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'Diabase' (IPA: ) is a mafic, holocrystalline, igneous rock equivalent to volcanic basalt or plutonic gabbro. Diabase is also called ''dolerite'' in many older British references.^[1][2] Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic ''chilled margins'' which may contain tachylite (dark mafic glass).
Diabase normally has a fine, but visible texture of euhedral lath shaped plagioclase crystals (62%) set in a finer matrix of clinopyroxene, typically augite (20 - 29%), with minor olivine (3% up to 12% in olivine diabase), magnetite (2%) and ilmenite (2%).^[3] Accessory and alteration minerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, and calcite. The texture is termed ''diabasic'' and is typical of diabases. This ''diabasic'' texture is also termed ''interstitial''.^[4] The feldspar is high in anorthite (as opposed to albite), the calcium end member of the plagioclase Anorthite-Albite solid solution series, most commonly labradorite.

Contents

Locations

Diabase/dolerite

Inscription controversy

References

Locations

Diabase is usually found in smaller relatively shallow intrusive bodies such as dikes and sills. Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundreds of individual dikes or sills radiating from a single volcanic center.
The Palisades Sill which makes up the New Jersey Palisades on the Hudson River, near New York City, is an example of a diabase sill. The dike complexes of the Hebridean Tertiary volcanic province which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion region of Ireland, and extends across northern England contains many examples of diabase dike swarms.

Diabase/dolerite

In older British usage ''dolerite'' was preferred and ''diabase'' was used to refer to an ''altered dolerite''. Dolerite (Greek: doleros, meaning "deceptive") was the name given by Haüy in his 1822 ''Traité de minéralogie''. In current geologic usage diabase is preferred.^[5]

Inscription controversy

During seven centuries a diabase formation called Runamo was famous in Scandinavia as a runic inscription, until it became the object of a famous scientific controversy in the first half of the 19th century.

References

1. Holmes, Arthur, 1974, ''Principles of Physical Geology'', Halsted Press, 3rd ed., p. 70 ISBN 0-471-07251-6
2. Encyclopædia Britannica Eleventh Edition
3. Klein, Cornelus andCornelius S. Hurlbut, Jr., 1986, ''Manual of Mineralogy'', Wiley, 20th ed., p. 483 ISBN 0-471-80580-7
4. Morehouse, W. W.,1959, ''The Study of Rocks in Thin Section'', Harper & Row, p. 160
5. http://www.britannica.com/eb/article-9030242/diabase Encyclopedia Britannica