Specific gravity: about 2.6 to 2.8
• Acid: generally no fizz unless calcareous; a slight reaction means a calcareous variety
• Water test: unlike weak claystone/mudstone, argillite does not slake (fall apart) readily in water

Common Look-Alikes and How to Tell Them Apart

• Shale: same clay-rich composition but shale is fissile, splitting into thin layers along bedding; argillite is massive and blocky.
• Slate: slate has a well-developed slaty cleavage (splits into flat plates), a silky sheen, and rings when tapped; argillite is duller, harder to split, and more weakly metamorphosed.
• Mudstone/claystone: softer, slakes in water, can be scratched or smeared more easily; argillite is more indurated.
• Siltstone: slightly coarser and grittier to the touch; argillite is smoother (more clay).
• Hornfels: a contact-metamorphic rock that is very hard and tough; can resemble argillite but is harder (often scratches glass) and forms near intrusions.

Where Argillite Is Typically Found

Argillite forms in basins where thick clay/silt sequences were buried and lightly compacted or weakly metamorphosed. Notable examples include the Belt Supergroup argillites of Montana and Idaho (red and green varieties), the Haida Gwaii (Queen Charlotte Islands) black argillite carved by Haida artisans, and many fold-belt sequences worldwide. Look in ancient marine and lacustrine sedimentary basins.