Research / Geoscience / Language
Alkali feldspar
A feldspar subgroup dominated by potassium and sodium end-members; in QAPF usage, A also includes albite-rich compositional fields by convention.
Language and route: English compound, ultimately Arabic + German. English alkali (ultimately Arabic al-qaly, plant ashes) + German-derived feldspar
Literal gloss: alkali-metal feldspar
The name and its history
No single introducer established · Established mineral classification
- What does the word literally mean?
- The gloss ‘alkali-metal feldspar’ describes the linguistic allusion. The original name and its modern scientific definition must be kept distinct.
- What did the original author mean?
- A chemical/mineralogical subgroup rather than a single mineral species.
- Why was the name needed?
- Mineralogists needed a chemical basis to differentiate sodium–potassium feldspars from calcium-rich plagioclase members.
- What evidence supported the original concept?
- Alkali metal occupancy, solid solution, symmetry and structure.
- How has geology changed the concept?
- The A corner of QAPF is a modal classification convention and should not be naively read as a list of approved mineral species.
Further historical context
This compositional label provides a useful contrast to the appearance- and cleavage-based feldspar names.
Related names
Feldspar · Albite · Orthoclase · Microcline · Sanidine · QAPF classification
Sources
- Streckeisen (1976), IUGS QAPF mineral-component definitions — QAPF classification
- University of Waterloo Earth Sciences Museum, feldspar family — Mineral-family teaching classification