Optical Mineralogy

Optical Mineralogy Lab 12 - Fall, 2014 Pleochroism, Interference Colors, and Extinction Angles

Pleochroism • The ability of a mineral to show different colors when viewed along differing crystallographic orientations

Pleochroism Video • Double click to play movie • Watch the grain in the very center

Interference Colors • Interference colors are the colors that you see when a rock thin section is placed between two pieces of polarizing film • This phenomenon is a side effect of light slowing down as it passes through different substances • You observe another effect of light slowing down when you see a straw in a glass of water appear to bend where it passes into the water

Index of Refraction • The slowing down of light as it passes through a substance is measured in a number called the index of refraction • Interference colors are an effect produced by the fact that most solids have more than one index of refraction

Retardation • As the two beams of polarized light pass through a crystal, they travel at different speeds and get out of phase • The slow ray is said to be retarded and the phase difference is called retardation

Integral Retardation • If the retardation is a whole number of wavelengths, the beams recombine with the same orientation as when they entered the crystal • These wavelengths will be blocked by the upper polarizer

Half-integral Retardation • If the retardation is a whole number of wavelengths plus one-half, the beams recombine with an orientation perpendicular to the original direction of polarization • These wavelengths will be fully transmitted by the upper polarizer

Birefringence • The greater the difference between the indices of refraction, the more intense are the interference colors produced • The difference in the index of refraction in two viewing directions is called the birefringence

Michel-Lévy Color Chart • First order colors are to the left, with orders increasing to the right

No Birefringence • Garnet is isometric • Birefringence is zero • Therefore the interference color is black • This is also known as being “in extinction”

Weak Birefringence • Quartz has very low birefringence • Colors are very muted • 1° grays and white

Low Birefringence • Kyanite shows some color • Kyanite grain in center is surrounded by muscovite, biotite, and opaque minerals • Lower image is in pp – note that kyanite is colorless

High Birefringence • Muscovite shows high interference colors • Highest colors (4° and beyond) are pastels

Extinction Angle • The angle formed by one line of the crystal with the extinction position • Either the longest dimension of the mineral or the system of cleavage lines are generally used as this line of reference

Determination of Extinction Angle • Initially, view the crystal in pp • The reference line is rotated so it coincides with the direction of the polarizer (E-W) • The analyzer is inserted • The stage is rotated and turned slowly until extinction occurs • The rotation angle is the extinction angle

Example Movie 1 • Double click to play the movie • The angle is actually determined by noting the position of the stage before and after rotation

Example Movie 2 • Double click to play movie

Parallel Extinction • When the analyzer is inserted, if the crystal is already dark, the extinction angle is 0° • The mineral is said to have a straight, or parallel extinction • Double click to play the movie

Optical Mineralogy