Transmitted Light
Very common monoclinic amphibole.
PPL: Strong pleochroism in green, yellow-green to bluish-green. 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: Green hornblende in PPL and in XPL.
Very common monoclinic mica.
PPL: Strong pleochroism in green (or brown or red-brown or yellow or red). Medium relief. One perfect cleavage if not altered. Basal sections without cleavage.
XPL: 3rd-4th order colors, masked by the strong mineral color. Parallel and mottled extinction. ES(+), B(-).
Images: Biotite in PPL and in XPL, showing some of the possible colors.
Typical monoclinic pyroxene alkaline igneous rocks.
PPL: Strong pleochroism between deep green, grass green and yellow-brown. High relief. Short columnar to acicular crystals.
XPL: Intense 3rd and 4th order colors. 0-10º extinction, ES(-), zoning and twins are common. B(+/-). Its deep colors mask the interference colors.
Images: Aegirine in PPL showing typical pleochroism.
Less common trigonal cyclosilicate of the Tourmaline Group (>40 different minerals).
PPL: Strong pleochroism in green (may be in many different colors like yellow, blue, 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(-), frequent zonation. U(-), can have 2V of 5º.
Images: zoned longitudinal and basal sections of tourmaline.
Rare monoclinic amphibole of alkaline rocks.
PPL: Very deep colors with pleochroism in brownish-green, blue-gray to brownish-green. It may look like an opaque mineral! Two cleavages at 56º/124º in the basal sections. Various habits.
XPL: Colors are almost the same. 5-9º extinction, ES(-), twins and zoning possible, B(+/-).
Images: Arfvedsonite in PPL.
Rare monoclinic pyroxene of syenites and alkaline granites.
PPL: Strong pleochroism between blue-green, dark blue, yellow-green and black. Medium to high relief, Prismatic, acicular and fibrous habit.
XPL: Colors up to end 1st order. Extinction 0-30º, ES (-), B(+/-).
Images: Hedenbergite in PPL and in XPL.
Rather rare monoclinic sorosilicate, typical of specific metamorphic rocks.
PPL: Weak to moderate pleochroism between colorless, yellowish, light green and blue-green. Zoning common. Lamellar, acicular or fibrous. Medium-high relief, 2 good cleavages.
XPL: 1st and 2nd order colors or anomalous (indigo blue or leather brown). SE(+/-), Twins possible. B(+), 2V 7º-110º.
Image not available yet.
Rare triclinic nesosilicate of specific types of metamorphic rocks.
PPL: Moderate to strong pleochroism between green, green-gray, colorless, bluish, 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.
