Orthoclase
A monoclinic K-feldspar mineral, KAlSi₃O₈, distinguished from sanidine and microcline by structural state.
Language and route: French/German ← Ancient Greek. Haüy’s French orthose (1801) from Greek ὀρθός, ‘straight, right’; Breithaupt’s German Orthoklas (1823) adds the ‘fracture/cleavage’ element.
Literal gloss: straight fracture / right cleavage
The name and its history
René Just Haüy (*orthose*); August Breithaupt (*Orthoklas*) · 1801; 1823
- What does the word literally mean?
- The transmitted name is Haüy’s French orthose (1801) from Greek ὀρθός, ‘straight, right’; Breithaupt’s German Orthoklas (1823) adds the ‘fracture/cleavage’ element. The short gloss ‘straight fracture / right cleavage’ is a guide to the naming allusion, not by itself a modern mineralogical definition.
- What did the original author mean?
- Haüy’s orthose described feldspar crystals with cleavage faces approaching a right angle. Breithaupt later adapted the term to Orthoklas; the two dates refer to distinct naming acts.
- Why was the name needed?
- Crystallographers needed observable angles and cleavage distinctions to order minerals when structural analysis and full chemical classification were unavailable.
- What evidence supported the original concept?
- Haüy’s cleavage-based crystallography, followed by Breithaupt’s mineral descriptions, supplied visible geometric evidence; neither was based on modern X-ray diffraction.
- How has geology changed the concept?
- Modern feldspar identification relies on symmetry and Al–Si order as well as composition. ‘Orthoclase’ is not a catch-all for all pink K-feldspars.
Further historical context
Haüy’s orthose described feldspar crystals with cleavage faces approaching a right angle. Breithaupt later adapted the term to Orthoklas; the two dates refer to distinct naming acts.
Crystallographers needed observable angles and cleavage distinctions to order minerals when structural analysis and full chemical classification were unavailable.
Haüy’s cleavage-based crystallography, followed by Breithaupt’s mineral descriptions, supplied visible geometric evidence; neither was based on modern X-ray diffraction.
Modern feldspar identification relies on symmetry and Al–Si order as well as composition. ‘Orthoclase’ is not a catch-all for all pink K-feldspars.
Related names
Sources
- Mindat, Orthoclase — Haüy 1801 and Breithaupt 1823 — Haüy’s orthose and later orthoklas; modern species identity
- Mindat, Microcline — Breithaupt (1830) — Name, original Norwegian material and structural meaning
- IMA–CNMNC, List of Mineral Names (updated September 2026) — Contemporary mineral-species standing; consult live master list
- Haüy (1801), Traité de minéralogie, digitised edition — Original-period mineralogical treatise; do not assume exact page verified