Earth & SkyGreek → English
Plagioclase
/ˈpleɪdʒioʊkleɪs/noun
The group of feldspar minerals whose two cleavage planes meet at an angle slightly off ninety degrees.
The Greek roots
πλάγιος
plágios
slanting, oblique
κλάσις
klásis
breaking, fracture
Literally: “oblique breaking”
The story of the word
Johann Breithaupt named it in 1847 for a geometric fault: its cleavages meet obliquely rather than square. The name was built as the answer to orthoclase, the right-angled feldspar he had named twenty-four years earlier, so the pair still divides the feldspars in every mineralogy course. Plagioclase makes up a large share of the Earth's crust and most of the pale lunar highlands.
- How it travelled
- Ancient Greek → German → English
- First recorded
- 19th century
Same family
orthoclasemicroclineiconoclastplagiarism
In a sentence
The thin section showed plagioclase laths in a dark groundmass.
Plagioclase composition tracks the temperature the magma cooled at.