Transmitted Light
Rare monoclinic amphibole of high P and low P metamorphic rocks.
PPL: Strong pleochroism in various shades of blue and lilac, may be greenish, gray to black. Medium to high relief. Prismatic, columnar to acicular.
XPL: 1st and 2nd order colors. 5-7º extinction, symmetrical in basal sections. ES(+) and no twins, but frequent zoning. B(+/-), 2V = 0-50º.
Images: Glaucophane in PPL.
Rare monoclinic inosilicate found in Na-rich igneous rocks and in some metamorphic rocks.
PPL: Strong pleochroism between dark blue, blue-green, yellow-green and black. Typical amphibole cleavage. Medium-high relief. Prismatic, acicular and fibrous (in BIFs).
XPL: Interference colors masked by the own blue color. ES(-), 0-30º extinction, B(+/-). Glaucophane and tourmaline may be similar.
Images: Riebeckite in PPL, rotated 90 degrees.
Very rare orthorhombic nesosilicate, of Al-rich metamorphic rocks.
PPL: Strong pleochroism between dark blue, yellow to red violet to colorless or very pale blue. High relief, cleavage not visible.
XPL: Upper 1st order colors. Parallel extinction, ES(-), twins are common. B(-), with 2V = 20 – 52º.
Images: Dumortierite in PPL, rotated 90 degrees.
Very rare monoclinic inosilicate of Al/Mg-rich, Si-poor metamorphic rocks.
PPL: Pleochroic between sky blue, sapphire blue, pinkish, brown and colorless. High relief, one distinct cleavage.
XPL: Interference colors between black and dark gray. Oblique extinction (6-9º), B(-) with 2V = 51 – 69º. Rare twins, no zonation.
Images: Sapphirine in PPL, rotated 90 degrees.
Less common trigonal cyclosilicate of the Tourmaline Group (>40 different minerals).
PPL: Moderate pleochroism in blue (many other colors are possible, like green, yellow, red, black and brown). High relief, cleavage not visible, fractures perpendicular to elongation. Basal sections (without pleochroism!) triangular or pseudo-hexagonal.
XPL: Paralell extinction, ES(-), almost always zoned. U(-), can have 2V of 5º.
Images: Tourmaline with blue colors in PPL.
Rare aluminium oxide.
PPL: May show weak, irregular pleochroism in blue (diagnostic!) and violet tones, but usually colorless. High relief. Prismatic, tabular or barrel-shaped. No cleavage. 2 partitions at 94º.
XPL: Gray colors. Extinction tends to parallel. ES(-) in prismatic crystals. Very frequent twins and zoning. U(-), 2V of 5-7º possible.
Images: Corundum in PPL and in XPL.
Very common cubic halide, often associated to granites, but can occur in many types of rocks.
PPL: Colorless, but very commonly exhibits irregular blue or violet zonation. Simulates pleochroism in blues, but is not pleochroic. Relief moderate. Perfect (111) cleavage: or two cleavages intercept at 70º/110º or three at 60º/120º. Usually anhedral crystals, sometimes cubes.
XPL: As it is a cubic mineral, it is isotropic (completely black). Anomalous birefringence is possible.
Images: Fluorite in PPL with zonation in blues.
Rare triclinic nesosilicate of specific types of metamorphic rocks.
PPL: Moderate to strong pleochroism between bluish, green, green-gray, colorless or yellow-green. Hourglass zoning possible. High relief, cleavage not visible.
XPL: Low 1st order colors or anomalous. Extinction tends to oblique. ES(-), twins common, B(+/-).
Images: Chloritoid in PPL, rotated 90 degrees.
Very common monoclinic amphibole.
PPL: Strong pleochroism in green, yellow-green, bluish-green and bluish. Two cleavages at 56º-124º in the basal sections (pseudo-hexagonal, rhombic). Long to short prismatic, medium-high relief.
XPL: 1st to end 2nd order colors, oblique extinction (14-22º), ES(+), twins and zoning are common, B(-).
Images:
ANTHOPHYLLITE: amphibole, paralell extinction, ES(+), B(+/-), no twins, no zonation.