BH250-226
Title
BH250-226
Subject
Diabase
Description
Major Minerals: plagioclase, augite, olivine
Diabase
This sample is a classic example of diabase (also called dolerite), a medium-grained mafic igneous rock that forms when basaltic magma cools in shallow subsurface environments such as dikes, sills, and small intrusive bodies. Diabase is compositionally similar to basalt and gabbro, but it differs primarily in cooling history and texture. Because it cools at an intermediate rate—slower than basalt erupted at the surface but faster than deeply crystallized gabbro—it typically develops a distinctive medium-grained texture visible in hand sample and thin section. This specimen is composed primarily of plagioclase, augite, and olivine, a mineral assemblage consistent with relatively magnesium- and iron-rich mafic magma.
In thin section, plagioclase commonly occurs as elongated laths that may form an interlocking framework, while augite fills spaces between feldspar crystals in a classic ophitic to subophitic texture, where larger pyroxene crystals partially enclose plagioclase laths. Olivine, if preserved, may occur as early-formed crystals and can sometimes show alteration to iddingsite, serpentine, or iron oxides if the rock has undergone weathering. Diabase plays an important role in both regional geology and plate tectonics because it commonly forms during crustal extension, continental rifting, and large igneous events.
Diabase
This sample is a classic example of diabase (also called dolerite), a medium-grained mafic igneous rock that forms when basaltic magma cools in shallow subsurface environments such as dikes, sills, and small intrusive bodies. Diabase is compositionally similar to basalt and gabbro, but it differs primarily in cooling history and texture. Because it cools at an intermediate rate—slower than basalt erupted at the surface but faster than deeply crystallized gabbro—it typically develops a distinctive medium-grained texture visible in hand sample and thin section. This specimen is composed primarily of plagioclase, augite, and olivine, a mineral assemblage consistent with relatively magnesium- and iron-rich mafic magma.
In thin section, plagioclase commonly occurs as elongated laths that may form an interlocking framework, while augite fills spaces between feldspar crystals in a classic ophitic to subophitic texture, where larger pyroxene crystals partially enclose plagioclase laths. Olivine, if preserved, may occur as early-formed crystals and can sometimes show alteration to iddingsite, serpentine, or iron oxides if the rock has undergone weathering. Diabase plays an important role in both regional geology and plate tectonics because it commonly forms during crustal extension, continental rifting, and large igneous events.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 226. Housed at Carleton College in Minnesota.
Contributor
John Goodge
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-226,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/299.