Optical Mineralogy in a Nutshell Use of the petrographic microscope in three easy lessons Part III Slides borrowed adapted from Jane Selverstone University of New Mexico and John Winter Whitman College Some review Optical mineral properties ONLY visible in PPL Color not an interference color for that see below Pleochroism is there a color change while rotating stage Relief low intermediate high very high Optical mineral properties visible in PPL or XPL Cleavage number and orientation of cleavage planes may need higher magnification and at different grains Habit characteristic form of mineral sometimes better in XPL Optical mineral properties ONLY visible in XPL Birefringence use highest order interference color to describe Twinning type of twinning orientation Extinction angle parallel or inclined Angle Some generalizations and vocabulary All isometric minerals e g garnet and glass are isotropic they cannot reorient light These minerals are always black in crossed polars All other minerals are anisotropic they are all capable of reorienting light All anisotropic minerals contain one or two special directions the optic axes that do not reorient light Minerals with one special direction are called uniaxial Minerals with two special directions are called biaxial Uniaxial and biaxial minerals can be subdivided into optically positive and optically negative depending on the orientation of fast and slow rays relative to the xtl axes All anisotropic minerals can resolve light into two plane polarized components that travel at different velocities and vibrate in planes that are perpendicular to one another Some light is now able to pass through the upper polarizer fast ray slow ray minera l grain plane polarized light W E lower polarizer When light gets split velocity changes rays get bent refracted 2 new vibration directions usually see new colors Calcite experiment and double refraction O E Fig 6 8 Bloss Optical Crystallography MSA Fig 6 7 Bloss Optical Crystallography MSA We ve talked about minerals as magicians now let s prove it calcite e it lc ca cal cit e calcite calcite ordinary ray extraordinary stays stationary ray rotates How light behaves depends on crystal structure there is a reason you took mineralogy Isotropic Isometric All crystallographic axes are equal Uniaxial Hexagonal trigonal tetragonal All axes c are equal but c is unique Biaxial Orthorhombic monoclinic triclinic All axes are unequal Let s use all of this information to help us identify minerals Simple guide to interference figures Get a good interference figure Distinguish uniaxial and biaxial figures Determine optic sign and Estimate 2V 1 Choose a grain showing the lowest interference colors 2 Move to the high powered objective lens and refocus 3 Open the sub stage diaphragm as wide as possible 4 Insert the condenser lens 5 Cross the polars 6 Insert the Bertrand lens Use of interference figures continued You will see a very small circular field of view with one or more black isogyres rotate stage and watch isogyre s or uniaxial biaxial If uniaxial isogyres define cross arms remain N S E W as stage is rotated If biaxial isogyres define curve that rotates with stage or cross that breaks up as stage is rotated Use of interference figures continued Now determine the optic sign of the mineral 1 Rotate stage until isogyre is concave to NE if biaxial 2 Insert gypsum accessory plate 3 Note color in NE immediately adjacent to isogyre Blue Yellow uniaxial biaxial Without plateGypsum plate inserted emember determining optic sign last week with the gypsum plat blue in NE ow l s Gypsum plate has constant of 530 nm 1storder pink Isogyres black Background gray 0 100 Add or subtract 530 nm 530 100 630 nm blue 530 100 430 nm yellowish Addition slow slow Subtraction slow fast Time for some new tricks the optical indicatrix ought experiment nsider an isotropic mineral e g garnet Imagine point source of light at garnet center turn light on for fixed amount of time then map out distance traveled by light in that time hat geometric shape is defined by mapped light rays Isotropic indicatrix Soccer ball or an orange Light travels the same distance in all directions n is same everywhere thus nhi nlo 0 black anisotropic minerals uniaxial indicatrix c axis c axis calcite quartz Let s perform the same thought experiment Uniaxial indicatrix c axis c axis tangerine uniaxial Spaghetti squash uniaxial quartz calcite Uniaxial ellipsoid and conventions Fig 6 11 Bloss Optical Crystallography MSA crystal oblate crystal prolate Propagate light along the c axis note what happens to it in plane of thin section n c Z n n n a X b Y n n 0 therefore 0 grain stays black same as the isotropic case Now propagate light perpendicular to c axis N n n 0 therefore 0 nn n nn W nnn E n n S Grain changes color upon rotation Grain will go black whenever indicatrix axis is EW or N S is orientation will show the maximum of the miner anisotropic minerals biaxial indicatrix clinopyroxene feldspar Now things get a lot more complicated Biaxial indicatrix triaxial ellipsoid OA Z 2Vz OA 2Vz n n Y n n n The potato X n n n n n n ere are 2 different ways to cut this and get a circ Alas the potato indicatrix can have any orientation within a biaxial mineral Y c a Z Z olivine c augite b Y b X a X but there are a few generalizations that we can make The potato has 3 perpendicular principal axes of different length thus we need 3 different RIs to describe a biaxial mineral X direction n lowest Y direction n intermed radius of circ section Z direction n highest horhombic axes of indicatrix coincide w xtl axes oclinic Y axis coincides w one xtl axis clinic none of the indicatrix axes coincide w xtl 2V a diagnostic property of biaxial minerals OA Z OA When 2V is acute about Z 2Vz When 2V is acute about X n n Y n X When 2V 90 sign is indeterminate When 2V 0 mineral is uniaxial 2V is measured using an interference figure More in a few minutes How interference figures work uniaxial example Converging lenses force light rays to follow different paths through the indicatrix Bertrand lens N S polarizer What do we see Sample looking down OA n substage condensor n n n n n n n Effects of multiple cuts thru indicatrix W E Biaxial interference figures re are lots of types of biaxial figures we ll concentrate on only t 1 Optic axis figure pick a grain that stays dark on rotation Will see one curved isogyre determine sign w gyps determine 2V from curvature of isogyre 90 60 40 See Nesse p 103