303A_Lec09 - GEOL 303A – Mineralogy LECTURE 9 OPTICAL...

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Unformatted text preview: GEOL 303A – Mineralogy LECTURE 9 OPTICAL MINERALOGY I Isotropic Minerals •All isometric minerals and volcanic glass •Determining Isotropic or Anisotropic minerals: total extinction in XP? Biotite Garnet Isotropic Minerals •All isometric minerals and volcanic glass •Determining Isotropic or Anisotropic minerals Æ total extinction in XP? YES Garnet (total extinction ISOTROPIC) sotropic Minerals Why do isotropic minerals isplay total extinction in XP in 60º? Because they do not affect the ibration direction of the light oming from the lower polarizer. Thus, all light is absorbed by the upper polarizer. nisotropic Minerals •All triclinic, monoclinic, hexagonal, tetragonal, and orthorhombic minerals •Determining Isotropic or Anisotropic minerals: total extinction in XP? Biotite Garnet Quartz Muscovite Isotropic Minerals •All isometric minerals and volcanic glass •Determining Isotropic or Anisotropic minerals Æ total extinction in XP? NO Biotite (birds eye extinction ANISOTROPIC) Garnet (total extinction ISOTROPIC) Muscovite Quartz Intro. to UNIAXIAL optics •What does uniaxial mean? •A mineral with only one optic axis •Uniaxial mineral commonalities: •Hexagonal or tetragonal crystals •High degree of symmetry of about the crystallographic c-axis and uniform chemical bonding in all directions 90° to the c-axis Intro. to UNIAXIAL optics •Light traveling along the c-axis is equally free to vibrate in any direction •No preferred vibration directions in this plane •Light is not split into two rays of unequal velocity- rather, it passes through the mineral as it would through an isotropic mineral. •Light traveling at an angle to the c-axis experiences a different atomic/electrical environment, which results in this light ray being split from the ordinary path and propagating at a different velocity Intro. to UNIAXIAL optics •If the cleavage of a calcite rhomb is placed on a dot two images appear, each composed of plane polarized light vibrating at 90° to each other and moving at different velocities . When rotated, one dot moves and one is stationary. •The stationary dot is the result of the “ordinary ray” (or ω- ray) because it is behaving as it would if it were in an isotropic mineral. The rotating dot is therefore referred to as the “extraordinary ray” (or ε- ray) •If light passes through the c-axis of the calcite (or any other anisoptropic) crystal, only one ray will pass through. This is hy the c-axis is referred to as the “Optic axis”, because this is the only direction along which light can propagate without being split into two rays. ptic Sign •In calcite, the ε-ray has a lower index of refraction than the ω-ray. In other uniaxial inerals, however, (like quartz) the ε-ray has a higher index of refraction than the ω-ray •This is the basis for defining an important optical property known s the “optic sign”...
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303A_Lec09 - GEOL 303A – Mineralogy LECTURE 9 OPTICAL...

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