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 n_w = 1.631-1.968 n_e = 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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