Where is Diabase found?
Diabase
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Open the appDiabase occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Diabase
Palisades Sill, USA
A prominent diabase sill forming the Palisades cliffs along the Hudson River in New York and New Jersey, USA. Part of the Central Atlantic Magmatic Province (CAMP).
Karoo Large Igneous Province, Southern Africa
Extensive diabase dikes and sills associated with the Jurassic Karoo LIP, found across South Africa, Lesotho, and other southern African countries.
Tasmanian Dolerite, Australia
Widespread Jurassic diabase (dolerite) intrusions forming significant topographic features across Tasmania, Australia.
Mid-Continent Rift System, USA
Diabase intrusions are common within this ancient rift system extending through the central United States.
Collecting tips
- Look for Intrusive Forms. Diabase typically occurs as dikes (tabular intrusions cutting across existing rock layers) or sills (tabular intrusions parallel to existing layers). Look for these linear or sheet-like features in road cuts, quarries, or eroded landscapes.
- Examine Texture. Use a hand lens to observe the characteristic ophitic or subophitic texture. This involves seeing lighter-colored, lath-shaped plagioclase crystals partially or completely enveloped by darker, anhedral pyroxene crystals.
- Check for Dark Color and Hardness. Diabase is a dark, dense, and hard rock. It will scratch glass and will not effervesce with dilute hydrochloric acid (unless altered).
- Consider Associated Rocks. Diabase often intrudes into sedimentary rocks (e.g., sandstones, shales) or metamorphic rocks. Look for contact metamorphic effects (e.g., baking, hardening) in the country rock adjacent to the diabase intrusion.
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.
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