“Tartan plaid”,  “plaid”, “gridiron,” or “cross-hatch” twinning is a combination of albite and pericline twinning. It is the dead giveaway of microcline.

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 Triclinic
Habit Prismatic, stubby to elongate
Cleavable masses
Irregular grains
Color White
Salmon pink
Hardness 6
Specific gravity 2.56
Cleavage Perfect (001), good (010)
Fracture Conchoidal to uneven
Luster Vitreous
Transparency Translucent
Streak White
Optical Properties
PPL Colorless
Low relief
XPL 1st order grays
Plaid twinning
δ 0.010
Twinning Plaid
after Perkins, 307-308

Microcline in Hand Sample

Amazonite (green microcline)
Scanning electron micrograph of amazonite

Microcline in Thin Section

We wouldn’t normally circle the mineral in question on the slide. For some reason, almost everyone has trouble identifying microcline at first, so we thought we’d throw you a bone.

Thin Section GigaPans

Microcline, plane polars
Microcline, crossed polars

Microcline, PPL

Microcline, XPL

Microcline, XPL

Further Reading

Microcline at webmineral.com
Microcline at mindat.org