Concentration Processes of Limestones and Cherts

Concentrated by Evaporation

• Limestones can be concentrated through evaporation, particularly in arid climate settings.

Crystalline Limestone

• Definition: Crystalline limestone is composed of interlocking crystals of carbonate (calcite).

• Formation: These crystals precipitate from a saturated solution and settle to form masses of layered crystals.

• Commonality:

  • While crystalline limestone is commonly concentrated by evaporation in arid climates, it is also found in various other environments.

• Biochemical Nature:

  • Limestones are primarily considered biochemical.

  • Although they can form through purely chemical processes, approximately 90% of limestones are formed by living organisms.

• Note on Identification:

  • For the purposes of this course, it is crucial to understand that while limestone can form chemically, specimens will not be recognized as such if they are strictly chemical in origin.

Travertine Limestone

• Formation: Travertine limestone forms near mineral springs and cave formations.

Chert

• Definition: Chert may consist of microcrystalline silica (quartz) that precipitates from a saturated solution.

• Dissolution: Groundwater often contains significant amounts of dissolved silica, which can precipitate into distinct nodules of microcrystalline chert.

• Differentiation:

  • Differentiating biochemical chert from crystalline chemical chert can be challenging unless the depositional setting is thoroughly understood.

  • For this course, chert should be regarded as a mixed source rock, inclusive of both biochemical and crystalline chemical origins.

Depositional Environments

Common Chemical Environments and Sediments

• Calote precipitating organisms (which produce limestone) are prevalent in common sedimentary environments.