How to identify Tafoni

To correctly identify Tafoni (Tafoni (weathering feature in sandstone or other granular rock)), 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

   Variable, depends on the host rock and presence of moisture, algae, or mineral staining. Interiors can be lighter or darker than the exterior.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Not applicable, as it is a weathering feature, not a mineral. The host rock's luster would apply.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The interior surfaces are typically smooth to granular, often softer and more friable than the surrounding rock due to ongoing weathering. The exterior rim can be harder and more resistant.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable, as it is a weathering feature.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable, as it is a weathering feature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in coastal areas (due to salt spray), arid and semi-arid regions (due to salt crystallization and rapid evaporation), and mountainous regions (due to freeze-thaw). They require exposed rock surfaces, often cliffs, boulders, or large outcrops, where moisture and weathering agents can act differentially.

Key Facts

Hardness

Variable, depends on the host rock. The interior is often softer (Mohs 1-4) than the exterior (Mohs 5-7) due to ongoing disintegration.

Specific Gravity

Variable, depends on the host rock (e.g., sandstone ~2.2-2.7, granite ~2.6-2.8).

Crystal System

Not applicable (weathering feature).

Color

Variable, depends on the host rock and environmental factors.

Luster

Not applicable (weathering feature).

Transparency

Opaque (host rock).

Fracture

Conchoidal to irregular (host rock).

Cleavage

Absent to poor (host rock).

Composition

Composed of the same minerals as the host rock (e.g., quartz, feldspar, mica in granite; quartz in sandstone) but with altered cementing agents or disaggregated grains in the hollows.

Physical characteristics

• Crystal Habit: Not applicable (weathering feature).
• Cleavage Type: Not applicable (weathering feature).
• Fracture Type: Not applicable (weathering feature).
• Tenacity: Brittle (host rock).
• Luster Type: Not applicable (weathering feature).

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