Darrell J. Henry and Charles V. Guidotti, 1985, Tourmaline as a petrogenetic indicator mineral: an example from the staurolite-grade metapelites of NW Maine, American mineralogist, vol 70 p 1-15

Franco Piranjno and R. H. Smithies, 1992, The FeO/(FeO + MgO) ratio of tourmaline: a useful indicator of spatial variations in granite-related hydrothermal mineral deposits, Journal of Geochemical Exploration, vol. 42, p 371-381

Vincent J. van Hinsberg et al., 2006, Hourglass sector zoning in metamorphic tourmaline and resultant major and trace-element fractionation. American mineralogist, v 91, p 717-728

Vincent J. van Hinsberg, John C. Schumacher. 2007, Intersectorelement partitioning in tourmaline: a potentially powerful single crystal thermometer. Contrib Mineral Petrol vol. 153. p. 289-301

TourmalineHardness : 7 – 7 ½

Density : 3.0 – 3.25

'Absence' of cleavage

Hexagonal: class 3m , space group R3m

Unit cell : a = 15.95 , c = 7.24

General Formula

XY3Z6T6O18(BO3)V3W(Na,Ca)(Li,Mg,Al)(Al,Fe,Mn)6

(BO3)3(Si6O18)(OH)4

In the Mg-Li-Fe solid solution : schorl (Fe), elbaite(Al-Li), dravite (Mg)

Schorl and elbaiteusually granitic and pegmatitic, whereas dravite is more commonly sedimentary (detrital) and metamorphic (Pirajno , 1992)

Related to weathering susceptibility of host rock?

Cyclosilicate: rings of SiO4 tetrahedron

(note the orientation)Reactive ions (Na,Ca) occupies the rings' centre (X site) as a dodecahedron

Densely packed unit cell

Bond length generally close to 2 Å

No obvious plane of weaker bonds

Symmetry

Absence of any symmetry element perpendicular to the long axis (C axis).

ZoningTourmaline composition factors

P, TChemical environmental− Bulk composition− Mineral paragenesis− Fluid composition

Core-rim zoning, perpendicular to C axis

What's happening along the C axis ?

Core to rim abrupt colour change

Zoned rim vs homogenous core

Constant colour variations

Composition change

Possible causeChange in host rock paragenesis.

Detrital provenance of the core.

Homogenization at thermal peak of metamorphism.

Respective problemsProperties are inconstant with continuous growth.

Similarities between core and rim, widespread occurrence

Variability of core to rim relative diameter ratio, textural relationship often inconsistent

Hourglass sector zoningVincent Van Hinsberg et al., 2006, Hourglass sector zoning in metamorphic tourmaline and resultant major and trace-element fractionation. American mineralogist, v 91, p 717-728

Hourglass sector zoning along the c axis

Observed in other mineralsNon symmetrical composition in both directionsInclusions may lead to the presence of shadows

Electron microprobe wavelength dispersive element X-ray maps

Many compositional variation between sectorsSuperimposition of sector zoning and growth zoningCompositional differences are variable relative to the distance along the c axis

Why ?Uneven incorporation of elements in different growth surfaces (which face does what?).

Dipolar charge of the tetrahedron ringsSite morphology of the r faces

Preservation of crystal disequilibrium caused by slow diffusion rate.

Is this type of sector zoning present tourmaline from other geological settings ?

.Hourglass sector zoning has now been observed in tourmaline from other geological settings.

Magmatic tourmaline seems to show the hourglass sector zoning along the o face more frequently.

Intersector element partitioningVincent j. Van Hinsberg, John C. Scumacher. 2007. Intersector element partitioning in tourmaline: a potentially powerful single crystal thermometer. Contrib Mineral Petrol vol. 153. p. 289-301

ConclusionProperties derived from the crystal structure of tourmaline make a good potential mineral indicator.

Abundance reduces the direct use of its presence as a discriminant information.

Variability in chemical composition may hint to the crystallization environment.

Sector zoning, along with growth zoning, need to be taken into account in order to obtain valid tourmaline chemistry assessment.

Intersector element partitioning may reveal information on crystallization environment history.

Other use: Spatial relation indicator in granite-related hydrothermal depositsHowever: needs a well constrained system, dependance on wall rock composition.Relation with zoning ?

(Pirajno, 1992)