Anorthosite
Plutonic rock made up predominantly (approximately ≥90%) of plagioclase, classified specially among gabbroic rocks.
Language and route: English scientific rock-name based on French anorthose. English scientific rock-name based on French anorthose
Literal gloss: not straight-angled feldspar rock
The name and its history
T. Sterry Hunt / Geology of Canada (1863) early printed use; Delesse attribution often misreported · 1863: Geology of Canada printed attestation
- What does the word literally mean?
- Literal naming sense: not straight-angled feldspar rock. Immediate language/formation: English scientific rock-name based on French anorthose. The original Canadian rock discussions did not simply mean an anorthite monomineralic rock.
- What did the original author mean?
- The Canadian Laurentian descriptions separated feldspar-dominated masses from ordinary gabbro and granite.
- Why was the name needed?
- Feldspathic rock masses in the Laurentian system were too distinctive to classify as ordinary granite or gabbro.
- What evidence supported the original concept?
- Feldspar mineralogy and field relations, initially interpreted through competing theories about intrusive versus sedimentary origin.
- How has geology changed the concept?
- The mineral term anorthose changed meaning; anorthosite now means a plagioclase-dominant rock rather than a rock necessarily rich in the species anorthite.
Further historical context
The Canadian Laurentian descriptions separated feldspar-dominated masses from ordinary gabbro and granite.
Feldspathic rock masses in the Laurentian system were too distinctive to classify as ordinary granite or gabbro.
Feldspar mineralogy and field relations, initially interpreted through competing theories about intrusive versus sedimentary origin.
The mineral term anorthose changed meaning; anorthosite now means a plagioclase-dominant rock rather than a rock necessarily rich in the species anorthite.
Editorial qualification
Delesse used anorthose, not demonstrably anorthosite; exact individual inventor uncertain.
Related names
Anorthite · Plagioclase · Gabbro · Orthoclase · QAPF classification
Sources
- Canadian Mineralogist, history of the term anorthosite — Discusses 1863 Geology of Canada and disputed Hunt/Delesse coinage
- BGS, Rock Classification Scheme — Modern descriptive rock-naming principles and definitions
- USGS, Lithologic classification of geologic map units — Modal definitions and named plutonic-rock categories
- IUGS / Le Maitre et al. (2002), Igneous Rocks: A Classification and Glossary of Terms, ch. 3 — Authoritative IUGS rock-name history and bibliographic attributions