Anorthite

Anorthite is the calcic end member of the plagioclase feldspar series.

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

Physical Properties
Chemical formula CaAl2Si2O8
Class Tectosilicate
3D frameworks of linked tetrahedra
Crystal system Triclinic
Habit Prismatic
Tabular
Bladed
Cleavable masses
Irregular grains
Color White
Grey
Hardness 6 to 6.5
Specific gravity 2.76
Cleavage Perfect (001), good (010),
poor {110}
Fracture Uneven
Luster Vitreous
Pearly
Transparency Translucent
Streak White
Optical Properties
PPL Colorless
Low relief
Non-pleochroic
XPL 1st order greys
Polysynthetic/albite twinning (“zebra stripes”)
δ 0.013
Twinning Polysynthetic/albite

Twins may be wider than the twins in albite,
but x-ray diffraction or other tests are
required to differentiate them for certain.

after Perkins, 309

Anorthite in Hand Sample

Anorthite is the dark mineral in this hand sample.

Scanning electron micrograph of anorthite

Anorthite in Thin Section

Thin Section GigaPans

Anorthite in plane polars

Anorthite in crossed polars

Anorthite, PPL

Anorthite, XPL

Further Reading

Anorthite at webmineral.com

Anorthite at mindat.org