“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 Green 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
Microcline
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
