How to identify Olivine Basalt

To correctly identify Olivine Basalt (Basalt (mafic volcanic rock) with Olivine (Mg,Fe)2SiO4), 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

   Groundmass is typically dark gray to black; olivine phenocrysts are green, yellowish-green, or brownish-green.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Groundmass is dull to sub-vitreous; olivine phenocrysts are vitreous (glassy).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, with larger, well-formed olivine crystals (phenocrysts) set in a fine-grained to aphanitic groundmass. The groundmass can be vesicular (containing gas bubbles) or amygdaloidal (vesicles filled with secondary minerals).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Olivine phenocrysts often exhibit euhedral to subhedral forms, typically equant or short prismatic. The groundmass crystals are too small to discern without a microscope.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Basaltic groundmass minerals (plagioclase, pyroxene) have cleavage but are too small to observe macroscopically. Olivine has poor to indistinct cleavage, often appearing as conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Mid-ocean ridges, oceanic islands (hotspots like Hawaii), continental flood basalts, rift zones, and volcanic arcs. It is a common product of mantle melting.

Key Facts

Hardness

Basalt (bulk rock) 5-6 on Mohs scale; Olivine 6.5-7 on Mohs scale.

Specific Gravity

Basalt 2.8-3.0 g/cm³; Olivine 3.27-3.37 g/cm³ (for forsterite-fayalite series).

Crystal System

Basalt (groundmass minerals): Plagioclase (triclinic), Pyroxene (monoclinic); Olivine (orthorhombic).

Color

Dark gray to black (groundmass), green to yellowish-green (olivine phenocrysts).

Luster

Dull to sub-vitreous (groundmass), vitreous (olivine).

Transparency

Opaque (groundmass), transparent to translucent (olivine).

Fracture

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

Cleavage

Absent to poor (basalt, due to fine grain size), poor to indistinct (olivine).

Composition

Primarily plagioclase feldspar (labradorite), pyroxene (augite), olivine, and often magnetite. SiO2 content typically 45-52 wt%.

Physical characteristics

• Crystal Habit: Porphyritic texture with euhedral to subhedral olivine phenocrysts in an aphanitic to fine-grained groundmass.
• Cleavage Type: Olivine: Poor to indistinct on {010} and {100}. Groundmass minerals: Plagioclase has two good cleavages, Pyroxene has two good cleavages at nearly 90 degrees, but these are rarely visible macroscopically in basalt.
• Fracture Type: Conchoidal to uneven for the bulk rock; conchoidal for individual olivine crystals.
• Tenacity: Brittle.
• Luster Type: Vitreous for olivine phenocrysts; dull to sub-vitreous for the basaltic groundmass.

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