How to identify Tafoni

To correctly identify Tafoni (Weathered Sandstone), 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 parent rock (e.g., tan, red, brown, grey for sandstone). Often, the interior may be lighter due to less exposure to staining, while the rim can be darker due to case hardening.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, reflecting the granular nature of the weathered rock. No inherent luster of its own, as it is a geomorphological feature.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The interior surfaces are typically smooth to slightly granular, often friable (easily crumbled) due to ongoing weathering. The exterior rim can be harder and more resistant. The overall texture is cavernous and pitted.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable, as Tafoni is a weathering feature, not a mineral with a defined crystal form. The constituent minerals (e.g., quartz in sandstone) will have their own crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable, as Tafoni is a weathering feature. The parent rock's minerals may exhibit cleavage (e.g., feldspar), but the feature itself does not.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in arid, semi-arid, coastal, and even alpine environments where salt weathering, freeze-thaw, and differential erosion are active. Requires exposed rock faces, often cliffs, boulders, or outcrops, particularly in porous, granular rocks like sandstone, but also in granite and other crystalline rocks. Often found in areas with fluctuating moisture levels.

Key Facts

Hardness

Variable, depends on the parent rock and degree of weathering. Interiors are often friable (Mohs hardness < 2-3), while rims can be harder (Mohs hardness 5-7 for quartz-rich sandstone).

Specific Gravity

Variable, depends on the parent rock. For sandstone, typically 2.0-2.6 g/cm³ (porous) to 2.65 g/cm³ (dense quartz).

Crystal System

Not applicable (geomorphological feature).

Color

Reflects the parent rock, often with iron oxide staining (reds, browns, yellows) or grey/white.

Luster

Dull to earthy.

Transparency

Opaque (as a rock feature).

Fracture

Irregular to conchoidal in individual grains, but the overall feature is a result of granular disintegration.

Cleavage

Not applicable (geomorphological feature).

Composition

Primarily composed of the minerals of the parent rock (e.g., quartz, feldspar, clay minerals in sandstone), along with secondary minerals like iron oxides, hydroxides, and evaporite salts (e.g., halite, gypsum) that contribute to weathering.

Physical characteristics

• Crystal Habit: Not applicable (geomorphological feature).
• Cleavage Type: Not applicable (geomorphological feature).
• Fracture Type: Granular disintegration, irregular.
• Tenacity: Friable to brittle, depending on the degree of weathering and cementation.
• Luster Type: Earthy to dull.

Explore Tafoni