Tourmaline

Physical Properties
Chemical formula (Na,Ca)(Fe,Mg,Al,Li)3Al6(BO3)3Si6O18(OH)4
Class Cyclocilicate
Rings of linked tetrahedra
Crystal system Hexagonal (rhombohedral)
Habit Elongate, striated trigonal prisms
Parallel or radiating aggregates
Massive and compact
Color Variable and often zoned
Black
Blue
Green
Red
Pink
Colorless
Hardness 7 to 7.5
Specific gravity 2.9
Cleavage Poor {101}, poor {110}
Fracture Subconchoidal
Luster Resinous
Vitreous
Transparency Translucent
Streak White
Optical Properties
PPL Strong and variable color
Strong and variable pleochroism
XPL Color usually masks birefringence
2nd to 3rd order colors
δ 0.020 to 0.030
after Perkins, 336-337

Tourmaline in Hand Sample

Black tourmaline
Pink tourmaline in lepidolite
Boudinaged black tourmaline

Tourmaline in Thin Section

Tourmaline in tourmaline schist, plane polars
Tourmaline in tourmaline schist, crossed polars

Tourmaline, plane polars

Tourmaline, Plane Polars

Tourmaline in plane polarized light. Magnification: 40x Rock: Tourmaline schist Click here to see this grain rotate in plane polars: https://youtu.be/pXGloKq…

Tourmaline, crossed polars

Tourmaline, Crossed Polars

Tourmaline in cross polarized light. Magnification: 40x Rock: Tourmaline schist Click here to see this grain rotate in plane polars: https://youtu.be/BiKnJbm…

Further Reading

Tourmaline at webmineral.com (links to a search results page)
Tourmaline at mindat.org