Earth & SkyGreek → English
Orthoclase
/ˈɔrθəkleɪs/noun
The common potassium feldspar, whose two cleavage directions meet at exactly ninety degrees.
The Greek roots
ὀρθός
orthós
straight, upright, right
κλάσις
klásis
breaking, fracture
Literally: “right-angled breaking”
The story of the word
Breithaupt named it in 1823 for the square corner where its cleavages meet, and twenty-four years later coined plagioclase for the feldspars that break obliquely. The pair still organises the whole family. Orthoclase is what gives many granites their pink, and the same ὀρθός stands at the front of orthodox, right opinion, and orthodontics, straightening teeth.
- How it travelled
- Ancient Greek → German → English
- First recorded
- 19th century
Same family
plagioclaseorthodoxorthodonticsiconoclast
In a sentence
Orthoclase sits at six on the Mohs scale, so a steel blade will not scratch it.
Pink orthoclase crystals stood out against the grey quartz.