How to identify Limestone with Caliche

To correctly identify Limestone with Caliche (Limestone (Calcium Carbonate) with Caliche (Calcium Carbonate)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Limestone is typically white, gray, tan, or light brown. Caliche is often white to off-white, sometimes yellowish or reddish due to impurities. The combination can show patches or veins of lighter material within the darker host rock.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy for both components, though crystalline calcite within limestone can exhibit a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Limestone can be fine-grained, fossiliferous, oolitic, or crystalline. Caliche often presents as powdery, chalky, nodular, or dense and concretionary. The overall texture will be a mix, with the caliche often filling pores or coating surfaces, giving a mottled or irregular appearance.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Limestone is composed of calcite crystals, often microcrystalline. Caliche is typically microcrystalline to cryptocrystalline calcite, forming irregular masses, coatings, or nodules rather than distinct euhedral crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Calcite (the primary mineral in both) exhibits perfect rhombohedral cleavage in three directions, though this is often not visible in fine-grained or cryptocrystalline forms. The rock itself does not typically show macroscopic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Limestone forms in marine environments. Caliche forms in arid to semi-arid terrestrial environments, typically in soil profiles (pedogenic) or as vadose zone cements, often overlying or within existing carbonate bedrock.

Key Facts

Hardness

3 on the Mohs scale (for calcite, the primary mineral). The overall hardness of the rock can vary depending on the degree of cementation and porosity, from soft and friable to dense and hard (calcrete).

Specific Gravity

2.71 g/cm³ (for calcite). The bulk specific gravity of the rock will vary with porosity and impurities, typically ranging from 2.0 to 2.7 g/cm³.

Crystal System

Trigonal (for calcite).

Color

Variable, typically white, gray, tan, or light brown for the limestone, with white to off-white, yellowish, or reddish secondary carbonate (caliche).

Luster

Dull to earthy, sometimes vitreous for larger calcite crystals.

Transparency

Opaque to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect rhombohedral (for calcite), but often not macroscopically visible in the rock.

Composition

Primarily Calcium Carbonate (CaCO3) in both the limestone and caliche components. Minor impurities can include clay minerals, quartz, iron oxides, and organic matter.

Physical characteristics

• Crystal Habit: Limestone: Microcrystalline to macrocrystalline calcite, often with fossil fragments. Caliche: Microcrystalline to cryptocrystalline calcite, forming irregular masses, nodules, coatings, or cements.
• Cleavage Type: Perfect rhombohedral (3 directions at 74° and 106°) for calcite.
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy, sometimes vitreous.

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