Transmitted Light
Strong pleochroism in various shades of blue, in greenish, can be gray to black. Monoclinic, amphibole with medium to high relief. Prismatic, columnar to acicular. In XPL, 1st and 2nd order colors. 5-7º extinction, symmetrical in basal sections. ES(+) and no twins, but frequent zoning. B(+/-), 2V = 0-50º. Both images in PPL.
Strong pleochroism between dark blue, blue-green, yellow-green and black. Monoclinic, amphibole with typical cleavage. Medium-high relief. Prismatic, acicular and fibrous (in BIFs). In XPL colors masked by the blue color. ES(-), 0-30º extinction, B(+/-). Glaucophane and tourmaline may be similar. Both images in PPL.
Strong pleochroism between dark blue, yellow to red violet to colorless or very pale blue. Orthorhombic. High relief, cleavage not visible. In XPL, upper 1st order colors. Parallel extinction, ES(-), twins are common. B(-), with 2V = 20-52o. Rare mineral, from aluminum-rich metamorphic rocks. Both images in PPL.
Pleochroic between sky blue, sapphire blue, pinkish, brown and colorless. High relief, one distinct cleavage. In XPL, colors between black and dark gray. Oblique extinction (6-9o), B(-) with 2V = 51-69o. Rare twins, no zonation. Rare mineral from silica-poor metamorphic and magmatic rocks. Both images in PPL.
Strong pleochroism in blue (many other colors are possible for tourmaline: yellow, orange, green, red, black, brown, etc.). Trigonal. High relief, cleavage not visible, fractures perpendicular to elongation. Only sometimes, triangular or pseudo-hexagonal basal sections. Paralell extinction, ES(-). Almost always zoned. U(-), can have 2V of 5º. Both images in PPL.
Weak, irregular pleochroism in blue (diagnostic!) and violet tones, but usually colorless. Trigonal. Prismatic, tabular or barrel-shaped. No cleavage. 2 partitions at 94º. Extinction tends to parallel. ES(-) in prismatic crystals. Very frequent twins and zoning. U(-), 2V of 5-7º possible. Left image: PPL. Right image: XPL.
Very commonly exhibits irregular blue or violet zonation, simulates pleochroism, but is not pleochroic. Relief moderate. In PPL, usually colorless. Cubic. Perfect (111) cleavage: or two cleavages intercept at 70º/110º or three at 60º/120º. Usually anhedral crystals, sometimes cubic. In XPL, isotropic, anomalous birefringence is possible. Both images in PPL.
Moderate to strong pleochroism between bluish, green, green-gray, colorless or yellow-green. Monoclinic/triclinic, pseudo-hexagonal. Hourglass zoning possible. High relief, cleavage not visible. In XPL, lower 1st order colors or anomalous. Extinction tends to oblique. ES(-), twins common, B(+/-). Left image in PPL, right image in XPL, rotated 90 degrees.
Amphiboles with strong pleochroism in green colors, may be bluish. Monoclinic. Basal sections with 2 cleavages at 56º-124º (pseudo-hexagonal!). Prismatic short to long, medium-high relief. In CPL, colors from upper 1st to lower 2nd order. Oblique extinction (14-22º), ES(+), twins and zoning common. B(-), 2V=34-90º.
ANTHOPHYLLITE: amphibole, paralell extinction, ES(+), B(+/-), no twins, no zonation.