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
![](http://blogs.nvcc.edu/mineralogy/files/2018/10/50HD0005-1024x768.jpg)
![](http://blogs.nvcc.edu/mineralogy/files/2018/10/50HD0007-1024x768.jpg)
Further Reading
Tourmaline at webmineral.com (links to a search results page)
Tourmaline at mindat.org