Transmitted Light
Moderate pleochroism from colorless to yellow (pale to intense) to golden. Monoclinic, pseudo-orthorhombic. Distinct cleavage, high relief. May have many inclusions (“sieve texture”). Short prisms with pseudo-hexagonal sections. In XPL, low interference colors (only up to 1st order orange - very diagnostic!). ES(+), B(+), 2V from 80-90º. Both images in PPL,
One of the pleochroism colors is yellow, the other one usually is brown or brownish or deep brown or green in light to deep green colors. Monoclinic. Moderate relief. A perfect cleavage if not altered. In XPL, 3rd and 4th order colors, masked by the strong mineral color. ES(+). Very common, alters to chlorite, which is green in PPL. Phlogopite is similar. Both images in PPL.
Strong pleochroism in various shades of yellow and brown to greenish. Triclinic. Medium-high relief, 1 perfect cleavage. Lamellae, fibers and needles. In CPL, 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. Both images in PPL.
Strong pleochroism in yellow and/or orange (many other colors are possible, like green, blue, red, black and brown). Trigonal. High relief, cleavage not visible, fractures perpendicular to elongation. Basal sections (without pleochroism!) triangular or pseudo-hexagonal. Paralell extinction. ES(-), almost always zoned. U(-), can have 2V of 5º. Both images in PPL.
Weak to strong pleochroism in yellow, usually brownish and reddish (orange or green possible). Tetragonal. Very high relief, cleavage not visible. In XPL, the color is almost the same as in PPL. Extinction tends to parallel, ES(+). Twins are common. U(+), anomalous 2V up to 38º.
Yellow to dark brown color with slight pleochroism. High relief, usually massive, usually in submicroscopic crystals. The yellow or brown color seen in PPL remains in XPL because it is very strong. Sulfate of secondary formation. Image in PPL, color in XPL is practically the same.
Intense yellow or green color. Clay mineral of secondary origin (alteration) often in volcanic rocks. Submicroscopic sizes, for conclusive identification other techniques must be used. In XPL, strong colors, masked by the yellow color of the mineral. Image in PPL.
Some other minerals may, rarely, present yellow colors with generally weak pleochroism, usually associated with other colors, which are the dominant ones:
