How to identify Iron-rich Metamorphic Rock

To correctly identify Iron-rich Metamorphic Rock (Metamorphic rock with iron oxides/hydroxides), 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

   Typically dark grey, black, reddish-brown, or silvery. The color is highly dependent on the dominant iron oxide (red for hematite, black for magnetite) and the presence of other minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Can range from dull to submetallic or metallic, especially if hematite or magnetite are prominent and well-crystallized.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often banded (relict from BIF protolith) or foliated, but can also be massive or granoblastic. Grain size varies from fine to coarse, depending on metamorphic grade. May show porphyroblastic textures with larger metamorphic minerals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral crystals (e.g., magnetite, hematite, quartz, amphiboles, garnets) may be anhedral to euhedral within the rock matrix. Magnetite often forms octahedral crystals, hematite can be platy or massive.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Cleavage is typically observed in individual mineral components rather than the rock as a whole. Amphiboles (e.g., grunerite) will show characteristic amphibole cleavage (two directions at ~56° and ~124°). Garnets are typically anhedral and lack cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in ancient cratonic shields and orogenic belts where Precambrian Banded Iron Formations (BIFs) or other iron-rich sedimentary rocks have undergone regional or contact metamorphism. Associated with other metamorphic rocks like schists, gneisses, and quartzites.

Key Facts

Hardness

Varies significantly depending on mineralogy. Quartz (7), Hematite (5-6), Magnetite (5.5-6.5), Amphiboles (5-6), Garnets (6.5-7.5). Overall rock hardness is moderate to hard.

Specific Gravity

High, typically 3.0 to 4.5, reflecting the high iron content. Can be higher if very rich in iron oxides.

Crystal System

Not applicable to the rock as a whole, but constituent minerals have their own crystal systems (e.g., trigonal for hematite, isometric for magnetite, monoclinic/orthorhombic for amphiboles).

Color

Dark grey, black, reddish-brown, silvery.

Luster

Dull to submetallic or metallic.

Transparency

Opaque.

Fracture

Conchoidal to uneven, depending on mineralogy and texture.

Cleavage

Absent in the rock as a whole, but present in constituent minerals like amphiboles.

Composition

Primarily iron oxides (hematite, Fe2O3; magnetite, Fe3O4) and quartz (SiO2), with varying amounts of iron silicates (e.g., grunerite, cummingtonite, almandine garnet), carbonates, and sulfides depending on protolith and metamorphic grade.

Physical characteristics

• Crystal Habit: Granular, massive, banded, or foliated. Individual mineral grains can be anhedral to euhedral.
• Cleavage Type: Not a rock property. Individual minerals may exhibit cleavage (e.g., amphiboles).
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Dull, submetallic, or metallic.

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