How to identify Basalt with Quartz Veins

To correctly identify Basalt with Quartz Veins (Basalt (mafic volcanic 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 gray to black; quartz veins are usually white, clear, or milky, sometimes stained by iron oxides (red/brown).

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) or porphyritic; quartz veins are crystalline, ranging from massive to granular to well-formed crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basalt minerals are typically microscopic; quartz in veins can be anhedral (massive) to euhedral (well-formed hexagonal prisms with pyramidal terminations).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basalt minerals (pyroxene, plagioclase) have cleavage, but it's rarely visible in hand specimen due to fine grain size. Quartz has no cleavage, exhibiting conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic terrains, rift zones, oceanic crust, large igneous provinces, often in areas that have experienced post-emplacement hydrothermal activity or tectonic fracturing.

Key Facts

Hardness

Basalt (variable, typically 5-6 on Mohs scale for 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 monoclinic, triclinic, or orthorhombic); Quartz (Trigonal)

Color

Basalt (dark gray, black, greenish-black); Quartz (colorless, white, milky, sometimes tinted by impurities)

Luster

Basalt (dull, earthy, sub-vitreous); Quartz (vitreous)

Transparency

Basalt (opaque); Quartz (transparent to translucent)

Fracture

Basalt (uneven, hackly); Quartz (conchoidal)

Cleavage

Basalt (poor to absent in hand specimen); Quartz (none)

Composition

Basalt (primarily plagioclase feldspar, pyroxene, olivine, magnetite); Quartz (SiO2)

Physical characteristics

• Crystal Habit: Basalt (aphanitic to porphyritic, massive); Quartz (massive, granular, prismatic crystals in veins)
• Cleavage Type: Basalt (not typically observed in hand specimen); Quartz (none)
• Fracture Type: Basalt (uneven, hackly); Quartz (conchoidal)
• Tenacity: Basalt (brittle); Quartz (brittle)
• Luster Type: Basalt (dull, earthy, sub-vitreous); Quartz (vitreous)

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