How to identify Basalt with Quartz Veins

To correctly identify Basalt with Quartz Veins (Basalt (igneous rock) with Quartz (SiO2) veins), 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

   Basalt matrix is typically dark (black, dark gray, greenish-black). Quartz veins are typically white, milky, or translucent.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt matrix is dull to sub-vitreous. Quartz veins are vitreous (glassy).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Basalt is fine-grained (aphanitic), sometimes porphyritic or vesicular. Quartz veins are crystalline, often granular or massive, sometimes showing euhedral crystal faces if space allowed.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basalt minerals are typically anhedral to subhedral, often too small to discern without magnification. Quartz in veins can be anhedral to euhedral (e.g., hexagonal prisms with pyramidal terminations) if grown in open spaces.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basalt minerals (pyroxene, plagioclase) have distinct cleavage, but it's rarely visible in the fine-grained matrix. Quartz has no true cleavage, exhibiting conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in areas of volcanic activity, mid-ocean ridges, oceanic islands, and continental flood basalt provinces. Quartz veins indicate subsequent hydrothermal activity, often associated with faulting, fracturing, or nearby intrusive bodies.

Key Facts

Hardness

Basalt: 5-6 on Mohs scale (average of constituent minerals). Quartz: 7 on Mohs scale.

Specific Gravity

Basalt: 2.8-3.0 g/cm³. Quartz: 2.65 g/cm³.

Crystal System

Basalt: Polycrystalline, constituent minerals are typically monoclinic (pyroxene, plagioclase). Quartz: Trigonal.

Color

Basalt: Black, dark gray, greenish-black. Quartz: Colorless, white, milky, sometimes smoky or amethystine.

Luster

Basalt: Dull to sub-vitreous. Quartz: Vitreous.

Transparency

Basalt: Opaque. Quartz: Transparent to translucent.

Fracture

Basalt: Irregular to conchoidal. Quartz: Conchoidal.

Cleavage

Basalt: Poor to distinct in constituent minerals, but not typically observed in hand sample. Quartz: None.

Composition

Basalt: Primarily plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor magnetite, ilmenite. Quartz: Silicon dioxide (SiO2).

Physical characteristics

• Crystal Habit: Basalt: Aphanitic (fine-grained), sometimes porphyritic or vesicular. Quartz: Massive, granular, or euhedral crystals (prismatic with pyramidal terminations) in veins.
• Cleavage Type: Basalt: Not readily observable in hand sample due to fine grain size. Quartz: None.
• Fracture Type: Basalt: Irregular to sub-conchoidal. Quartz: Conchoidal.
• Tenacity: Basalt: Brittle. Quartz: Brittle.
• Luster Type: Basalt: Dull to sub-vitreous. Quartz: Vitreous.

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