Tourmaline -  Crystallography, Crystal Chemistry and Provenance Potential

Tourmaline is one of the chief seats for boron in the Earth's crust

It is stable from essentially Earth surface conditions to well over 800 C and 60 kbar

It is complex structure with many possible substituents that will reflect the environment of formation of the tourmaline

It is extremely refractory in clastic sedimentary environments - being one of the most prominent heavy minerals

Consequently, it is an exceptionally good indicator of provenance of the detritus found in sedimentary and metasedimentary rocks

Overview of the crystallography and crystal chemistry on the Tourmaline Today website: http://www.geol.lsu.edu/henry/Research/tourmaline/TourmalineToday.htm

Tourmaline provenance studies - classic study (Krynine, 1946) and more recent approach (Henry and Dutrow, 1992)

Krynine (1946) The tourmaline group in sediments. Journal of Geology, 54, 65-87.

Distinguished 5 types of tourmalines based on color, zoning and inclusions

• textural relations such as degrees of roundness and authigenic overgrowths are additional useful descriptors

• used for correlation, interpretation and differentiation of sedimentary units in the Appalachians

Krynine suggests that the similar distribution of tourmaline types of the Bellefonte and Gatesburg formations indicates that the Upper Cambrian Gatesburg formation is reworked and incorporated in the Ordivician Bellefonte formation

Monopolar tourmaline in red beds (Henry et al. 1994)

Rosehill Formation - quartz, chlorite, hematite, tourmaline (+overgrowths)

- Low T (<300 C)

Sector-zoned tourmaline from the cap rock of a sale dome (Gulf of Mexico)

- Henry et al. (1999) EJM

Carbonate-rich cap-rock of salt dome

Mineralogy: calcite, native sulfur, pyrite, quartz, dolomite, hematite, tourmaline (all are oil-coated).

Basis for more recent chemical fingerprinting of detrital tourmaline sources.

Likely optical zoning of metamorphic tourmaline with a detrital core

"Cut" effect of tourmaline with detrital core and multiple stages of metamorphic overgrowth

Zoned tourmaline from matrix of staurolite zone metapelitic schist from Farmington Quad, Maine (Henry and Dutrow, 1996)

Zoning trends from core to rim at the (+) and (-) c-pole of the tourmaline.

Henry and Dutrow (1992) - Tourmaline in a low grade clastic metasedimentary rock: an example of the petrogenetic potential of tourmaline. Contributions to Mineralogy and Petrology, 112, 203-218.

A single sample of a lithic wacke from Farmington Quadrangle (Maine) was metamorphosed to chlorite zone conditions. Retains a variety of optically-, texturally- and chemically-distinct detrital grains Small asymmetric tourmaline overgrowths develop on the detrital cores, and retain two distinct growth episodes.

Varieties of detrital tourmaline

• 10 sources identified in this single thin section (29 grains)

Metamorphic overgrowth signature

• Compositional polarity on tourmaline at either pole

• No volume diffusion noted

• Two stages of metamorphic overgrowth

  • Stage 1 - largest overgrowth volume of intermediate X(Mg) inner rims

  • Stage 2 - lesser volume of growth after pyrite converted to pyrrhotite resulting in Mg-rich outer rims

Detrital blue
BSE image and traverse line (plotted on right). Note the two-staged metamorphic overgrowth. Detrital tourmaline grain chemistry indicates a low Li-granitoid as a likely source.