sedimentary
Limestone (Weathered)
Calcium Carbonate (CaCO3)
Hardness: 3 on Mohs scale; Color: typically grey, tan, or white, often covered with dark organic lichen or dirt as seen here; Luster: dull to earthy; Crystal structure: orthorhombic or hexagonal; Cleavage: perfect rhombohedral in calcite crystals, but usually massive in rock form.
- Hardness
- 3 on Mohs scale
- Luster
- dull to earthy
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Physical properties
Hardness: 3 on Mohs scale; Color: typically grey, tan, or white, often covered with dark organic lichen or dirt as seen here; Luster: dull to earthy; Crystal structure: orthorhombic or hexagonal; Cleavage: perfect rhombohedral in calcite crystals, but usually massive in rock form.
Formation & geological history
Formed primarily in marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also be formed by the precipitation of calcium carbonate from lake or ocean water. Most limestone formed during the Paleozoic and Mesozoic eras.
Uses & applications
Extensively used in construction as aggregate, cement production, soil conditioner in agriculture, and as a decorative building stone. It is also a primary source of lime used in steel manufacturing and water treatment.
Geological facts
Limestone accounts for about 10% of the total volume of all sedimentary rocks. It is soluble in weak acids, which leads to the formation of karst topography, caves, and sinkholes over geological time.
Field identification & locations
Identify in the field by its softness (can be scratched by a steel knife) and its reaction to dilute hydrochloric acid, which causes it to effervesce (fizz). This specimen shows significant surface weathering and biological growth (crustose lichen), common for rocks found in humid outdoor environments.
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