How to identify Iron-stained Quartz on Shale

To correctly identify Iron-stained Quartz on Shale (SiO2 (Quartz) with Fe-oxides on Shale), 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

   Quartz: typically colorless, white, or translucent, but stained yellow, orange, red, or brown by iron oxides. Shale: typically dark gray, black, green, or red.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz: vitreous (glassy). Iron oxides: earthy to submetallic. Shale: dull to earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Quartz: crystalline, granular, or massive. Iron oxides: powdery, earthy, or botryoidal. Shale: fine-grained, fissile (splits into thin layers).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: often prismatic crystals with hexagonal cross-sections, or massive/vein-filling. Iron oxides: amorphous coatings, earthy masses, or microcrystalline. Shale: platy, layered.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: none (conchoidal fracture). Iron oxides: none. Shale: excellent basal cleavage (fissility).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in sedimentary basins, particularly in areas where shales have undergone diagenesis, weathering, or hydrothermal alteration. Often associated with fault zones, fractures, or areas of groundwater flow through iron-rich strata.

Key Facts

Hardness

Quartz: 7 on Mohs scale. Shale: 2.5-4 (variable, depending on composition and compaction). Iron oxides: 1-6 (variable, e.g., goethite 5-5.5, hematite 5-6).

Specific Gravity

Quartz: 2.65 g/cm³. Shale: 2.0-2.8 g/cm³. Iron oxides: 3.3-5.3 g/cm³ (variable).

Crystal System

Quartz: Trigonal. Iron oxides: Orthorhombic (goethite), Trigonal (hematite). Shale: Not applicable (rock, not a single mineral).

Color

As described above, a combination of shale's natural color, quartz's natural color, and the staining from iron oxides.

Luster

As described above, a combination of vitreous, earthy, and dull.

Transparency

Quartz: transparent to translucent. Iron oxides: opaque. Shale: opaque.

Fracture

Quartz: conchoidal. Iron oxides: uneven to earthy. Shale: splintery or platy.

Cleavage

Quartz: none. Iron oxides: none. Shale: excellent basal cleavage (fissility).

Composition

Primarily SiO2 (quartz), clay minerals (illite, kaolinite, smectite) and quartz silt (shale), and various iron oxides/hydroxides (e.g., FeO(OH) - goethite, Fe2O3 - hematite).

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular, drusy. Iron oxides: Earthy, botryoidal, reniform, stalactitic, massive. Shale: Fissile, platy.
• Cleavage Type: Quartz: None. Iron oxides: None. Shale: Perfect basal cleavage (fissility).
• Fracture Type: Quartz: Conchoidal. Iron oxides: Uneven to earthy. Shale: Splintery, platy.
• Tenacity: Quartz: Brittle. Iron oxides: Brittle to earthy. Shale: Brittle.
• Luster Type: Quartz: Vitreous. Iron oxides: Earthy to submetallic. Shale: Dull to earthy.

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