Poikilitic
An intergrowth texture in which a larger crystal (oikocryst) encloses many smaller crystals (chadacrysts) of another mineral or minerals.
Language and route: English scientific term, ultimately formed from Ancient Greek poikilos. English scientific term, ultimately formed from Ancient Greek poikilos
Literal gloss: mottled or variegated
The name and its history
G. H. Williams, 1886; historical discussion 1893 · G. H. Williams, 1886; historical discussion 1893
- What does the word literally mean?
- Immediate route: English scientific term, ultimately formed from Ancient Greek poikilos. Linguistic gloss: ‘mottled or variegated’. This gloss describes language, not a demonstrated early geological theory.
- What did the original author mean?
- G. H. Williams applied the petrographic name in 1886 (initially poicilitic); his 1893 paper reviewed spellings and distinguished this use from earlier terms applied to red strata.
- Why was the name needed?
- Petrologists sought to name mottled optical appearances produced by several grains enclosed in a single crystal without treating them as separate rock species.
- What evidence supported the original concept?
- Williams cited microscopic and macroscopic inclusion patterns in igneous minerals, discussing Pumpelly’s earlier “luster-mottling” and South Mountain rocks.
- How has geology changed the concept?
- It is now a spatial relation observed across igneous settings and may admit several crystallisation pathways; petrographic poikilitic is distinct from the historical red-bed name.
Further historical context
G. H. Williams applied the petrographic name in 1886 (initially poicilitic); his 1893 paper reviewed spellings and distinguished this use from earlier terms applied to red strata.
Petrologists sought to name mottled optical appearances produced by several grains enclosed in a single crystal without treating them as separate rock species.
Williams cited microscopic and macroscopic inclusion patterns in igneous minerals, discussing Pumpelly’s earlier “luster-mottling” and South Mountain rocks.
It is now a spatial relation observed across igneous settings and may admit several crystallisation pathways; petrographic poikilitic is distinct from the historical red-bed name.
Editorial qualification
Poikilitic petrographic term first applied as poicilitic; Williams warns of older unrelated red-bed applications.
Related names
Sources
- Williams (1893), poikilitic / micropoikilitic, p. 177 — Accessible historical text with Williams’s explanation of 1886 name and earlier alternative meanings
- Geological Survey of India, Rock Texture Atlas — Photomicrographs and modern texture definitions
- University of Freiburg, describing thin sections — Petrographic grain relations and texture terminology