sedimentary

Sulfidic Mudstone with Pyrite/Goethite

Sulfidic Siltstone/Mudstone (FeS2)

AI-generated identification · may be incorrect

Hardness: 3-5 (variable), Color: Weathered brownish-yellow to dark grey, Luster: Earthy to sub-metallic where crystals are exposed, Structure: Finely-grained clastic, Specific Gravity: 2.6-4.0 depending on metal content.

Identified June 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in low-oxygen marine or lacustrine environments where organic matter and iron-rich sediments reacted with sulfate-reducing bacteria. The orange/yellow staining is a result of the oxidation of iron-sulfide minerals into goethite or limonite.

Uses & applications

Secondary source of iron or sulfur in some contexts; primarily used as construction fill or studied for environmental geochemical impact (acid mine drainage potential).

Geological facts

Due to the presence of iron sulfides like pyrite, these rocks can produce sulfuric acid when exposed to air and water, leading to the yellow 'sulfur' looking crusts which are actually iron oxides/hydroxides.

Field identification & locations

Identify by the distinct orange-yellow weathered 'rind' and the dark grey, dense interior. Common in coal-bearing regions and ancient seabed deposits. Check for a faint sulfur smell if struck.