sedimentary
Sulfidic Mudstone with Pyrite/Goethite
Sulfidic Siltstone/Mudstone (FeS2)
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.
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Physical properties
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.
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.
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