Diabase is a dark-colored igneous rock. It is compositionally equivalent to gabbro and basalt but texturally between them.
Groundmass of diabase usually resembles basalt — it is fine-grained although frequently somewhat coarser (often visible to the naked eye). Characteristic feature of diabase is larger crystals (phenocrysts) of plagioclase and less frequently pyroxene (augite mostly).
This rock (when fresh) is sometimes called dolerite as well, especially in Britain.
How is this rock formed? It started to form when it was still in the Crust as part of magma body. The phenocrysts were formed then. The groundmass crystallized later and in different conditions, where cooling was rapid and crystals didn’t have time to grow large. Dikes and sills are often composed of diabase because these magma bodies are narrow and host rock that surrounds them cools it pretty quickly into a solid rock.
Diabase is an interesting rock type because it is so decorative. That is probably one of the contributing factors why it was defined as a distinct rock type. Here are two diabase samples that I have an honour to have in my collection. One is from Tenerife and the other from the Isle of Mull, Scotland.
White phenocrysts are plagioclase and black phenocrysts are pyroxene crystals. Tenerife, The Canary Islands. The rock is 14 cm in length.
White phenocrysts are plagioclase crystals. Brown spots are left by sea animals that were attached to the rock. The Isle of Mull, Scotland. The rock is 8 cm in length.
