sedimentary
Limestone (specifically Bituminous or Carbonaceous Limestone)
Calcium Carbonate (CaCO3) with Organic Impurities
Hardness: 3 (Mohs scale); Color: Dark gray to black; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture common); Specific gravity: 2.3-2.7
- Hardness
- 3 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull to earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: Dark gray to black; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture common); Specific gravity: 2.3-2.7
Formation & geological history
Formed through the accumulation of organic remains, calcium carbonate from marine organisms, and chemical precipitation in shallow marine environments. Often dates from the Paleozoic to Mesozoic eras.
Uses & applications
Used in the manufacturing of cement, as a soil conditioner in agriculture, as crushed stone for road construction, and for carbon sequestration research.
Geological facts
Dark-colored limestones often contain trapped organic matter or bitumen. When struck with a hammer, some varieties release a characteristic 'oily' or sulfurous odor due to these organic impurities.
Field identification & locations
Identify in the field by its effervescence (fizzing) when exposed to dilute hydrochloric acid. Common in karst topography and sedimentary basins worldwide.
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