Guide <strong>to</strong> Thin Section Microscopy Alteration phenomena Raith, Raase & Reinhardt – February 2012 Figure 3-28. Pleochroic halos around minerals containing radioactive iso<strong>to</strong>pes A-L: Pleochroic halos around inclusions <strong>of</strong> zircon, monazite and xenotime in biotite (A-C), cordierite (D-F), amphibole (G-H) and chlorite (J-L). 59
Guide <strong>to</strong> Thin Section Microscopy Optical properties: basic principles 4. Optical properties 4.1 Some basic principles 4.1.1 Nature <strong>of</strong> light, refraction In order <strong>to</strong> describe the interaction <strong>of</strong> light rays with matter, two physical models can be applied: (a) light as a wave, and (b) light as energy quanta. Most optical phenomena which are observed during microscopic investigation <strong>of</strong> amorphous or crystalline substances (glass phase, minerals), can be adequately explained with the wave model. Wave model: Light rays propagate as electromagnetic waves. In each wave electric and magnetic vec<strong>to</strong>rs oscillate orthogonal <strong>to</strong> each other and orthogonal <strong>to</strong> the propagation direction. The optical behaviour <strong>of</strong> light when passing through amorphous or crystalline substances is essentially controlled by the interaction <strong>of</strong> the electric vec<strong>to</strong>r with the electric field <strong>of</strong> the ions. Interactions with the magnetic vec<strong>to</strong>r are negligible. Thus, each light wave can be described as a harmonic oscillation [y = A sin(x)] (Fig. 4-1). Colour: The human eye can only see a small part <strong>of</strong> the large spectrum <strong>of</strong> electromagnetic radiation, namely the spectral domain between about 400 and 800 nm (visible light). This is the colour spectrum from violet <strong>to</strong> blue, green, yellow, orange and red (Fig. 4-1). Sunlight consists <strong>of</strong> various proportions <strong>of</strong> these colours, the combination <strong>of</strong> which is perceived as white light. In <strong>thin</strong> <strong>section</strong>, colour effects are caused if the spectral composition <strong>of</strong> originally white light is changed as light passes minerals, either by depletion <strong>of</strong> specific wavelengths (absorption), or by dispersion <strong>of</strong> white light as a result <strong>of</strong> refraction or diffraction <strong>of</strong> light at grain boundaries, inclusions and rough surfaces. Intensity: The intensity <strong>of</strong> light, <strong>of</strong> a specific colour, for example, is determined by the amplitude <strong>of</strong> the light wave. It can be modified by absorption. Raith, Raase & Reinhardt – February 2012 Polarization: Sunlight or the light emitted from the light source in the microscope consists <strong>of</strong> waves which vibrate in random directions. In plane-polarized light, the light waves vibrate in a defined direction. Plane-polarized light is generated in modern microscopes by a polarization filter which reduces light <strong>of</strong> random vibration directions from natural or artificial sources <strong>to</strong> light <strong>of</strong> a single vibration direction (Fig. 4-1). The bundle <strong>of</strong> light waves entering the <strong>thin</strong> <strong>section</strong> consists entirely <strong>of</strong> E-W vibrating light waves if the polarizer is adjusted precisely. Interference: Two coherent light waves generated by the same light source can overlap (i.e., interfere) if they vibrate in the same plane and have the same velocity. This is realised in optically anisotropic minerals when the two orthogonally vibrating light rays, generated through double refraction in the crystal plate, are brought <strong>to</strong> interference in the analyzer after leaving the <strong>thin</strong> <strong>section</strong> (see Ch. 4.2.3). The degree <strong>of</strong> phase shift (Φ) determines whether the interfering waves are eliminated or produce a resultant wave <strong>of</strong> decreased or increased intensity (Fig. 4-1). If certain <strong>section</strong>s <strong>of</strong> the white light spectrum are eliminated, diminished or amplified, interference colours are generated (see Ch. 4.2.3). 60
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