How to identify Diabase

To correctly identify Diabase (Diabase), 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

   Typically dark gray to black, sometimes with a greenish or brownish tint due to alteration.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to sub-vitreous on fresh surfaces, becoming duller with weathering.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine- to medium-grained (phaneritic), with a characteristic ophitic or subophitic texture visible with a hand lens, where plagioclase laths are partially or wholly enclosed by pyroxene.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals are generally anhedral to subhedral, with plagioclase often forming lath-like shapes and pyroxene filling interstitial spaces.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Minerals within diabase (e.g., pyroxene, plagioclase) exhibit cleavage, but the rock itself does not have a rock cleavage. Fracture is typically irregular to conchoidal.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Occurs as dikes, sills, and small intrusive bodies, often associated with regional extensional tectonics, continental rifting, and large igneous provinces. It is commonly found cutting through older sedimentary or metamorphic rocks.

Key Facts

Hardness

6-7 on Mohs scale (for constituent minerals, rock is tough)

Specific Gravity

2.8-3.1 g/cm³

Crystal System

Monoclinic (pyroxene), Triclinic (plagioclase)

Color

Dark gray to black, sometimes greenish or brownish

Luster

Dull to sub-vitreous

Transparency

Opaque (as a rock)

Fracture

Irregular to sub-conchoidal

Cleavage

Not applicable to the rock as a whole; constituent minerals exhibit cleavage (e.g., pyroxene: two at ~90°, plagioclase: two at ~90°)

Composition

Primarily calcic plagioclase (labradorite to bytownite) and pyroxene (augite), with minor olivine, magnetite, ilmenite, and sometimes biotite or hornblende.

Physical characteristics

• Crystal Habit: Ophitic to subophitic texture, where lath-shaped plagioclase crystals are partially or completely enclosed by larger anhedral pyroxene crystals.
• Cleavage Type: Not applicable to the rock; constituent minerals have distinct cleavage planes.
• Fracture Type: Irregular to sub-conchoidal, tough.
• Tenacity: Brittle, but very tough and resistant to abrasion.
• Luster Type: Dull to sub-vitreous on fresh surfaces.

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