Smith College - Geology 222b - Petrology
Petrographic Data File

Tourmaline
Property
Value
Comments
Formula Na(Mg,Fe,Mn,Li,Al)3 Al6 [Si6 O18](BO3 )3 (O,OH,F)4

Tourmaline is a Ring Silicate (Si:O ratio = 1:3). It is a solid solution series with multiple end members including dravite, schorl, and elbaite.
Crystal System Hexagonal (trigonal), 3m  
Crystal Habit Striated hexagonal prisms  
Cleavage {110} and {101} Very poor cleavages
Color/Pleochroism Blue, red, green, yellow, black, brown. (Schorl, the most common tourmaline, is black) Strongly pleochroic
Optic Sign Uniaxial (-)  
Refractive Indices
omega =
  epsilon =
1.631-1.650
1.635-1.675 		 
Max Birefringence 0.017-0.035 Cannot be used as a reliable guide to composition
Extinction Longitudial sections show parallel extinction and are length fast.  
Unit Cell

a = 15.8-16.0 Å
c = 7.1-7.25 Å
Z = 3

 
Physical Properties
H = 7
G = 2.90-3.22		  
Distinguishing Features

- In hand sample: columnar crystals with rounded triangular cross sections.
- Concentric color zoning maybe observed in end sections.
- Deep color may mask interference colors.
- In plane polarized light, tourmaline crystal experiences maximum absorption perpendicular to polarizer (and c-axis). This means that they appear darkest when the long grain is oriented perpendicular to the polarizer. (Strongly Dichroic).

Occurrence Tourmaline is a characteristic mineral in granitic pegmatites. It is an accessory mineral in granite, granodiorite, and related felsic rocks. Tourmaline is also a common mineral in schist, gneiss, quartzite, and phyllite. Also found in metasomatically altered limestone and dolomite in contact metamorphic zones.
References Minerals in Thin Section (2000) Perkins, Dexter and Henke, Kevin. Introduction to Opticial Mineralogy (1991) Nesse, William. Introduction to Mineralogy (2000) Nesse, William.
Editors

Data sources: Emma Anderson ('05), Clementine Hamelin ('13), Olivia Leadbetter ('19).

 
Plane polarized image of tourmaline, which is the small green grain in the center of the image. Note the concentric zoning of colors. Also visible are biotite (tan), muscovite (colorless), quartz, and plagioclase. Click on the image to see a larger version with rollover to crossed polarized light.
Tourmaline crystal in plane polarized light and cross polarized light. Tourmaline, the olive-green colored grain, presents zoning. Click on the link for a PPL and XPL rollover view.
Concentric zoning of Tourmaline crystal in plane polarized light. Click on the link for a PPL and XPL rollover view.
Tourmaline crystal oriented with long grain direction parallel to polarizer in plane polarized light. Minimum absorbance observed. Click on the link to see change color as grain is rotated so that its long direction is perpendicular to the E-W polarizer direction in the rollover view.
WWW Images

University of North Carolina
University College London
Hunterian Museum

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