Plagioclase
A series of tectosilicate feldspars between albite (sodium-rich) and anorthite (calcium-rich), usually recognised petrographically by composition, cleavage and twinning.
Language and route: German scientific mineralogical coinage. German Plagioklas (Breithaupt, 1847), from Greek πλάγιος (plágios), ‘oblique’, and κλάσις (klásis), ‘fracture’.
Literal gloss: oblique fracture / oblique cleavage
The name and its history
August Breithaupt (attributed) · 1847 (German name; original page unexamined)
- What does the word literally mean?
- ‘Oblique fracture’ or, in mineralogical context, ‘oblique cleavage’.
- What did the original author mean?
- Breithaupt used the name for feldspars whose prominent cleavage directions deviate from a right angle, distinguishing them from the ‘straight-cleaving’ orthoclase.
- Why was the name needed?
- Researchers needed a mineralogical distinction within superficially similar feldspars before modern composition analysis and crystallography provided a full structural scheme.
- What evidence supported the original concept?
- Measured cleavage geometry and crystallographic observations; an older mineralogical reference explicitly identifies this as the naming rationale.
- How has geology changed the concept?
- Modern plagioclase denotes a compositional series in sodium–calcium feldspars, not every feldspar with non-orthogonal cleavage; microcline is a useful counterexample.
Further historical context
The 1911 Encyclopaedia Britannica explains the Breithaupt 1847 choice and cautions that other triclinic feldspars are excluded.
Our 1847 attribution is source-linked through historical lexicography and Britannica; Breithaupt’s exact 1847 original page has not been checked here.
Editorial qualification
An historical reference supports date and rationale; do not mark Breithaupt’s original publication directly consulted.
Related names
Feldspar · Igneous · Metamorphic · Granite · Granitic · Albite · Anorthite · Oligoclase · Labradorite · Andesine · Bytownite · QAPF classification · Alkali feldspar · Granodiorite · Tonalite · Anorthosite · Gabbro · Trondhjemite
Sources
- Encyclopaedia Britannica (1911), Plagioclase — Historical explanation of oblique versus right-angle cleavage
- Online Etymology Dictionary, plagioclase — Breithaupt 1847 attribution and Greek components
- Le Maitre et al. (2002), Igneous Rocks: A Classification and Glossary of Terms (IUGS) — International igneous nomenclature and modal proportion conventions