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