Optical_Properties_of_Minerals_F10

Optical_Properties_of_Minerals_F10 - Optical Properties of...

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1 Optical Properties of Minerals GLY 4200 Fall, 2010
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2 Path Differences In Crystals Waves entering an anisotropic crystal will generally experience two indices of refraction in two perpendicular directions Even plane-polarized radiation will be split into the Ordinary (O) and Extraordinary (E) vibrating
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3 Retardation Path differences upon passing through a crystal are called retardation (of the slower ray relative to the faster) Δ = λ in this case
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4 Accessory Plates Quartz sensitive tint, gypsum plate, 1 st order red has Δ = 550 nm Mica or quarter-wave plate has Δ = 150 nm Quartz wedge has variable Δ Generally, the slow direction is indicated on the plate (N) – the fast direction (n) is unmarked, but is perpendicular
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5 Accessory Plate Photo Left to right: Quarter wave plate; full wave sensitive tint plate; quartz wedge Viewed in crossed- nicols
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6 Quartz Sensitive Tint Plate Usually made of gypsum, mica, or quartz Mineral is cut parallel to the optic axis of the crystal, to such a thickness that the O-rays and E-rays for green light ( l = 540 nm) are out of phase by exactly one wavelength The analyzer therefore extinguishes green light, but permits other wavelengths to pass through to some extent When using white light this causes the field of view to appear red (white light minus green light) Isotropic, non-birefringent materials also appear red
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7 Quarter-wave Plate Made form a flake of mica that is cleaved to such a thickness that the O-rays and E-rays emerge a quarter of a wavelength out of phase This corresponds to a pale grey interference color This plate is especially useful for examining specimens showing bright interference colors, because they are moved only a short distance along the scale The plate can be used to enhance the contrast between regions of the specimen
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8 Quartz Wedge Plate The quartz wedge is cut so that it varies in thickness from about 0.01 mm to about 0.08 mm and covers several orders of retardation colors As the wedge is inserted into the slot in the microscope it produces progressively higher retardations, and the position at which complete extinction occurs is noted Michel Levy produced a color chart which plots the thickness of an isotropic specimen, its birefringence and its retardation in nanometers Once two of these variables is known, the third can be easily determined
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