sedimentary
Limestone with Algal/Lichen Colonization
Calcium Carbonate (CaCO3)
Hardness: 3 (Mohs scale), Color: tan to grayish with black surface spotting, Luster: dull to earthy, Structure: massive or bedded, Cleavage: none (conchoidal fracture possible)
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Physical properties
Hardness: 3 (Mohs scale), Color: tan to grayish with black surface spotting, Luster: dull to earthy, Structure: massive or bedded, Cleavage: none (conchoidal fracture possible)
Formation & geological history
Formed in marine environments through the accumulation of skeletal fragments of marine organisms like coral or foraminifera. The black spots on the surface are biological (likely crustose lichens or cyanobacteria colonies) typical of subaerial weathering.
Uses & applications
Used primarily in building construction as dimension stone, road base, and as a primary ingredient in Portland cement. Also used in soil conditioner for agriculture.
Geological facts
Limestone contains roughly 10 percent of the total volume of all sedimentary rocks. It is highly susceptible to chemical weathering by acid rain, which creates karst landscapes like sinkholes and caves.
Field identification & locations
Identify in the field by applying weak cold hydrochloric acid; the rock will effervesce (fizz). Found globally in sedimentary basins. Collectors often look for fossil inclusions within the matrix.
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