How to identify Limonite with Quartz

To correctly identify Limonite with Quartz (FeO(OH)·nH2O (Limonite) with SiO2 (Quartz)), 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

   Variable, ranging from yellow, orange, brown, to reddish-brown, depending on the concentration and hydration state of the limonite. The underlying quartz may be visible as colorless or white.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy to dull for the limonite component; vitreous to greasy for the quartz component. The overall appearance can be dull if limonite predominates, or vitreous if quartz is dominant and merely stained.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, botryoidal, stalactitic, or earthy for limonite, often coating or intergrown with granular or crystalline quartz. The texture will be a combination of the two minerals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Limonite is typically amorphous or cryptocrystalline, forming botryoidal, mammillary, or stalactitic masses, or as earthy coatings. Quartz forms hexagonal prisms with pyramidal terminations, or as massive, anhedral grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Limonite has no true cleavage, exhibiting irregular fracture. Quartz has no cleavage, exhibiting conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in the oxidized zones of iron ore deposits, gossans, weathered igneous and metamorphic rocks, and as cementing material in sedimentary rocks (e.g., ferruginous sandstones). Also found in bog iron deposits and as a product of hydrothermal alteration.

Key Facts

Hardness

Limonite: 4-5.5 (Mohs); Quartz: 7 (Mohs). The aggregate will have variable hardness depending on the dominant mineral.

Specific Gravity

Limonite: 2.7-4.3; Quartz: 2.65. The aggregate will have a specific gravity between these values.

Crystal System

Limonite: Amorphous or Orthorhombic (for goethite/lepidocrocite components); Quartz: Trigonal.

Color

Yellow, orange, brown, reddish-brown, often with colorless or white areas.

Luster

Earthy to dull (limonite); vitreous to greasy (quartz).

Transparency

Opaque (limonite); Transparent to translucent (quartz).

Fracture

Irregular to conchoidal (limonite); Conchoidal (quartz).

Cleavage

None (limonite); None (quartz).

Composition

Hydrated iron(III) oxide-hydroxide (Limonite) and Silicon Dioxide (Quartz).

Physical characteristics

• Crystal Habit: Limonite: Massive, earthy, botryoidal, stalactitic, reniform, oolitic. Quartz: Prismatic, massive, granular.
• Cleavage Type: None for both minerals.
• Fracture Type: Limonite: Irregular to conchoidal. Quartz: Conchoidal.
• Tenacity: Limonite: Brittle to earthy. Quartz: Brittle.
• Luster Type: Limonite: Earthy, dull. Quartz: Vitreous, greasy.

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