How to identify Weathered Rock

To correctly identify Weathered Rock (Indeterminate Weathered 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

   Highly variable, often discolored compared to the fresh parent rock. Common colors include reddish-browns, yellows, and oranges due to iron oxidation; grays, whites, or pale greens if leaching or hydration is dominant. Can also retain original rock colors but with a duller appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or powdery due to the presence of clay minerals, iron oxides, and fine-grained alteration products. Original mineral lusters (e.g., vitreous, pearly) are typically diminished or absent.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable. Can range from friable (easily crumbled) to soft and clay-rich, or highly fractured and granular. Original textures (e.g., crystalline, clastic) may be partially preserved but are often obscured by alteration. Pores and voids may be enlarged or filled with secondary minerals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms of primary minerals are often degraded, rounded, or replaced by secondary minerals (e.g., euhedral feldspars may be altered to anhedral clay masses). New, often microscopic, crystal forms of secondary minerals may be present.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original mineral cleavages may be preserved in relict grains but are often obscured or destroyed by chemical alteration and fracturing. The rock mass itself typically lacks coherent cleavage planes, instead exhibiting irregular fracture patterns or crumbling.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Occurs ubiquitously at the Earth's surface and in the shallow subsurface, wherever rocks are exposed to atmospheric, hydrologic, and biological agents. Common in all climatic zones, but rates and types of weathering vary significantly with climate (e.g., intense chemical weathering in humid tropics, strong physical weathering in arid or glacial environments).

Key Facts

Hardness

Highly variable, generally much softer than the parent rock. Can range from 1 (talc-like) to 7 (quartz-like) for relict minerals, but the bulk rock mass is typically soft and friable.

Specific Gravity

Variable, generally lower than the parent rock due to increased porosity and replacement by lighter secondary minerals (e.g., clays). Typically ranges from 1.8 to 2.8 g/cm³.

Crystal System

Indeterminate for the bulk rock. Individual relict minerals retain their original crystal systems. Secondary minerals (e.g., clays) may be monoclinic or triclinic, often cryptocrystalline.

Color

Highly variable, often discolored (reds, yellows, browns, grays, whites).

Luster

Dull, earthy, powdery.

Transparency

Opaque to translucent, depending on the degree of alteration and mineral composition.

Fracture

Irregular, earthy, or conchoidal in relict minerals. The bulk rock often crumbles or breaks along irregular planes of weakness.

Cleavage

Absent in the bulk rock mass. Relict minerals may retain original cleavage, but it is often obscured or degraded.

Composition

Highly variable. Consists of relict primary minerals (e.g., quartz, feldspar, mica) and newly formed secondary minerals (e.g., clay minerals like kaolinite, illite, smectite; iron oxides/hydroxides like goethite, hematite; aluminum hydroxides like gibbsite). Soluble components may have been leached away.

Physical characteristics

• Crystal Habit: Indeterminate for the bulk rock. Relict primary minerals may show original habits. Secondary minerals often form cryptocrystalline aggregates, earthy masses, or microscopic flakes.
• Cleavage Type: Absent in the bulk rock. Relict minerals may exhibit their characteristic cleavage (e.g., perfect, good, poor).
• Fracture Type: Irregular, earthy, or crumbly. Relict minerals may show conchoidal, uneven, or splintery fracture.
• Tenacity: Brittle, friable, or earthy. Much less tenacious than fresh rock.
• Luster Type: Dull, earthy, powdery.

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