Diabase

Description

Diabase is a dark-colored, fine- to medium-grained mafic intrusive igneous rock. It is characterized by its distinctive ophitic or subophitic texture, where lath-shaped plagioclase feldspar crystals are partially or completely enclosed by larger anhedral pyroxene crystals. Its mineralogical composition is similar to basalt and gabbro, primarily consisting of calcic plagioclase, pyroxene (augite), and often olivine, with minor amounts of magnetite, ilmenite, and sometimes biotite or hornblende. The term 'dolerite' is synonymous with diabase, particularly in British geological literature.

How to Identify

Color

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

Luster

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

Texture

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.

Crystal Form

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

Cleavage

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.

Geological Environment

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.

Quick Check

• Color: Dark gray to black
• Luster: Dull to sub-vitreous
• Streak: Grayish-white (due to plagioclase) or colorless (due to pyroxene), but typically not useful for the rock as a whole.

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.

Formation

Diabase forms from the relatively rapid cooling of basaltic magma intruded into shallow crustal fissures, dikes, sills, and laccoliths. The cooling rate is intermediate between that of extrusive basalt (fine-grained) and intrusive gabbro (coarse-grained), leading to its characteristic ophitic or subophitic texture.

Usage

Diabase is widely used as crushed stone for road construction, railway ballast, concrete aggregate, and riprap due to its hardness, durability, and resistance to weathering. It is also used as dimension stone for building facades, paving, and monuments. In some regions, it has been used for traditional tools and weapons.

Age Distribution

Found in rocks of various ages, from Precambrian to Cenozoic, often associated with continental rifting and large igneous provinces.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock

Group

Mafic Intrusive/Hypabyssal Rock

Crystal System

Polycrystalline aggregate; constituent minerals are monoclinic (pyroxene) and triclinic (plagioclase)

Chemical Formula

Complex silicate, primarily (Ca,Na)(Al,Si)₂Si₂O₈ (plagioclase) and (Ca,Mg,Fe)SiO₃ (pyroxene)

Composition

Dominantly plagioclase feldspar (typically labradorite) and pyroxene (augite), with accessory minerals such as olivine, magnetite, ilmenite, and sometimes biotite or hornblende. Silica content is typically 45-52 wt% SiO₂.

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