Ocellus (petrology)

Ocellus (amphibole-coated quartz) in altered granodiorite (thin section)

As Ocellus ( lat. "Little eyes" plurality Ocelli or Ocellen ) are in the Petrologie eye-shaped, ovoid or roundish structures referred to, which greatly differ in their mineral constituents and their texture from the surrounding host rock. They should not be confused with similar looking tonsils , eyes , porphyroblasts or porphyroclasts .

Distinguishing criteria and synonyms

Monchiquit storage corridor with various ocelliforms, Sainte-Dorothée, Québec , Canada

Ocelli are bound to igneous rocks and must therefore be separated from metamorphic textures such as eyes, porphyroblasts and porphyroclasts due to the way they are formed. Compared to the almonds of the volcanic rocks, they usually differ in terms of their mineral filling and the more fluid transition to the host rock. Almonds mostly consist of agate , calcite , quartz or zeolites , whereas ocelli are mostly composed of ordinary igneous rock minerals such as amphibole , epidote , potassium feldspar , quartz, plagioclase , titanite or glass.

Some authors also use the term "globulite" or "globuide" in the same sense.

Occurrence

Ocelli occur in meso- to melanocratic igneous rocks such as quartz diorite , diorite and gabbro . They are very common in lamprophyric dike rocks .

Emergence

The origin of the ocelli has not yet been fully clarified, but two hypotheses are usually discussed:

Immiscibility of two different types of magma coming together in a magma chamber, or separation of a parent agma into two immiscible partial magmas (e.g. when cooling down)
Relics, unassimilated foreign rock remnants or foreign crystals ( xenolites )

Criticism of the term

Due to the fact that the term in its present version comprises at least two genetically different possible origins, it has already been suggested in the literature to dispense with its use entirely. The term is also not found in the recommendations of the International Union of Geological Sciences on the nomenclature of igneous rocks.

Practical meaning

The occurrence of such structures in basaltic rocks is held responsible for the occurrence of the sunburn phenomenon.

swell

Wimmenauer, W. (1985). Petrography of igneous and metamorphic rocks. Ferdinand Enke Verlag Stuttgart ISBN 3-432-94671-6

Web links

Ocelli in the Central Bohemian Pluton PDF file; 123.44 kB

Individual evidence

↑ ^a ^b Ulrich Schreiber: New research results on the characteristic patchy weathering phenomenon of basaltic rocks (basaltic "sunburn") . In: New Yearbook for Geology and Paleontology, monthly books . No. 6 . Stuttgart 1990, p. 353-366 .
^ David Shelley: Igneous and Metamorphic Rocks under the Microscope . Chapman & Hall, London 1993, ISBN 0-412-44200-0 , pp. 201, 419, 431 .
↑ Ron Vernon: A Practical Guide to Rock Microstructure . 3. Edition. Cambridge University Press, Cambridge 2011, ISBN 978-0-521-89133-2 , pp. 486 .
↑ RW Le Maitre (Ed.): Igneous Rocks - A Classification and Glossary of Terms . 2nd Edition. Cambridge University Press, Cambridge 2004.

[:0-1] Ulrich Schreiber: New research results on the characteristic patchy weathering phenomenon of basaltic rocks (basaltic "sunburn") . In: New Yearbook for Geology and Paleontology, monthly books . No. 6 . Stuttgart 1990, p. 353-366 .

[2] David Shelley: Igneous and Metamorphic Rocks under the Microscope . Chapman & Hall, London 1993, ISBN 0-412-44200-0 , pp. 201, 419, 431 .

[3] Ron Vernon: A Practical Guide to Rock Microstructure . 3. Edition. Cambridge University Press, Cambridge 2011, ISBN 978-0-521-89133-2 , pp. 486 .

[4] RW Le Maitre (Ed.): Igneous Rocks - A Classification and Glossary of Terms . 2nd Edition. Cambridge University Press, Cambridge 2004.