How to identify Quartz Veined Basalt

To correctly identify Quartz Veined Basalt (Basalt with Quartz 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: Dark gray to black. Quartz veins: White, clear, sometimes milky, or stained by iron oxides (yellow, brown, red).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt matrix: Dull to sub-vitreous. Quartz veins: Vitreous (glassy).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Basalt matrix: Aphanitic (fine-grained), sometimes porphyritic, vesicular, or amygdaloidal. Quartz veins: Crystalline, often granular or massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basalt: Microscopic crystals of plagioclase, pyroxene, olivine. Quartz veins: Anhedral to euhedral quartz crystals, often interlocking.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basalt: No distinct cleavage in the rock mass, but constituent minerals (e.g., pyroxene, plagioclase) have cleavage. Quartz: No cleavage (conchoidal fracture).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic terrains, rift zones, oceanic crust, large igneous provinces, areas subjected to post-magmatic hydrothermal activity, often associated with fault zones or shear zones that act as conduits for hydrothermal fluids.

Key Facts

Hardness

Basalt matrix: 5-6 (Mohs). Quartz veins: 7 (Mohs).

Specific Gravity

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

Crystal System

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

Color

Dark gray to black (basalt), white to clear (quartz).

Luster

Dull to sub-vitreous (basalt), vitreous (quartz).

Transparency

Opaque (basalt), transparent to translucent (quartz).

Fracture

Conchoidal to uneven (basalt), conchoidal (quartz).

Cleavage

None in the rock mass; constituent minerals have cleavage. Quartz has no cleavage.

Composition

Basalt: Predominantly plagioclase feldspar, pyroxene, and often olivine, with minor iron-titanium oxides. Quartz veins: SiO2 (silicon dioxide).

Physical characteristics

• Crystal Habit: Basalt: Aphanitic to porphyritic, sometimes vesicular. Quartz: Massive, granular, or prismatic crystals within veins.
• Cleavage Type: Basalt: Not applicable to the rock as a whole; constituent minerals have varying cleavage. Quartz: None.
• Fracture Type: Basalt: Uneven to conchoidal. Quartz: Conchoidal.
• Tenacity: Brittle (both basalt and quartz).
• Luster Type: Basalt: Dull to sub-vitreous. Quartz: Vitreous.

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