Caliche

Sedimentary Rock (Pedogenic Carbonate)

Caliche

Also known as: Hardpan, Calcrete, Kankar, Duricrust

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Description

Caliche is a sedimentary deposit of calcium carbonate that forms in the soil profile, typically in arid and semi-arid regions. It is characterized by its variable appearance, ranging from soft, powdery accumulations to hard, cemented layers or nodules. The color can vary from white to reddish-brown, depending on the presence of iron oxides and other impurities. It is a common component of paleosols and can significantly impact soil hydrology and engineering properties.

How to Identify

Color: Typically white, off-white, cream, light gray, or reddish-brown due to iron oxide impurities.
Luster: Dull to earthy, sometimes chalky.
Texture: Highly variable, from powdery and friable to dense, hard, and cemented. Can be nodular, platy, or massive. Often contains embedded soil particles, rock fragments, and root casts.
Crystal Form: Microcrystalline to cryptocrystalline calcite, often forming irregular masses, nodules, or coatings. Individual crystals are rarely macroscopic.
Cleavage: Not applicable in a macroscopic sense due to its fine-grained, amorphous to microcrystalline nature. Individual calcite grains within caliche exhibit perfect rhombohedral cleavage.
Geological Environment: Arid and semi-arid regions with distinct wet and dry seasons, where evaporation exceeds precipitation for significant periods. Found in soil profiles (pedogenic), alluvial fans, playas, and ancient paleosols.

Key Facts

Hardness: Variable, typically 3-4 on Mohs scale (for the calcite component), but the overall rock can be softer if powdery or harder if well-cemented.
Specific Gravity: 2.5-2.7 (for the calcite component), but the bulk density of caliche can be lower due to porosity and impurities.
Crystal System: Trigonal (for the constituent calcite mineral)
Color: White, off-white, cream, light gray, reddish-brown
Luster: Dull, earthy, chalky
Transparency: Opaque
Fracture: Conchoidal to irregular, depending on cementation and purity.
Cleavage: Not observed macroscopically; constituent calcite has perfect rhombohedral cleavage.
Composition: Primarily calcium carbonate (CaCO3) in the form of calcite, with varying amounts of clay, silt, sand, and other impurities.

Quick Check

Color: White to reddish-brown
Luster: Dull to earthy
Streak: White

Physical Characteristics

Crystal Habit: Massive, nodular, platy, or as coatings; microcrystalline to cryptocrystalline aggregates.
Cleavage Type: Not applicable for the rock mass; constituent calcite has perfect rhombohedral cleavage in three directions.
Fracture Type: Irregular to sub-conchoidal.
Tenacity: Brittle to friable, depending on degree of cementation.
Luster Type: Earthy to dull.

Formation

Caliche forms through the precipitation of calcium carbonate (CaCO3) in the soil profile, typically in arid and semi-arid climates. This process, known as calcification, involves the dissolution of calcium-bearing minerals in the upper soil horizons by percolating rainwater, followed by the downward movement of the dissolved calcium bicarbonate (Ca(HCO3)2). As the water evaporates in the drier, lower horizons, or as CO2 degasses, calcium carbonate precipitates, accumulating to form distinct layers or nodules. Biological activity, such as root respiration, can also contribute to CO2 levels, influencing precipitation.

Usage

Caliche has various uses, including as a natural aggregate for road construction, a base material for foundations, and a source of lime for agricultural soil amendment. In some regions, it has been used as a low-grade building material. Historically, it was used in some areas for adobe construction. It can also be a source of calcium carbonate for industrial processes.

Age Distribution

Quaternary to Recent, but can be found in older paleosols.

Where to Find

Southwestern United States

Extensive deposits in states like Arizona, New Mexico, Texas, and California, particularly in desert and steppe environments.

Australia

Widespread in arid and semi-arid interior regions, often referred to as calcrete.

North Africa and Middle East

Common in desert and semi-desert landscapes.

Mediterranean Basin

Present in areas with suitable climatic conditions.

Parts of South America

Found in arid regions, such as the Atacama Desert.

Finding Tips

Look for Arid/Semi-Arid Climates

Caliche is characteristic of regions with low rainfall and high evaporation rates. Focus on desert, steppe, and savanna environments.

Examine Soil Profiles

Caliche typically forms within the B or C horizons of soils. Look for exposed soil cuts, road cuts, or excavations.

Test with Acid

Since caliche is primarily calcium carbonate, it will effervesce (fizz) vigorously when a drop of dilute hydrochloric acid (HCl) is applied. This is a key diagnostic test.

Observe Landscape Features

In some areas, caliche can form resistant caps that influence topography, creating mesas or plateaus.

Similar Rocks

Travertine

Also known as: Tufa

Limestone

Also known as: Calcium Carbonate Rock

Dolomite

Also known as: Dolomitic Limestone

Scientific Classification

Mineral Class: Carbonate
Group: Calcite Group (as the primary mineral component)
Crystal System: Trigonal (for calcite)
Chemical Formula: CaCO3 (primary component)
Composition: Calcium carbonate (calcite) with variable amounts of detrital minerals (quartz, feldspar, clays) and organic matter.

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