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
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.|
|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|
|moderate to high positive
nw = 1.631-1.968
ne = 1.610-1.675
|Cleavage||poorly developed, fractures are conchoidal|
up to upper second order, but commonly masked by mineral's colour
|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.|