1. Tourmaline general formula
XY3Z6(BO3)3Si6O18(OH)3(OH,F)
X = Na, Ca, K
Y = Mg, Fe, Mn, Li, Cr+3, V+3, Ti, Al
Z = Al, Mg, Fe, V+3, Cr+3

2. Tourmaline

3. Elbaite Na(Li,Al)3Al6(BO3)3Si6O18(OH)4

4. Dravite NaMg3(Al,Fe)6(BO3)3Si6O18(OH)4

5. Schorl NaFe3(Al,Fe)6(BO3)3Si6O18(OH)4

6. Uvite Ca(Mg,Fe)3Al5Mg(BO3)3Si6O18(OH)4

7. Buergerite NaFe3Al6(BO3)3Si6O18(OH)3F

8. Some tourmaline end members
Elbaite Na(Li,Al)3Al6(BO3)3Si6O18(OH)4
Dravite NaMg3(Al,Fe)6(BO3)3Si6O18(OH)4
Schorl NaFe3(Al,Fe)6(BO3)3Si6O18(OH)4
Uvite Ca(Mg,Fe)3Al5Mg(BO3)3Si6O18(OH)4
Buergerite NaFe3Al6(BO3)3Si6O18(OH)3F
Foitite Na(Fe,Al)3Al6(BO3)3Si6O18(OH)4
Liddicoatite Ca(Li,Al)3Al6(BO3)3Si6O18(OH,F)4
Rossmanite LiAl3Al6(BO3)3Si6O18(OH)4
Feruvite (Ca,Na)(Fe,Mg,Ti)3(Al,Fe)6(BO3)3Si6O18(OH)4
Chromdravite NaMg3Cr6(BO3)3Si6O18(OH)4
Olenite NaAl3Al6(BO3)3Si6O18(OH)4
Povondraite NaFe3Fe6(BO3)3Si6O18(OH)4

9. Physical properties of tourmaline
Density (g/cm3) 3.0 (elbaite) (schorl)
Hardness
Refractive indices 1.62, 1.68 (elbaite)
Birefringence
Piezoelectric and pyroelectric
Often displays color zoning

10. Unit cell data for tourmaline
Space group R3m
a = Å
c = Å
V = Å3
a = 90°
g = 120°
Z = 3

11. Atomic positional parameters of tourmalineX Y Z Si B H O O O O O O O O

12. Tourmaline: looking down (001)

13. Tourmaline: looking down (001)

14. Solid solution between tourmaline end members

15. Chemistry of tourmaline
From Dyar et al.
B: pfu
Li: pfu
F: pfu
H: pfu
Si: pfu
Fe+3/Total Fe:
Fe+2 occupies 3 different Y sites.
Fe+2 and Fe+3 can be in both Y and Z octahedral sites.
There may also be a small amount of [4]Fe+3

16. Tourmaline structure is flexible
From Foit et al.
Tetrahedral bond angles inversely proportional to alkali-O bond length
Distortion of silica ring (ditrigonality) inversely proportional to 9b octahedron angle variance
Unit cell dimensions increase as 9b and 18c octahedra dimensions increase

17. Uses of tourmaline Gemstones
Electronics (pyroelectric and piezoelectric)