How to identify Granite with Iron Staining

To correctly identify Granite with Iron Staining (Granite (felsic intrusive igneous 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

   Typically light-colored (white, gray, pink, red) due to primary minerals, but overlaid with secondary reddish, orange, yellow, or brown staining from iron oxides/hydroxides. The staining can be patchy, pervasive, or concentrated along fractures.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to dull for quartz and feldspar, pearly for mica, with the iron staining often imparting a dull or earthy luster to the affected areas.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained), meaning individual mineral crystals are visible to the naked eye. The texture is interlocking and granular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Anhedral to subhedral crystals of quartz, feldspar, and mica. Quartz often appears as glassy, irregular grains. Feldspars are typically blocky. Micas are platy.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Feldspars exhibit two distinct cleavages at or near 90 degrees. Micas have perfect basal cleavage, forming thin sheets. Quartz has no cleavage but exhibits conchoidal fracture. The iron staining does not affect the inherent cleavage of the primary minerals.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in the continental crust, typically associated with orogenic belts, subduction zones, and continental rifting. Iron staining occurs in surficial or near-surficial environments where the granite is exposed to weathering, oxygen, and water, allowing for the oxidation of iron-bearing minerals.

Key Facts

Hardness

6-7 on Mohs scale (due to quartz and feldspar). The iron oxide staining itself is softer (e.g., goethite 5-5.5, hematite 5-6).

Specific Gravity

2.6-2.7 g/cm³ (typical for granite). The iron staining has a negligible effect on bulk specific gravity.

Crystal System

Monoclinic (feldspars), Hexagonal (quartz), Monoclinic (biotite), Orthorhombic (muscovite). The iron oxides/hydroxides can be amorphous or crystalline (e.g., hematite is trigonal, goethite is orthorhombic).

Color

Variable, typically light-colored (white, gray, pink, red) with secondary reddish, orange, yellow, or brown staining.

Luster

Vitreous to dull for primary minerals; earthy to dull for iron staining.

Transparency

Opaque to translucent for primary minerals; opaque for iron staining.

Fracture

Conchoidal (quartz), uneven to splintery (feldspar).

Cleavage

Good in feldspars (two directions at ~90°), perfect basal in micas. No cleavage in quartz.

Composition

Primarily quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase feldspar (10-65% of total feldspar), and micas (biotite, muscovite) or amphiboles. Secondary iron oxides/hydroxides (e.g., Fe2O3, FeO(OH)) are present as staining.

Physical characteristics

• Crystal Habit: Granular, anhedral to subhedral interlocking crystals.
• Cleavage Type: Feldspars: two distinct cleavages. Micas: perfect basal cleavage. Quartz: no cleavage.
• Fracture Type: Conchoidal (quartz), uneven to splintery (feldspar).
• Tenacity: Brittle.
• Luster Type: Vitreous to dull (primary minerals), earthy to dull (iron staining).

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