How to identify Olivine Basalt

To correctly identify Olivine Basalt (Basalt with Olivine phenocrysts), 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

   Dark gray to black groundmass with distinctive yellowish-green to green olivine phenocrysts.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the groundmass; vitreous to greasy on fresh olivine phenocrysts.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, with larger, often euhedral to subhedral olivine crystals (phenocrysts) embedded in a very fine-grained to aphanitic (microcrystalline) groundmass. Vesicular textures (containing gas bubbles) are common.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Olivine phenocrysts often exhibit characteristic stubby prismatic or equant forms. Groundmass minerals are too small to discern individual crystal forms without magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Olivine has poor to indistinct cleavage. Pyroxene in the groundmass has two distinct cleavages at nearly 90 degrees. Plagioclase has two distinct cleavages at nearly 90 degrees. These are typically not visible in the fine-grained groundmass.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Mid-ocean ridges, oceanic islands (hotspots), continental rift zones, and continental flood basalt provinces. It is a primary component of oceanic crust.

Key Facts

Hardness

6-7 on Mohs scale (for olivine phenocrysts; groundmass is similar but variable)

Specific Gravity

2.9-3.3 g/cm³

Crystal System

Orthorhombic (olivine); Monoclinic (pyroxene, plagioclase in groundmass)

Color

Dark gray to black with green olivine phenocrysts

Luster

Vitreous to greasy (olivine), dull to earthy (groundmass)

Transparency

Opaque (rock), transparent to translucent (olivine phenocrysts)

Fracture

Conchoidal to uneven (olivine), irregular (rock)

Cleavage

Poor to indistinct in olivine; two distinct cleavages at nearly 90 degrees in pyroxene and plagioclase (not typically visible in groundmass)

Composition

Predominantly plagioclase feldspar (labradorite-bytownite), pyroxene (augite), and olivine, with minor magnetite and other accessory minerals. SiO2 content typically 45-52 wt%.

Physical characteristics

• Crystal Habit: Olivine phenocrysts are typically euhedral to subhedral, stubby prismatic or equant. Groundmass is aphanitic.
• Cleavage Type: Olivine: Poor {010} and {100}. Pyroxene and Plagioclase: Two distinct cleavages at nearly 90 degrees.
• Fracture Type: Conchoidal to uneven for olivine; irregular for the bulk rock.
• Tenacity: Brittle
• Luster Type: Vitreous to greasy for olivine; dull to earthy for the fine-grained groundmass.

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