How to identify Weathered Metamorphic Rock

To correctly identify Weathered Metamorphic Rock (Metamorphic rock with iron oxide staining), 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

   Dominantly red, orange, brown, or yellow due to iron oxide staining. Original rock colors may be visible beneath the staining or in less weathered areas.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or sometimes sub-vitreous on fresh fracture surfaces. Iron oxide coatings can be dull to earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, depending on the protolith. Can be foliated (schistose, gneissic), massive (quartzite, marble), or granular. The texture may be softened or friable due to weathering. Iron oxide staining often highlights existing textures or forms coatings.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original mineral crystal forms may be obscured or pseudomorphed by iron oxides. Individual crystals are typically not well-formed due to weathering processes.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   May retain the cleavage of original minerals (e.g., mica in schist) or the rock's foliation. However, weathering can weaken or obscure these features.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in outcrop exposures, road cuts, quarries, and drill cores in regions where metamorphic rocks are exposed to surface weathering processes. Common in humid and tropical climates where chemical weathering is intense, but also present in temperate and arid regions.

Key Facts

Hardness

Variable, typically softer than the fresh protolith. Can range from 1-7 on Mohs scale, depending on the degree of weathering and the nature of the iron oxides and remaining primary minerals.

Specific Gravity

Variable, generally lower than the fresh protolith due to porosity and hydration of iron oxides. Can range from 2.0 to 3.5.

Crystal System

Not applicable to the rock as a whole; individual minerals within the rock retain their crystal systems, but iron oxides are often amorphous or cryptocrystalline.

Color

Red, orange, brown, yellow.

Luster

Dull, earthy.

Transparency

Opaque.

Fracture

Irregular, earthy, or conchoidal if quartz-rich.

Cleavage

May exhibit relict cleavage from original minerals or foliation, but often obscured by weathering.

Composition

Composed of original metamorphic minerals (e.g., quartz, feldspar, mica, amphibole, garnet) partially or wholly altered, with significant amounts of secondary iron oxyhydroxides (goethite, limonite) and possibly hematite, clay minerals, and other weathering products.

Physical characteristics

• Crystal Habit: Not applicable to the rock; iron oxides typically form coatings, stains, or earthy masses.
• Cleavage Type: Variable, depending on the protolith and degree of weathering. May show relict foliation or mineral cleavage.
• Fracture Type: Irregular, earthy, or splintery.
• Tenacity: Brittle to friable.
• Luster Type: Dull, earthy.

Explore Weathered Metamorphic Rock