Feldspar
Feldspar is not a single mineral but a group of related aluminosilicate minerals, comprising the potassium feldspars, orthoclase, microcline, and sanidine, and the plagioclase series running from albite to anorthite, together making up roughly 60 percent of Earth's crust by volume. All share a similar framework silicate structure, two cleavage directions near 90 degrees, and moderate hardness, but differ in composition, color, and crystal system. As the most abundant mineral group on the planet, feldspar defines granite, basalt, gneiss, and countless other rock types.
Feldspar properties at a glance
| Type | Feldspar Group |
|---|---|
| Chemical formula | (K,Na,Ca)AlSi3O8 – CaAl2Si2O8 (solid solution series) |
| Mohs hardness | 6–6.5 |
| Crystal system | Monoclinic to Triclinic (depending on species) |
| Luster | Vitreous to pearly |
| Streak | White |
| Cleavage | Perfect in two directions (near 90° or 86°) |
| Fracture | Uneven to conchoidal |
| Specific gravity | 2.55–2.76 |
| Transparency | Transparent to translucent |
| Common colors | White, Pink, Gray, Green (amazonite), Colorless |
| Rarity | common |
Geology & background
Feldspars crystallize across an extraordinary range of geologic settings, from slowly cooled granite and pegmatite bodies, where large well-formed crystals develop, to rapidly chilled volcanic rocks where they appear only as tiny groundmass grains or phenocrysts. Potassium feldspar and plagioclase also form during metamorphism and, less commonly, through sedimentary processes such as arkose forming from feldspar-rich sand. Notable crystal localities include pegmatites in Madagascar and Brazil for large gem-quality specimens, and Norway and Finland for labradorite and moonstone varieties prized for their optical play of color.
Collectors are often drawn to the gem varieties within the feldspar group, including labradorite's iridescent labradorescence, moonstone's adularescent sheen, and sunstone's glittering copper inclusions, all arising from internal structural features rather than trace-element color. Because feldspar species can look nearly identical without close inspection, rockhounds rely on twinning patterns, cleavage angles, and associated minerals to tell orthoclase from plagioclase out in the field. Weathering matters too: feldspar breaks down over time into clay minerals, a process fundamental to soil formation.
How to identify Feldspar
Feldspars are generally white, gray, pink, or green with a vitreous to pearly luster on cleavage faces and a white streak, and they show two cleavage directions meeting at or near 90 degrees, with hardness around 6. Plagioclase varieties typically display fine parallel striations on one cleavage face from polysynthetic twinning, while potassium feldspars like microcline instead show a cross-hatched tartan twin pattern under magnification. Crystal habit ranges from blocky to tabular. Feldspar gets confused with quartz fairly often, though quartz lacks cleavage and has a distinctly greasier luster.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Feldspar is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Feldspar worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Feldspar minerals collectively make up roughly three-fifths of Earth's continental crust, making the group the most abundant of any mineral family.
- The word feldspar derives from German terms for field and a word for a certain crystal type, reflecting its common appearance as loose grains in plowed farmland.
- Weathered feldspar is the primary source of clay minerals like kaolinite, linking this common mineral group directly to soil formation and ceramics.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
