Dolerite, diabase, or formally microgabbro, is a dark-colored, medium-grained subvolcanic rock equivalent to basalt or gabbro in composition.
Subvolcanic or hypabyssal rock is a plutonic rock that forms at shallower depths less than 2 km (1.2 mi) from the Earth’s surface.
Cooling is a bit faster at these shallower depths than other intrusive rocks. Therefore, these will have a medium-grained texture, not fine-grained like volcanic rocks or coarse-grained like other plutonic rocks.
On the other hand, basalt is fine-grained volcanic or extrusive rock, and gabbro is coarse-grained plutonic.
Lastly, basalt, dolerite, and gabbro are all mafic rocks. Mafic rocks are relatively low in silica and rich in iron and magnesium.
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Properties and description
Diabase is a massive, dense, hard, dark-colored rock with a specific gravity of 2.9-3.3g/cm3 and a Mohs hardness scale of 6-7.
This rock is usually dark gray to black but can have a bluish or greenish tinge. However, it may appear tan, brown, or reddish brown on a weathered surface.
The usual color index M or mafic mineral content by volume is 35-90. Those with M less than 35 are known as leucodiabase.
What about its texture? Dolerite or diabase is medium-grained. Medium-grained rocks are phaneritic rocks with crystals measuring 1-5mm. You can see and identify mineral crystals in these rocks without a microscope.
Sometimes, diabase may also show other textures like porphyritic, ophitic, and subophitic.
Porphyritic diabase has large crystals in a finer but medium-grained matrix or groundmass.
On the other hand, ophitic has pyroxene or sometimes olivine surrounding plagioclase laths completely. If plagioclase is only partially surrounded, the texture will be ophitic.
Chemical composition
Diabase is a basic or silica-poor rock. It is high in iron, calcium, and magnesium oxides and low in sodium and potassium oxides (alkali oxides).
The typical weight percentage diabase chemical composition range is 45-52% silica, less than 5% alkalis (Na2O + K2O), about 10% calcium oxide, 5-12% magnesium oxide, and 5-15% iron oxides.
Dolerite mineral composition
Diabase or dolerite is a mafic rock dominated by plagioclase and augite. It may or may not have a smaller amount of olivine, enstatite, quartz, feldspathoids, alkali feldspar, and hornblende.
Accessory minerals in diabase may include zircon, ilmenite, magnetite, sulfides, apatite, chromite, etc.
The calcic plagioclase in this rock has more than 50 mol% anorthite. It is usually laths of bytownite or labradorite.
On the other hand, augite, a clinopyroxene, is the dominant pyroxene. However, this may have pigeonite or enstatite. These are calcium-poor clinopyroxene and orthopyroxenes, respectively.
On the QAPF classification, diabase is defined as a medium-grained igneous rock with no more than 10% and 20% feldspathoids and quartz of the QAPF by volume, respectively, with plagioclase no less than 90% of total feldspar.
How does diabase form
Dolerite or diabase forms when magma intrudes through deep fractures, zones of weakness, or faults and solidifies at depths less than 2 km from the Earth’s surface.
The cooling rate at these depths is moderate, allowing the growth of medium-sized crystals. However, the margins of these intrusions will chill quickly, forming fine-grained basalt rocks.
Later, tectonic movements like uplift or erosion can expose these intrusions.
Lastly, diabase or gabbro rocks can also form inside thick basaltic lava flows. The thick overlaying layer offers insulation, slowing down the cooling rate.
Where is dolerite or diabase found?
Diabase is most common on dikes and sills, including on sheeted dikes in oceanic. Also, it can occur as plugs, lopoliths, laccoliths, and in thick basalt lava flows like basalt floods.
Other less common places include ophiolite sequences like Semail ophiolites in UAE and Orman. Ophiolites are exposed to ancient oceanic crust thrust or obducted on the continental crust.
To specific locations, the Tasmania Complex in Australia boasts of the largest exposed dolerite sills and dikes, some forming columns or cliffs. Also, Oenpelli, Galiwinku, Norseman-Wiluna greenstone belt, and Yalgoo-Singleton in the Northern Territory in Australia have dike and sill swarms.
Some basalt floods in large igneous provinces have dolerite dikes and sills, including swarms. Examples include Karoo, Ferrar, Paraná traps, and Deccan traps.
In the US, diabase intrusions, dikes, and sills occur in Palisades Sill in New Jersey, Black diabase dike in Wyoming, Death Valley intrusion in California, and Schoodic Point in Maine.
Other notable emplacements are Scania in Sweden, Mackenzie and Nipissing in Canada, and Poya terrane in New Caledonia. Also, Great Whin Sill, Skye, Mull, Slieve Gullion, and Rum in the British Tertiary Igneous Province have dolerite dike complexes and swarms.
Dolerite uses
Dolerite is a hard, dense, durable rock that resists chemicals and weathering. Also, it is safe, i.e., not toxic.
These ideal properties give it many uses in the construction and dimensional industry, where it is often sold as black granite, trap, whin, traprock, or whinstone.
Its use spans to prehistoric times. For instance, about 80 pieces of spotted and non-spotted dolerite bluestone pillars from Carn Meini Quarry in Preseli Mountains were used about 2000-3000 BC to construct the Stonehenge monument in Wiltshire, England.
Crushed dolerite aggregate is used in various construction projects like building highways, making concrete, asphalt, railroad ballast, etc. You can also use this gravel on unpaved patios, walkways, or driveways.
Cut and polished dolerite is perfect for pavers, window lintels, floor tiles, kitchen countertops, windowsills, facing stones, headstones, monuments, memorials, etc.
On the other hand, unpolished cut dolerites have uses in masonry, making pavers, and other building works.
More use of dolerite rocks is in landscaping, riprap, and lining kilns/furnaces. Also, they are perfect sauna stones and can make stepping stones, statues, sculptures, etc.
Frequently Asked Rocks (FAQs)
Diabase, microgabbro, or dolerite are accepted synonyms, and all refer to the same rock, with diabase preferred in the US and Canada and dolerite in the UK. Dolerite isn’t an altered microgabbro, and neither is diabase a medium-grained tholeiitic basalt. However, the IUGS recommends the name microgabbro, i.e., the word micro + plutonic form name.
Olivine diabase rock has a considerable amount of olivine, i.e., up to 12%. However, calcic plagioclase and augite still dominate the minerals in this rock.
It is a dolerite rock in which quartz accounts for 5-20% of the QAPF content by volume.