Diabase, dolerite, or microgabbro is a medium-grained, dark-colored, hypabyssal, or subvolcanic rock with a composition similar to plutonic gabbro or volcanic basalt. It commonly occurs in dikes, sills, and sometimes in plugs, sheeted dikes, laccoliths, or lopoliths.

Subvolcanic or hypabyssal rocks are a kind of intrusive igneous rock emplaced at shallower depths within the Earth’s crust, usually at depths of less than 1.2 miles (2km). The relatively shallow emplacement allows for slightly faster cooling rates. Thus, they will have medium-mineral grain sizes.

Learn more about diabase or dolerite rock, including its properties (texture and color), chemical and mineral composition, how it forms, where it is found, and its uses.

Quick properties and characteristics  

• Name: Diabase, dolerite or microgabbro
• Rock type: Igneous
• Origin: Subvolcanic or hypabyssal
• Texture: Medium-grained (between fine-grained or aphanitic to coarse-grained or phaneritic.
• Grain size: Intermediate between the fine-grained basalt and coarse-grained gabbro, usually 1-5mm
• Colors: Dark gray to black and sometimes greenish or bluish
• Silica content: 45-52%
• Chemical composition: Mafic or basic
• Specific gravity: 2.9 and 3.3 g/cm³ – Haldar (2014)
• Extrusive equivalent: Basalt
• Intrusive equivalent: Gabbro
• Mohs hardness scale: 6-7
• Tectonic environment: All. Divergent (continental and island arcs above subduction zones), divergent (mid-ocean ridges and continental rifts), and hotspots (ocean and continental). Essentially, where basalt forms, microgabbro can also form.

Is it microgabbro, diabase or dolerite?

The IUGS Subcommission on the Systematics of Igneous Rocks has a convection for naming medium-grained rock. It recommends using the prefix micro + their course-grained intrusive version.

For instance, medium-grained granite, diorite, and gabbro are microgranite, microdiorite, or microgabbro. Only volcanic and extrusive or plutonic igneous rocks have unique names, not subvolcanic.

Therefore, the correct name for a medium-grained rock with a basaltic composition is microgabbro.

However, diabase (preferred in the US and Canada) and dolerite (British usage) are accepted microgabbro synonyms. Why? Because they have long-established and widespread usage, notes Le Maitre (2002). But there are recommendations to end using the names dolerite or diabase.

Some authors indicate that dolerite is unaltered microgabbro while slightly altered or metamorphized as diabase. Some emphasis on diabase being a medium-grained equivalent of tholeiitic basalt. Others consider it an altered dolerite with plagioclase albitized or saussuritized and pyroxenes replaced by chlorite or amphibole.

However, in the US, diabase is applied to unaltered dolerite. And since it is an approved synonym, you should be free to use the names microgabbro, dolerite, and diabase interchangeably.

What is diabase rock?

You can define dolerite or diabase as a medium-grained, dark-colored subvolcanic rock with a composition similar to basalt or gabbro. It is a basic (low in silica 45-52 wt. %) or mafic rock rich in magnesium and iron and low in felsic minerals.

Diabase has mainly calcic plagioclase and augite. However, it may have minor amounts of olivine, hornblende, quartz, or feldspathoids.

What does it look like? Dolerite is a massive dark gray to black, with some rocks having a bluish or greenish hue.

It has a medium-grain texture, i.e., transitional between fine-grained or aphanitic basalt and coarse-grained or phaneritic gabbro rock. Typical grain sizes are 1-5mm.

Also, it often shows subophitic to ophitic texture, a variation of poikilitic texture, with some being porphyritic.

Ophitic texture is where larger anhedral augite surrounds or overgrows lath-shaped plagioclase. If augite partially encloses plagioclase, the texture is known as subophitic.

Ophitic texture is sometimes known as the doleritic texture since it is common in this rock.

On the other hand, porphyritic dolerite will have large crystals in a finer-grained matrix or groundmass. It indicates two cooling stages: a slower one that grows the larger minerals and a faster one that forms the groundmass.

Dolerite will show mainly tubular or lath-shaped plagioclase on the thin section, usually white grains with dark strips due to crystal twinning. Also, you expect partly or fully enclosed by larger dark green to black anhedral augite, opaques (magnetite or ilmenite), olive green olivine, hornblende, etc.

Lastly, diabase rocks have more than 35% mafic content with a typical color index M > 35. Those with M < 35 are leucodiabase, while those with > 90% mafic content are ultramafic rocks like peridotites.  

Chemical composition?

Diabase is a basic rock with 45-52% silica (SO₂) by weight. It is rich in iron oxides, magnesium oxide, and calcium oxide and relatively low in alkalis (sodium and potassium oxides).

