sedimentary
Limestone
Sedimentary Calcium Carbonate (CaCO3)
Hardness: 3 (Mohs scale); Color: White, tan, cream, or light gray; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline calcite); Cleavage: None (rhombohedral in calcite crystals but often invisible in massive rock); Specific Gravity: 2.3–2.7
- Hardness
- 3 (Mohs scale)
- Color
- White, tan, cream, or light gray
- Luster
- Dull to earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: White, tan, cream, or light gray; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline calcite); Cleavage: None (rhombohedral in calcite crystals but often invisible in massive rock); Specific Gravity: 2.3–2.7
Formation & geological history
Formed primarily through the accumulation of shell, coral, algal, and fecal debris in marine environments. It can also form chemically through the precipitation of calcium carbonate from lake or ocean water. Significant deposits range from the Paleozoic era to the present.
Uses & applications
Extensively used in construction as an aggregate for road bases, a primary ingredient in Portland cement, a flux in steel making, and as a soil conditioner in agriculture to neutralize acidity.
Geological facts
Limestone is the primary rock type found in karst landscapes, leading to the formation of spectacular cave systems like Mammoth Cave. Over 10% of all sedimentary rocks are limestones.
Field identification & locations
Identify in the field by its softness (can be scratched by a copper penny or steel knife) and its strong effervescence (bubbling) when a drop of diluted hydrochloric acid is applied. Many specimens contain visible small fossils or shell fragments.
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