1 / 10

Optical Mineralogy

Optical Mineralogy. WS 2007/2008. Last week - Uniaxial interference figures. without gypsum plate: same for (+) and (-). (+) with gypsum plate blue in I. quadrant. (-) with gypsm plate yellow in I. quadrant. Biaxial Interference Figures. black.

arthur-espinoza
Download Presentation

Optical Mineralogy

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Optical Mineralogy WS 2007/2008

  2. Last week - Uniaxial interference figures without gypsum plate: same for (+) and (-) (+) with gypsum plate blue in I. quadrant (-) with gypsm plate yellow in I. quadrant

  3. Biaxial Interference Figures black • Biaxial negative (acute bisectrix looking down X) • Condensor forms a cone of light through sample at O • OX  OS  OA decreasing  n n • OA  OT  OU increasing  n n • OX  OQ  OP increasing  n n black

  4. Biaxial Interference Figures Fig 10-15 Bloss, Optical Crystallography, MSA The result is an interference figure with ‘figure-of-8’ isochromes….

  5. Biaxial Interference Figure Upper row: Cut perpendicular to acute bisectrix (2V approx. 30°); Middle row: cut close to an Optic Axis; Lower row: Cuts nearly perpendicular to the obtuse bisectrix.

  6. Determining the optical sign (+ or -) In A-D, sections are perpendicular to the acute bisectrix. In E and F, they are perpendicular to one of the optic axes.

  7. Measuring 2V Maximum separation of isogyres Curvature of isogyres 15o 15o 30o 5o 90o 60o 45o 60o 30o

  8. How do we get an OAF? • In XN, find a grain that remains in extinction through 360º - centre it • Change to high-powered objective and focus • Make sure grain stays in field of view • Maximise light (open diaphragm, remove sub-stage lens) • Remove left ocular and adjust condensor settings • You should see an interference figure - draw it • Rotate isogyre so it is bent towards NE quadrant • Insert gypsum plate and note optic sign

  9. How do we get a BISECTRIX interference figure? • In XN, find a grain that shows low polarisation colour (1°) …. a bit of a guess …. • Change to high-powered objective and focus • Make sure grain stays in field of view • Maximise light (open diaphragm, remove sub-stage lens) • Remove left ocular and adjust condensor settings • You should see an interference figure - draw it • Rotate as shown • Insert gypsum plate and note optic sign

  10. Conoscopic observations - summary Find an isotropic section (remains black) • Optical character • No interference figure  cubic or amorphous • Uniaxial interference figure  hexagonal, trigonal, tetragonal • Biaxial interference figure  orthorhombic, monoclinic, triclinic • Using the gypsum plate • Uniaxial positive or negative • Biaxial positive, negative or neutral • Estimate the 2V angle (curvature or separation of isogyres)

More Related