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Caliche is a terrestrial sedimentary deposit, specifically a pedogenic carbonate, characterized by the accumulation and cementation of calcium carbonate within soil profiles. It is not a true mineral but rather a rock-like material composed predominantly of calcite (CaCO3) along with varying amounts of detrital minerals (quartz, feldspar, clay minerals) and organic matter. Its appearance is highly variable, ranging from soft, powdery accumulations to dense, hard, rock-like layers. It is a diagnostic feature of arid and semi-arid environments and provides valuable paleoclimatic information.
How to Identify
- Color
- Typically white, off-white, cream, light gray, or pale brown, depending on impurities.
- Luster
- Dull to earthy, sometimes chalky.
- Texture
- Highly variable; can be powdery, nodular, platy, laminar, or massive and hard. Often contains embedded clasts of other rocks or soil particles.
- Crystal Form
- Microcrystalline to cryptocrystalline calcite, often forming coatings, cements, or nodules. Individual crystals are rarely visible to the naked eye.
- Cleavage
- Not applicable as a rock; individual calcite grains within caliche exhibit rhombohedral cleavage.
- Geological Environment
- Forms in the vadose zone of soils in arid to semi-arid regions, often associated with grasslands, deserts, and Mediterranean climates. Found in ancient soil profiles (paleosols).
Key Facts
- Hardness: Variable, from 1 (powdery) to 3-4 (hard, cemented) on the Mohs scale, depending on the degree of cementation.
- Specific Gravity: 2.0 - 2.7, depending on porosity and impurities.
- Crystal System: Not applicable as a rock; constituent calcite is trigonal.
- Color: White, cream, light gray, pale brown.
- Luster: Dull, earthy, chalky.
- Transparency: Opaque.
- Fracture: Conchoidal to irregular in hard varieties; crumbly in soft varieties.
- Cleavage: Not applicable as a rock; constituent calcite has perfect rhombohedral cleavage.
- Composition: Primarily calcium carbonate (CaCO3) in the form of calcite, with varying amounts of quartz, feldspar, clay minerals, and other detrital components.
Quick Check
- Color: White to light brown/gray
- Luster: Dull to earthy
- Streak: White
Physical Characteristics
- Crystal Habit: Microcrystalline to cryptocrystalline aggregates, coatings, nodules, or massive layers.
- Cleavage Type: Not applicable for the rock; calcite exhibits perfect rhombohedral cleavage.
- Fracture Type: Irregular to conchoidal (for hard varieties), earthy (for soft varieties).
- Tenacity: Brittle when hard; friable when soft.
- Luster Type: Dull to earthy.
Formation
Caliche is a secondary accumulation of calcium carbonate (CaCO3) in the soil or subsoil, forming a hardened layer. It forms in arid to semi-arid climates where evaporation exceeds precipitation. Dissolved calcium carbonate, derived from the weathering of silicate minerals or from atmospheric CO2, is transported downwards by percolating rainwater. As the water evaporates in the vadose zone (unsaturated zone above the water table), CaCO3 precipitates, cementing soil particles, gravel, and other detritus. This process is known as pedogenesis. The morphology can range from powdery coatings on grains to massive, laminar, or nodular layers. The formation is often cyclical, with dissolution during wet periods and precipitation during dry periods, leading to complex internal structures.
Usage
Caliche is primarily used as a local construction material, particularly as a base course for roads, as aggregate, and for building foundations. In some regions, it has been used as a low-grade cement or as a source of lime for agricultural soil amendment. Historically, it was used for building materials in areas where other rock types were scarce. It can also act as an aquifer in some settings.
Age Distribution
Quaternary to Tertiary, but can form in any geological period with suitable climatic conditions.
Where to Find
Southwestern United States
Extensive deposits in states like Texas, New Mexico, Arizona, Nevada, and California, particularly in desert and semi-arid regions.
Australia
Widespread in the arid and semi-arid interior, often referred to as 'calcrete'.
North Africa and Middle East
Common in desert environments, forming significant surface and subsurface layers.
Mediterranean Basin
Present in countries like Spain, Italy, Greece, and Turkey, reflecting past and present arid/semi-arid conditions.
India
Known as 'kankar', it is widely distributed in the drier parts of the subcontinent.
Finding Tips
Look for Arid/Semi-Arid Climates
Caliche is a hallmark of dry regions. Focus your search in deserts, steppes, and areas with distinct wet and dry seasons.
Exposed Soil Profiles
Road cuts, stream banks, and construction sites often expose soil horizons where caliche layers are visible.
White to Light-Colored Layers
Look for distinct white, cream, or light gray layers within the soil profile, often harder than the surrounding soil.
Effervescence with Acid
A drop of dilute hydrochloric acid (HCl) will cause caliche to effervesce (fizz) due to its calcium carbonate content. This is a key diagnostic test.
Associated with Desert Pavement
In some areas, caliche can be found beneath desert pavement, where it helps cement the surface clasts.
Similar Rocks
Travertine
Travertine
Also known as: Tufa (when porous)
Limestone
Limestone
Also known as: Calcium Carbonate Rock
Dolomite
Dolomite
Also known as: Dolomitic Limestone
Scientific Classification
- Mineral Class
- Not a mineral, but a rock-like pedogenic deposit.
- Group
- Pedogenic Carbonate
- Crystal System
- Not applicable for the rock; constituent calcite is trigonal.
- Chemical Formula
- Primarily CaCO3 (calcite) with impurities.
- Composition
- Calcium carbonate (calcite) with detrital minerals (quartz, feldspar, clay) and organic matter.
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