Orthoclase is frequently known as potassium feldspar or K-spar, and is easily distinguished from the plagioclase feldspars in hand sample by its lack of striations.

In thin section, alkali feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). You can find the explanation on the alkali feldspar page.

Physical Properties
Chemical formula KAlSi3O8
Class Tectosilicate
3D framework of linked tetrahedra
Crystal system Monoclinic
Habit Prismatic
Stubby to elongate
May be flattened or doubly
Color White
Pink, salmon
Hardness 6
Specific gravity 2.56
Cleavage 90° cleavage angle; perfect (001),
good (010), poor {110}
Fracture Uneven
Luster Pearly
Transparency Translucent
Streak White
Optical Properties
PPL Colorless
Low relief
If altered, may display brownish
speckling or clouding
XPL 1st order gray
Uneven stripes or “wisps”
If altered, may look colorfully speckled
δ 0.007
Twinning Penetration and contact twins are common.
after Perkins, 306

Orthoclase in Hand Sample

Classic peachy-pink orthoclase
More classic peachy-pink orthoclase
Orthoclase with green apatite
Scanning electron micrograph or orthoclase

Orthoclase in Thin Section

Thin Section GigaPans

Thin section, plane polars
Thin section, crossed polars

Orthoclase, PPL

Orthoclase, XPL

Further Reading

Orthoclase at webmineral.com
Orthoclase at mindat.org