Transmitted Light
Less common pseudo-orthorhombic nesosilicate, occurs in metamorphic rocks.
PPL: Moderate, sometimes weak, pleochroism from colorless to yellow (pale to intense) to golden. Distinct cleavage, high relief. May have many inclusions (“sieve texture”). Short prisms with pseudo-hexagonal sections.
XPL: Low interference colors (only up to 1st order orange - very diagnostic!). ES(+), B(+), 2V from 80-90º.
Images: Staurolite in PPL, rotated 90 degrees.
Rarer triclinic phyllosilicate of some specific metamorphic rocks.
PPL: Strong pleochroism in various shades of yellow and brown to greenish. Medium-high relief, 1 perfect cleavage. Lamellae, fibers and needles.
XPL: Intense 2nd to 4th order colors. Paralell extinction. B(-) with 2V from 0º (pseudo-uniaxial) to 30º. Looks like biotite, but has no mottled extinction!
Images: Stilpnomelane in PPL, rotated 90 degrees.
Relatively rare tetragonal Sn-oxide, occurs associated with granites.
PPL: Weak to strong pleochroism in brownish and reddish (may be orange or green). Very high relief, cleavage not visible.
XPL: Its own color masks the very high interference colors. Extinction tends to parallel. ES(+), twins are very common. U(+), can have anomalous 2V up to 38º.
Images: Cassiterite in PPL, rotated 90 degrees.
Rare monoclinic sorosilicate of Mn-rich metamorphic rocks.
PPL: Strongly pleochroic in red (may be pink, yellow, orange or violet). High to extremely high relief, 1 perfect cleavage. Long prisms, sections with 6 sides.
XPL: Intense colors, masked by the red color of the mineral. 0-34º or parallel extinction. ES(+/-), B(+/-).
Images: Piemontite in PPL.
Somewhat rarer monoclinic phyllosilicate, typical mineral of rocks undersaturated in silica and Ca-rich.
PPL: colorless, but may have moderate brown pleochroism in sections perpendicular to (001); sections parallel to (001) almost without pleochroism. Moderate relief, perfect cleavage, lamellar habit.
XPL: Paralell and mottled extinction. ES(+) and B(-) with small 2V, can simulate to be uniaxial. Very similar to biotite.
Images: Phlogopite with calcite in PPL, rotated 90 degrees.
Most common iron hydroxide, occurs in many types of rocks.
PPL: Strong orange colors, can be red-brown. Pleochroism of variable intensity. It forms spherulites or radiated fibrous aggregates. The structure often resembles that of chalcedony.
XPL: The color is almost the same. It has high birefringence and parallel extinction.
Images: Goethite in PPL.
A somewhat rarer monoclinic amphibole, of intermediate and acidic igneous rocks.
PPL: Strong pleochroism between deep brown and reddish brown. Basal sections with 2 cleavages at 56º and 124º. Moderate to high relief.
XPL: Extinction 0-19º. Intense 1st to 2nd order colors, frequent zoning, B(-). Similar to biotite, but without mottled extinction.
Images: Brown hornblende in PPL, rotated 90 degrees.
APATITE: usually colorless, but may appear brownish to blue-gray and orange.
