How to identify Iron-rich Sedimentary Rock

To correctly identify Iron-rich Sedimentary Rock (Hematitic Sandstone or Siltstone), 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 red, reddish-brown, purplish-red, or maroon due to the presence of hematite. The intensity of the color depends on the concentration of iron oxides.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, reflecting the fine-grained nature of the hematite cement and detrital grains. Rarely sub-metallic if hematite is present as specularite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, ranging from arenaceous (sandy) to silty. Grains are typically sub-angular to rounded. Can be well-sorted or poorly sorted. The cement is often pervasive, filling pore spaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual hematite crystals are typically microscopic and anhedral, forming a pervasive cement. Detrital grains retain their original clastic forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No rock cleavage. Individual mineral grains (e.g., quartz, feldspar) may exhibit their characteristic cleavage, but the rock as a whole does not.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in various depositional environments where iron-rich fluids were present and oxidizing conditions prevailed. This includes fluvial (river), deltaic, shallow marine, and lacustrine (lake) settings. Often associated with continental red beds, which indicate arid to semi-arid climates.

Key Facts

Hardness

Variable, typically 5.5 to 7 on the Mohs scale, depending on the proportion and crystallinity of quartz and hematite cement. Pure hematite is 5-6.

Specific Gravity

Variable, typically 2.6 to 3.5. Higher iron content leads to higher specific gravity. Pure quartz sandstone is ~2.65, pure hematite is ~5.26.

Crystal System

Hematite (the primary iron mineral) is trigonal. Detrital grains (e.g., quartz) are hexagonal.

Color

Red, reddish-brown, purplish-red, maroon.

Luster

Dull to earthy, sometimes sub-metallic if specular hematite is present.

Transparency

Opaque.

Fracture

Conchoidal to irregular in quartz grains; generally irregular to splintery for the rock as a whole.

Cleavage

None for the rock. Hematite has no cleavage. Quartz has no cleavage.

Composition

Primarily quartz (SiO2) and/or other detrital silicates (e.g., feldspar, rock fragments) cemented by hematite (Fe2O3). Minor amounts of other iron oxides/hydroxides (e.g., goethite) may be present.

Physical characteristics

• Crystal Habit: Detrital grains are clastic (rounded to angular). Hematite cement is typically microcrystalline to cryptocrystalline, forming a pervasive matrix.
• Cleavage Type: Absent in the rock. Individual detrital minerals may exhibit cleavage (e.g., feldspar).
• Fracture Type: Irregular to conchoidal (for quartz grains).
• Tenacity: Brittle.
• Luster Type: Earthy to dull.

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