How to identify Quartz in weathered rock

To correctly identify Quartz in weathered rock (SiO2 (Quartz) in a matrix of weathered rock), 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, milky, gray, or smoky. Can be pink (rose quartz), purple (amethyst), yellow (citrine), or brown. Host rock: Highly variable, often discolored by weathering (e.g., reddish-brown, yellowish, grayish-white) due to oxidation of iron-bearing minerals or formation of clay minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz: Vitreous (glassy) to greasy. Host rock: Dull, earthy, or sometimes waxy if clay-rich.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Quartz: Crystalline, often with well-formed hexagonal prisms or massive. Host rock: Friable, granular, earthy, or crumbly due to weathering. Original textures (e.g., igneous, sedimentary) may be obscured.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: Hexagonal prisms terminated by rhombohedra, or massive/anhedral. Host rock: Original mineral grains may be visible but often altered and indistinct.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: None (conchoidal fracture). Host rock: Variable, depending on original rock type and degree of weathering. May exhibit relict cleavage planes of original minerals, but often obscured by alteration.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in hydrothermal veins cutting through various rock types (igneous, metamorphic, sedimentary). Also found in pegmatites, granites, and as a detrital component in sedimentary rocks. The 'weathered' aspect implies exposure to surface or near-surface conditions, often in areas of active erosion or chemical alteration.

Key Facts

Hardness

Quartz: 7 on Mohs scale. Host rock: Variable, often significantly softer than quartz (e.g., 1-5) due to weathering.

Specific Gravity

Quartz: 2.65 g/cm³. Host rock: Variable, typically 2.0-2.8 g/cm³ depending on original rock type and degree of weathering.

Crystal System

Quartz: Trigonal. Host rock: Depends on the original rock type and its constituent minerals.

Color

Quartz: Colorless, white, milky, gray, smoky, or various other colors due to impurities. Host rock: Highly variable, often discolored by weathering (e.g., reddish-brown, yellowish, grayish-white).

Luster

Quartz: Vitreous to greasy. Host rock: Dull, earthy, or sometimes waxy.

Transparency

Quartz: Transparent to opaque. Host rock: Opaque.

Fracture

Quartz: Conchoidal. Host rock: Variable, often irregular or earthy.

Cleavage

Quartz: None. Host rock: Variable, depending on original minerals, but often obscured by weathering.

Composition

Quartz: Silicon dioxide (SiO2). Host rock: Silicates, oxides, carbonates, etc., in various proportions, often altered to clay minerals, iron oxides, and other secondary minerals.

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular, cryptocrystalline. Host rock: Granular, earthy, friable, or relict textures of original rock.
• Cleavage Type: Quartz: Absent. Host rock: Variable, often poor or absent due to weathering.
• Fracture Type: Quartz: Conchoidal. Host rock: Irregular, earthy.
• Tenacity: Quartz: Brittle. Host rock: Friable, brittle, or earthy.
• Luster Type: Quartz: Vitreous to greasy. Host rock: Dull, earthy.

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