How to identify Quartz in Basalt

To correctly identify Quartz in Basalt (SiO2 in Basalt), 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 is typically dark gray to black. Quartz inclusions can be colorless, white, milky, gray, or various colors if impurities are present (e.g., purple amethyst, yellow citrine, red jasper, banded agate).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt is dull to sub-vitreous. Quartz is vitreous (glassy) to waxy (chalcedony).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Basalt is typically fine-grained to aphanitic, sometimes porphyritic or vesicular. Quartz inclusions will have a crystalline or cryptocrystalline texture within the vesicles or fractures.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz can form euhedral to anhedral crystals, drusy coatings, or massive to botryoidal (chalcedony/agate) aggregates within the basaltic matrix. Basalt itself has a granular or interlocking texture of its constituent minerals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basalt minerals (pyroxene, feldspar) have cleavage, but the rock as a whole does not exhibit prominent cleavage. Quartz has no cleavage, exhibiting conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in extrusive igneous environments, specifically in basaltic lava flows, sills, and dikes that have undergone secondary alteration by silica-rich hydrothermal fluids. Common in amygdaloidal basalts.

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 triclinic, monoclinic, or orthorhombic); Quartz (Trigonal)

Color

Dark gray to black (basalt); Colorless, white, milky, or various colors due to impurities (quartz)

Luster

Dull to sub-vitreous (basalt); Vitreous to waxy (quartz)

Transparency

Opaque (basalt); Transparent to translucent (quartz)

Fracture

Conchoidal to uneven (basalt); Conchoidal (quartz)

Cleavage

Absent in quartz; Basalt minerals have cleavage but not prominent in the rock mass.

Composition

Basalt (Plagioclase feldspar, pyroxene, olivine, magnetite); Quartz (SiO2)

Physical characteristics

• Crystal Habit: Basalt: Aphanitic to porphyritic, massive. Quartz: Prismatic, massive, granular, cryptocrystalline (chalcedony, agate), drusy.
• Cleavage Type: Quartz: None. Basalt: Constituent minerals have cleavage (e.g., pyroxene: two at ~90°, plagioclase: two at ~90°).
• Fracture Type: Conchoidal (quartz); Uneven to sub-conchoidal (basalt).
• Tenacity: Brittle (both basalt and quartz).
• Luster Type: Vitreous to waxy (quartz); Dull to sub-vitreous (basalt).

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