What happens as light moves through the scope propagatio n direction plane of vibration vibration direction light vibrates in all planes that contain the light ray i e all planes perpendicular to the propagation direction 3 Now insert a thin section of a rock west left Unpolarized light east right Light vibrating E W Light vibrating in many planes and with many wavelengths Light and colors reach eye How does this work Some generalizations and vocabulary All isometric minerals e g garnet are isotropic they cannot reorient light Light does not get rotated or split propagates with same velocity in all directions These minerals are always black in crossed polars All other minerals are anisotropic they are all capable of reorienting light transmit light under cross polars All anisotropic minerals contain one or two special directions that do not reorient light Minerals with one special direction are called uniaxial Minerals with two special directions are called biaxial Isotropic minerals light does not get rotated or split propagates with same velocity in all directions Anisotropic minerals Uniaxial light entering in all but one special direction is resolved into 2 plane polarized components that vibrate perpendicular to one another and travel with different speeds Biaxial light entering in all but two special directions is resolved into 2 plane polarized components Along the special directions optic axes the mineral thinks that it is isotropic i e no splitting occurs Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative depending on orientation of fast and slow rays relative to xtl axes How light behaves depends on crystal structure Isotropic Isometric All crystallographic axes are equal Uniaxial Biaxial Hexagonal tetragonal All axes c are equal but c is unique Orthorhombic monoclinic triclinic All axes are unequal Anisotropic crystals Calcite experiment and double refraction O O ray Ordinary Double images E Obeys Snell s Law and goes Ray 2 rays with straight different Vibrates plane containing propagation and ray and c axis optic axis vibration directions E ray Extraordinary Each is polarized each other deflected Vibrates in plane containing ray and c axis Fig 6 7 Bloss Optical Crystallography MSA also doesn t vibrate propagation but we ll ignore this O E IMPORTANT A given ray of incoming light is restricted to only 2 mutually perpendicular Both raysvibration vibrate directions parallel to oncethe it enters an anisotropic crystal incident surface for normal incident light so Called privileged directions the interface x section of Each theray indicatrix has a different is still valid n even for the E ray no Thus our simplification of vibration nE propagation works well enough in the case of calcite From now on we ll treat which makes the O ray dot appear Fig 6 7 Bloss Optical these two rays as collinear Crystallography MSAabove E ray dot but not interacting If each ray has a different velocity because it s the vibration then each has a different wavelength direction that counts because velocity If I slow down 1 ray and then recombine it with another ray that is still going faster what happens Splitting of light what does it mean For some exceptionally clear minerals where we can see this is hand sample this is double refraction calcite displays this Light is split into 2 rays one traveling at a different speed and this difference is a function of thickness and orientation of the crystal Norden Bombsight patented in 1941 utilized calcite in the lenses to gauge bomb delivery based on speed altitude of plane vs target ALL anisotropic minerals have this property and we can see that in thin sections with polarized light Difference between our 2 rays Apparent birefringence difference in refractive index speed between the 2 rays Retardation distance separating the 2 rays Retardation therefore is a function of the apparent birefringence and the thickness of the crystal ideally all thin sections are 0 3 mm but mistakes do happen Polarized light going into the crystal splits into two rays going at different velocities and therefore at different wavelengths colors one is O ray with n other is E ray with n When the rays exit the crystal they recombine When rays of different wavelength combine what things happen polarizer Michel L vy Color Chart Plate 4 11 Estimating birefringence 1 Find the crystal of interest showing the highest colors depends on orientation 2 Go to color chart thickness 30 microns use 30 micron line color follow radial line through intersection to margin read birefringence Suppose you have a mineral with second order green What about third order yellow 1 553 1 544 1 553 Example Quartz 1 544 Data from Deer et al Rock Forming Minerals John Wiley Sons Example Quartz 1 544 1 553 Sign because 0 009 called the birefringence maximum interference color when seen What color is this Use your chart Colors one observes when polars are crossed XPL Color can be quantified numerically nlow nhigh Rotation of crystal Retardation also affected by mineral orientation As you rotate a crystal observed birefringence colors change Find maximum interference color for each in practice Extinction When you rotate the stage extinction relative to the cleavage or principle direction of elongation is extinction angle Parallel inclined symmetric extinction Divided into 2 signs of elongation based on the use of an accessory plate made of gypsum or quartz which has a retardation of 550 nm which changes the color for a grain at 45 from extinction look for yellow fast or blue slow Twinning and Extinction Angle Twinning is characteristic in thin section for several common minerals especially feldspars The twins will go from light to dark over some angle This is characteristic of the composition Stage of the petrographic microscope is graduated in degrees with a vernier scale to measure the angle of extinction precisely Vernier scale 1 23 Appearance of crystals in microscope Crystal shape how well defined the crystal shape is Euhedral sharp edges well defined crystal shape Anhedral rounded edges poorly defined shape Subhedral in between anhedral and euhedral Cleavage just as in hand samples Physical character often note evidence of strain breaking etching on crystals you will notice some crystals show those features better than others So far all of this has been orthoscopic the normal way All light rays are parallel and vertical as they pass through the crystal