sedimentary
Limestone (Ocherous)
Calcium Carbonate (CaCO3) with Iron Oxide (Limonite)
Hardness: 3 (Mohs scale); Color: Yellow to brownish-orange (due to iron content); Luster: Dull, earthy; Structure: Fine-grained, microcrystalline; Cleavage: None (rhombohedral cleavage only in large crystals); Specific Gravity: 2.3-2.7
- Hardness
- 3 (Mohs scale)
- Color
- Yellow to brownish-orange (due to iron content)
- Luster
- Dull, earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: Yellow to brownish-orange (due to iron content); Luster: Dull, earthy; Structure: Fine-grained, microcrystalline; Cleavage: None (rhombohedral cleavage only in large crystals); Specific Gravity: 2.3-2.7
Formation & geological history
Formed primarily from the accumulation of shell, coral, algal, and fecal debris in marine environments. The yellow color indicates the presence of secondary limonite (hydrated iron oxide) deposited through groundwater infiltration or oxidation.
Uses & applications
Used in construction for cement and road base, in agriculture as a soil conditioner (lime), and historically as a pigment (yellow ocher) for paints and cosmetics.
Geological facts
Iron-stained limestone like this specimen is often found in karst landscapes. It is chemical evidence of the presence of ancient water tables and the weathering of iron-rich minerals nearby.
Field identification & locations
Identified in the field by its reaction to dilute hydrochloric acid (effervescence) and its yellow-orange streak. Commonly found in regions with extensive ancient sedimentary beds such as the American Midwest or the Alps.
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