How to identify Veined Basalt

To correctly identify Veined Basalt (Basalt with Quartz/Calcite 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

   The basalt matrix is typically dark gray to black. Quartz veins are white to translucent, sometimes smoky. Calcite veins are white, cream, or light gray, sometimes with a pinkish or yellowish tint.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt matrix is dull to sub-vitreous. Quartz veins exhibit a vitreous (glassy) luster. Calcite veins show a vitreous to pearly luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Basalt matrix is fine-grained (aphanitic) to porphyritic. Veins are typically crystalline, massive, or banded. The contact between the basalt and the vein material is usually sharp.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basalt minerals are typically anhedral to subhedral. Quartz in veins often forms euhedral to subhedral crystals (e.g., drusy quartz) or massive aggregates. Calcite in veins can form rhombohedral crystals, scalenohedra, or massive aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basalt minerals (e.g., pyroxene, plagioclase) have distinct cleavage planes but are often too small to observe macroscopically. Quartz has no cleavage but exhibits conchoidal fracture. Calcite has perfect rhombohedral cleavage, which is often visible in larger crystals within veins.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas of past or present basaltic volcanism, including oceanic crust, continental flood basalts, volcanic islands, and rift zones. The veins form post-emplacement, often due to hydrothermal alteration or diagenesis in fractured or vesicular basalt.

Key Facts

Hardness

Basalt matrix: 5-6 (Mohs). Quartz veins: 7 (Mohs). Calcite veins: 3 (Mohs).

Specific Gravity

Basalt matrix: 2.8-3.0. Quartz: 2.65. Calcite: 2.71.

Crystal System

Basalt minerals: various (e.g., plagioclase - triclinic, pyroxene - monoclinic). Quartz: trigonal. Calcite: trigonal.

Color

Dark gray to black (basalt), white to translucent (quartz), white to cream (calcite).

Luster

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

Transparency

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

Fracture

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

Cleavage

Basalt minerals have cleavage but often not macroscopically visible. Quartz: none. Calcite: perfect rhombohedral.

Composition

Basalt: primarily plagioclase feldspar, pyroxene, olivine. Veins: SiO2 (quartz) or CaCO3 (calcite).

Physical characteristics

• Crystal Habit: Basalt: fine-grained, massive, sometimes porphyritic. Quartz veins: massive, crystalline, drusy. Calcite veins: massive, crystalline, rhombohedral.
• Cleavage Type: Basalt minerals: variable (e.g., pyroxene - two at ~90 degrees, plagioclase - two at ~90 degrees). Quartz: none. Calcite: perfect rhombohedral.
• Fracture Type: Basalt: conchoidal to uneven. Quartz: conchoidal. Calcite: conchoidal to uneven.
• Tenacity: Brittle for all components.
• Luster Type: Basalt: dull to sub-vitreous. Quartz: vitreous. Calcite: vitreous to pearly.

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