Atlas of Optical Crystallography Interference figures, part II: biaxial crystals Olaf Medenbach Institut für Geologie, Mineralogie und Geophysik Ruhr-Universität Bochum 44780 Bochum, Germany olaf.medenbach@rub.de © by Olaf Medenbach

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix acute bisectrix (nX or nZ) ny = optical normal (ON) extinction position 45° off extinction position

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix trace of optic plane emergence point of the optic axes (melatopes) extinction position 45° off extinction position

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix extinction position 45° off extinction position isogyres (location of vibration directions in extinction positions)

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix after BLOSS, 1999 isochromes (Bertin's surfaces) (curves of equal retardation = equal interference color bands)

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix nx' nz= acute bisectrix (AB) ny = optical normal (ON) nx= obtuse bisectrix (OB) nz' nZ' compensator Red I addition subtraction optic sign (+)

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix nx' nz' nx= acute bisectrix (AB) ny = optical normal (ON) nz= obtuse bisectrix (OB) nZ' compensator Red I addition subtraction optic sign (-)

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix, in circular polarized light the isogyres disappear, the melatope und the isochromes are still visible

Interference figures of biaxial crystals cut perpendicular to the acute bisectrix, for crystals with different axial angles extinction position 45° off extinction position 2E = 114,5° Topaz 2E = 71,5° Anhydrite 2E = 17,4° Cerussite 2E = 30,8° Aragonite 2E = 7,7° KNO3 trace of optic plane (OP)

Interference figures of biaxial crystals The animation starts a few seconds after the next click. nZ' compensator Red I perpendicular to the acute bisectrix, optic sign (-), 2V ≈ 20°

Interference figures of biaxial crystals The animation starts a few seconds after the next click. nZ' compensator Red I perpendicular to the acute bisectrix, optic sign (-), 2V ≈ 40°

Interference figures of biaxial crystals The animation starts a few seconds after the next click. perpendicular to the acute bisectrix, large axial angle

Interference figures of biaxial crystals cut perpendicular to one axis ny = optical normal (ON) nz' nx' nZ' compensator Red I addition subtraction optic sign (+)

Interference figures of biaxial crystals cut perpendicular to one axis ny = optical normal (ON) nz' nx' nZ' compensator Red I addition subtraction optic sign (-)

Interference figures of biaxial crystals cut perpendicular to one axis, in circular polarized light the isogyres disappear, the melatopes und the isochromes are still visible

Interference figures of biaxial crystals The animation starts a few seconds after the next click. perpendicular to one axis, 2V small

Interference figures of biaxial crystals The animation starts a few seconds after the next click. nZ' compensator Red I perpendicular to one axis, optic sign (-), 2V large

Interference figures of biaxial crystals The animation starts a few seconds after the next click. perpendicular to one axis, 2V ≈ 90° (the isogyre is not curved in 45° off extinction position!)

Interference figures of biaxial crystals The animation starts a few seconds after the next click. semi-random orientation, large axial angle