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
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
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.