Optical Mineralogy WS 2008/2009

Theory Exam…. Thursday 18th 13:30 90 minutes Answer 3 questions from 5 Total of 30% of the course

Last week…. BIAXIAL INDICATRIX EXTINCTION ANGLES

Biaxial indicatrix - summary

Extinction Angle Extinction angle  = I – II = 29,5° I = 153,0° II = 182,5° For MONOCLINIC and TRICLINIC crystals…. Only the MAXIMUM extinction angle is diagnostic of a mineral  measure lots of grains

Compensator (Gypsum plate) Vibration direction of the higher n ray (slow ray) is NE-SW Vibration direction of the lower n ray (fast ray) is NW-SE = 550nm Retardation  = 550nm (= 1 order) Observed retardation (in diagonal position):  Addition  obs =  Mineral +  Gyps  Subtraction  obs =  Mineral -  Gyps Gypsum plate ( -plate) = helps in measuring the relative size of n (e.g. difference between fast and slow rays)

Compensator (Gypsum plate) slow // slow constructive interference colour increases fast // slow destructive interference colour decreases

Addition Example: Minerals with small birefringence (e.g. Quartz, Feldspar)  Mineral = 100 nm (1 o Grey) in diagonal position: N With analyser only With analyser and compensator 1 o Grey2 o Blue  Mineral = 100 nm (1 o Grey)  Gips = 550 nm (1 o Red)  obs =  Mineral +  Gyps   obs = 650 nm (2 o Blue) When the interference colour is 1 o higher (addition), then the NE- SW direction is the higher n - slow ray (parallel to n  of the gypsum plate).

Subtraction N Turn the stage through 90° (  Mineral stays at 100 nm )  Mineral = 100 nm (1 o Grey)  Gips = 550 nm (1 o Red)  obs = |  Mineral –  Gips |   obs = 450 nm (1 o Orange) When the interference colour is 1 o lower (subtraction), then the NE- SW direction is the lower n - fast ray. With analyser only With analyser and compensator 1o Grey1o Orange

Marking on vibration directions 1 – NE-SW diagonal position (extinction +45°), XPL Note the interference colour 2 – insert the gypsum plate Note the interference colour (addition or subtraction) 3 – rotate the mineral 90º Note the interference colour (addition or subtraction) 4 – Mark on the fast and slow rays How do these relate to pleochroic scheme? Also a helpful way to tell the order of the polarisation colour ….

Hauptzone + or - = Length fast or length slow? nnnn If n  parallel to slow ray (higher n) = addition  Length slow  Hauptzone + nn nn If n  perpendicular to slow ray (lower n) = subtraction  Length fast  Hauptzone - ALWAYS align length of mineral NE-SW

Optical v Hauptzone character Prismatic crystals: Optical and Hauptzone sign are the same…. Tabular crystals: Optical and Hauptzone sign are different…. Uniaxial minerals….

Some examples…. Prismatic crystal: Long dimension of mineral is parallel to the slow ray (n  ) = length slow = Hauptzone + Optically positive + Tabular crystal: Long dimension of mineral is parallel to the slow ray (n  ) = length slow = Hauptzone + Biaxial negative - sillimanite muscovite

Exsolution (XN) Exsolution lamellae of orthopyroxene in augite Exsolution lamellae albite in K-feldspar (perthite)

Undulose extinction (XN) Undulose extinction in quartz, the result of strain

Zoning (XN) Reflects compositional differences in solid solution minerals

Zoning

Twinning (XN) simple (K-feldspar) polysynthetic (plagioclase) cross-hatched or ‘tartan‘ (microcline) sector (cordierite)

Orthoscopic properties - summary Orthoscopic, PPL  Crystal shape/form  Transparent or opaque  Colour and pleochroism  Relief and (variable) refractive index  Cleavage, fracture Orthoscopic, XN (in the diagonal position)  Isotropic or anisotropic  Maximum polarisation colour  birefringence (  n)  Extinction angle  crystal system  Length fast or slow  Zoning (normal, oscillatory, etc.)  Twinning (simple, polysynthetic, sector)