Using data from Whyte (1989), the typical percentage weight composition of dolerite or diabase is SiO₂: 50.83%, TiO₂: 2.03%, Al₂O₃: 14.07%, Fe₂O₃: 2.88%, FeO: 9.00%, MnO: 0.18%, MgO: 6.34%, CaO: 10.42 %, Na₂O: 2.23%, K₂O: 0.0.82%, and P₂O₅: 0.23%.

Mineral composition

Dolerite or diabase is a mafic rock. It has primarily calcic plagioclase and augite with or without minor amounts of olivine, hornblende, enstatite, quartz, and feldspathoids (nepheline, analcime, etc.). Also, it may have a small amount of alkali feldspar.

Common accessory minerals in this rock include apatite, zircon, ilmenite, sulfides, magnetite, and chromite.

Usually, the calcium-rich plagioclase labradorite to bytownite with > 50% molar anorthite. It commonly has lath-shaped crystals and is the main felsic mineral.

On the other hand, augite is the dominant clinopyroxene and mafic mineral. However, some dolerite varieties may have calcium-poor pyroxenes. i.e., pigeonite (clinopyroxene) and enstatite (orthopyroxene).

Also, some varieties may have olivine. Olivine occurs as rounded crystals but often undergoes weathering to an orange-brownish color.

Common secondary minerals include serpentine and iddingsite (replaces olivine), chlorite or uralite (replaces pyroxenes), and sericite (replacing plagioclase or feldspathoids).

On the other hand, low-grade metamorphism and hydrothermal alteration may form zeolites, including heulandite, prehnite, etc. 

Lastly, on the QAPF diagram, diabase has up to 20% quartz and up to 10% feldspathoids of the QAPF content by volume, while plagioclase accounts for at least 90% of the feldspar content. However, quartz and feldspathoids don’t coexist.

How to name this rock

You can use modifier prefix terms such as texture and mineralogy in naming diabase if it is a dominant trait or is present in considerable amounts.

Thus, you may have names like porphyritic, quartz, analcime, olivine, or nepheline diabase. Examples of such rocks include:

• Olivine diabase or dolerite: It has considerable olivine, i.e., up to 12%, and often has augite as the only pyroxene.
• Quartz diabase or dolerite: A diabase with 5-20% quartz is known as quartz-diabase. It will have mainly plagioclase and pyroxenes, interstitial or micro-pegmatitic quartz with no feldspathoids.

Classification

Diabase may be classified into tholeiitic or alkali varieties, just like gabbro. This classification depends on the presence or absence of low-Ca pyroxenes, nepheline, analcite, quartz, and olivine (including amount). Also, some distinctive accessories may be an indicator.

1. Alkali diabase

Alkali diabase rocks are rich in alkali oxides (sodium and potassium oxides) and poorer in iron and calcium oxide than other diabases. These rocks often have olivine, interstitial feldspathoids nepheline, or analcite with no quartz or alkali feldspar.

Also, they have distinctive minerals like phlogopite and kaersutite (Ti-rich, brown amphibole).

Lastly, alkali diabses occur in oceanic islands, continental rifts, and other volcanic fields.

2. Tholeiitic diabase

Tholeiitic diabase is quartz-saturated or oversaturated relative to alkalis. Besides calcic plagioclase and augite, they will have interstitial quartz and calcium-poor pyroxenes (enstatite and pigeonite) with little or no olivine.

Where does it occur? Tholeiitic diabases occur in LIPs and mid-ocean ridges.

Varieties and related rocks

1. Trachydolerite

Trachydolerite is a medium-grained rock with a chemical composition between basaltic and trachyte. It has calcic plagioclase (labradorite), alkali feldspar (orthoclase), clinopyroxene, olivine, and sometimes analcime or leucite.

2. Ophite

As Mindat.org notes, ophite is a term originated by Palasson in 1819. It describes diabase or dolerite that has retained its ophitic texture, but its pyroxene has been altered to uralite. This rock usually has a hypidiomorphic-granular texture.

How does diabase differ from basalt, gabbro and diorite?

The main difference between diabase and basalt is grain size. Diabase has a medium grain size, while basalt has a fine-grained or aphanitic texture. Thus, you can see individual diabase minerals without a magnifier, while you cannot see them in basalt. However, dolerite shows more mineral diversification than basalt.  

Diabase differs from gabbro since it is medium grain size while gabbro has a coarse-grained texture. Also, diabase forms close to the Earth’s surface and gabbro deep in the Earth’s crust.

Lastly, dolerite differs from diorite since it has medium grains, while diorite has a coarse-grained texture. Also, it has more mafic minerals (color index >35% vs. <35% for diorite) and lower silica content (45-52 wt.%) compared to diorite, which as an intermediate amount, i.e., 52-63 wt.%.

