Alkaline amphibole: Arfvedsonite

Refractive indices and pleochroism 

nα=1.693 (dark blue green)

nβ=1.696 (brown yellow)
nγ=1.698 (greyish blue green)

Birefringence 
δ= 0.005 

difficult to observe interference colors due to the strong interference colors

Optical axes angle
2V= -70 to -30 ° (γ is the obtuse bisectrix)

Special diagnostic properties

Occurs in peralkaline rocks, particularly in peralkaline granites and nepheline syenites. Commonly associates with aegirine (sodic pyroxene). Similar to riebickite and is difficult to distinguish. However, riebickte has a larger angle between α and c. Glaucophane (another alkaline amphibole) has different violet and purple blue pleochroism colors, and the slow direction is aligned along elongation. Glaucophane occurs in metamorphic rocks (blue schists and eclogites).

Arfvedsonite thin section gallery

A grain of arfvedsonite (center of each image) surrounded by quartz-feldspar aggregate. The upper row demonstrates pleochroism in PPL. The middle row images are in XPL. And the bottom middle image is taken with gypsum plate inserted. The grain goes extinct in XPL at an angle of ~10°. The birefringence colors are low, it is difficult to name the color and order. Oriented parallel to the fast direction of the gypsum plate, the birefringent colors difficult to observe. One little white-ish spot at the lower left part of the grain went from white to blue, indicating length fast. So, the alpha direction (in this particular arbitrary cut grain) is oriented at a small angle to elongation and cleavage, and exhibits steel greenish grey pleochroism color, while the slow direction is dark blue (almost opaque).

A cluster of arfvedsonite crystals surrounded by quartz-feldspar matrix with needles of astrophyllite. Arfvedsonite shows the typical amphibole cleavage and dark blue color to brown yellow pleochroism. Two planes of cleavage intersect at ~60&120° angles. Top two panels are taken in PPL, bottom panels are in XPL. The images on the right are rotated 90° anti-clockwise compared to those on the left.