How to identify Vesicular Basalt with Sandstone Inclusion

To correctly identify Vesicular Basalt with Sandstone Inclusion (Basalt with sedimentary inclusion), 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; sandstone inclusion is usually light-colored (tan, buff, reddish, white, or gray), contrasting sharply with the matrix.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt matrix is dull to earthy; sandstone inclusion can be dull to sugary.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Basalt matrix is aphanitic (fine-grained) with abundant vesicles (porous). Sandstone inclusion is clastic, granular, and typically medium-grained, often showing individual sand grains.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Basalt minerals are too fine-grained to show distinct crystal forms without magnification. Sandstone grains are typically sub-angular to rounded.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basalt minerals (e.g., pyroxene, plagioclase) have cleavage, but it's rarely visible in hand samples due to fine grain size. Sandstone grains do not exhibit cleavage, but the rock may fracture along grain boundaries.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic settings where basaltic lava flows or pyroclastic eruptions interact with pre-existing sedimentary rock layers (e.g., continental rifts, volcanic plateaus, subduction zones with overlying sedimentary basins).

Key Facts

Hardness

Basalt matrix: 5-6 (Mohs); Sandstone inclusion: 6-7 (quartz grains), but overall rock hardness can be lower due to porosity and cement.

Specific Gravity

Basalt matrix: 2.7-3.0 (less if highly vesicular); Sandstone inclusion: 2.6-2.7 (depending on porosity and cement).

Crystal System

Basalt: Monoclinic (pyroxene, plagioclase); Sandstone: Quartz is hexagonal, feldspars are monoclinic/triclinic.

Color

Dark gray to black (basalt) with light tan, white, or reddish (sandstone).

Luster

Dull to earthy.

Transparency

Opaque (basalt); Translucent to opaque (sandstone grains).

Fracture

Basalt: Conchoidal to irregular; Sandstone: Granular to irregular.

Cleavage

Not typically observed in hand samples for basalt; absent in sandstone grains.

Composition

Basalt: Predominantly plagioclase feldspar, pyroxene, olivine, and iron-titanium oxides. Sandstone: Primarily quartz, with varying amounts of feldspar, rock fragments, and cementing minerals (e.g., silica, calcite, iron oxides).

Physical characteristics

• Crystal Habit: Basalt: Microcrystalline to cryptocrystalline, often with phenocrysts of olivine or pyroxene. Sandstone: Detrital grains, typically sub-angular to rounded.
• Cleavage Type: Basalt: Not visible in hand sample; minerals have good cleavage. Sandstone: None.
• Fracture Type: Basalt: Conchoidal to irregular. Sandstone: Granular.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

Explore Vesicular Basalt with Sandstone Inclusion