How is diabase formed?

Dolerite or diabase forms when basaltic magma cools moderately fast at shallower depths within the Earth’s crust.

This magma will intrude into weak zones, faults, or fractures in preexisting rock bodies, cool, and solidify within the crust, forming this rock. Afterward, erosion and uplift will eventually expose dolerite rocks to the surface.

The moderately fast cooling allows crystals to grow but not so large as the plutonic gabbro counterparts. Some of these intrusions may have chilled basalt contact margins.

Where is dolerite found?

Dolerite or diabase occurs mainly in dikes, sills, sometimes plugs, lopoliths, laccoliths, and very thick flood basalt. Dikes and sills can exist individually or in swarms associated with a single magma source, especially in flood basalts in large igneous provinces (LIPs).  

Also, dolerite can occur in ophiolites in sequences with gabbro, dunite, peridotite. Examples are Semail ophiolites (Oman and the United Arab Emirates) and Lajishankou ophiolite complex (China).

Another place where diabase occurs is in sheeted dikes where it feeds pillow lavas, and sheet flows in mid-ocean ridges.

It is worthwhile noting that dolerite occurs all over the world. Some notable places include the following:

• Tasmania Complex in Australia represents the largest diabase exposure in the world. It has Jurassic dikes and sills (sheets) across the entire complex and formations, including Tasmania cliffs or columns. These rocks are associated with the Gondwana supercontinent breakup and emplacements in Southern Africa and Antarctica.
• Northern Territory (Australia) has Oenpelli dolerite curved ridges, radial Galiwinku dolerite dike swarms, Norseman-Wiluna greenstone belt (200 km long associated with nonalluvial gold mining), and Yalgoo-Singleton greenstone.
• Dolerite sill and dike swarms are common in basaltic large igneous provinces (LIPs) like Karoo (southern Africa), Ferrar (Antarctica), and Paraná traps (south America), and part of Deccan traps in India.   
• In the US, notable diabase sills, dikes, and other intrusions include Palisades Sill (New Jersey), Death Valley intrusions (California), Black diabase dike at Mount Moran (Wyoming), and Schoodic Point (Maine).
• Also, diabase dike complexes and swarms occur in the British Tertiary Volcanic Province and the larger North Atlantic Igneous Province, including Eastern Greenland. Notable areas include Great Whin Sill, Skye, Rum, Mull, and Slieve Gullion.
• More locations include Scania (South Sweden), Mackenzie dike swarm and Nipissing diabase (Canada), Poya terrane (New Caledonia), Curaçao Island (Venezuela), Mongo (Chad), Steinerne Rose (Germany), etc.

Diabase rock uses

Diabase is dense, hard, and durable rock. It makes construction aggregate and dimensional stones (building and architectural stone). Also, you can use it for landscaping, riprap (soil erosion control), paving, etc. 

In the construction and dimension industries, dolerite and other rocks like basalt, gabbro, anorthosite, norite, and diorite are known as black granite.  Also, it goes by various names like trap, traprock, whin, or whinstone.

Let us discuss more on diabase or dolerite uses.

1. It is crushed to make aggregate

Crushed diabase rock makes aggregate for construction industries, including highway subbase, concrete, asphalt, railroad ballast, and subbase for concrete slaps.

Also, crushed dolerite stones are perfect for sewer filtration, riprap (soil erosion control), and landscaping. You can also use this gravel for unpaved roads, patios, walkways, stabilizing landscapes, etc.

2. Dimensional stone industry  

Diabase is cut and used in masonry, paving (cobblestones), and retaining walls in the dimension stone industry.

On the other hand, cut and polished dolerite stones are perfect for window lintels, windowsills, flooring tiles, cladding or facing stones, columns, and kitchen countertops.

Also, these stones are perfect ornamental stones for making monuments, headstones, and memorials, thanks to their durability.

3. More uses

More uses include sauna stones, making sea and marine walls, breakwaters, lining furnaces or kilns, etc.

4. Stonehenge monument (spotted & non-spotted dolerite)

The prehistoric (built in 3000 BC to 2000 BC) monumental Stonehenge at Salisbury Plain in Wiltshire, England, has about 80 pieces of spotted and non-spotted dolerite bluestone pillars. It remains a treasured architectural feature and was listed as a UNESCO Heritage Site in 1986.

The Preseli Bluestone came from Carn Meini Quarry in the Preseli Mountains in SW Wales. Spots are due to saussuritized plagioclase glomerocrysts or phenocrysts.

References

• Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell
• Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press
• Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
• Whyte, F. (1989). Dolerite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 130-131). New York: Van Nostrand Reinhold.  
• Haldar, S. K., & Tisľjar, J. (2014). Introduction to Minerology and petrology (1st ed.). Elsevier.
• Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub.