TOURMALINE

General Formula:
Na(Mg,Fe,Li,Al)3Al6(Si6O18)(BO3)3(OH,F)4
Sample: PT-123 System:
Hexagonal (trigonal)
Radiating Tourmaline
The range of pleochroic colours displayed by the radiating tourmaline crystals is evident. Note that the grains exhibit their lightest and darkest pleochroic colour when the long axis is parallel and perpendicluar, respectively, to the lower polar vibration direction (N-S).
Field of View 2.7 mm, plane light
Radiating Tourmaline
The extinction of the individual grains varies across the grain aggregate, when the long axis is parallel to the polars the grain is extinct.
Field of View 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
highly variable, blue, green, pink, yellow
stongly pleochroic with w > e, basal sections are uniformly dark.
Form euhedral, stubby columnar to acicular crystals with a rounded triangular to crudely hexagonal cross section
Relief
RI
moderate to high positive
nw = 1.631-1.968
ne = 1.610-1.675
Cleavage poorly developed, fractures are conchoidal
Birefringence
Interference Colours
0.015-0.035
up to upper second order, but commonly masked by mineral's colour
Twinning rare
Interference Figure
Optic Sign
2V
uniaxial
negative
Optic Orientation longitudinal sections show parallel extinction and are length fast
Composition highly variable, RI and birefringence increase generally with increasing Fe Alteration fairly stable in weathering environments
Occurrence characteristic mineral in granites and related rocks, in schists, gneisses and phyllites and as a detrital mineral Distinguishing Features crystal habit, distinct pleochrosm. Tourmaline exhibits its darkest pleochroic colour when the long axis of the grain is aligned perpendicular to the lower polar.


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