optical mineralogy ws 2012/2013. the week before last…. l biaxial indicatrix l extinction angles
TRANSCRIPT
Extinction Angle
Extinction anglee = 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
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. allows identification of fast and slow rays)
Addition
Example: Minerals with small birefringence (e.g. Quartz, Feldspar)
Mineral = 100 nm (1o Grey) in diagonal position:
With analyser only
With analyser and compensator
1o Grey 2o Blue
GMineral = 100 nm (1o Grey)
GGips = 550 nm (1o Red)
Gobs = GMineral + GGyps
Gobs = 650 nm (2o Blue)
When the interference colour is 1o higher (addition), then the NE-SW direction is the higher
n - slow ray (parallel to n of the gypsum plate).
?
Subtraction
Turn the stage through 90° (Mineral stays at 100 nm)
GMineral = 100 nm (1o Grey)
GGips = 550 nm (1o Red)
Gobs = |GMineral – GGips|
Gobs = 450 nm (1o Orange)
When the interference colour is 1o lower (subtraction), then the NE-
SW direction is the lower n - fast ray.
With analyser only
With analyser and compensator
1o Grey 1o Orange
?
Marking on vibration directions• 1 – Rotate into extinction and draw the grain and its privileged
vibration directions• 2 – Rotate 45° until the polarisation colour is brightest• Note the interference colour• 3 – insert the gypsum plate• Note the interference colour (addition or subtraction)• 4 – rotate the mineral 90º• Note the interference colour (addition or subtraction)• 5 – Mark the fast (short line) and slow (long line) rays• How do these relate to pleochroic scheme?• Also a helpful way to tell the order of the polarisation colour
….
Length fast or length slow?
n g
If slow ray (n) of compensator is parallel to the slow ray of the mineral (higher n) (Addition)
Length slow
ngna
If slow ray (n) of compensator is perpendicular to slow ray of the mineral (lower n) (Subtraction)
Length fast
ALWAYS align length of mineral NE-SW
= Hauptzone + = Hauptzone -
Optical character and Hauptzone
Prismatic crystal:If HZ + and Optically +If HZ - and Optically -
Tabular crystal:If HZ + and Optically -If HZ - and Optically +
Uniaxial minerals….
Long dimension of mineral is parallel to the slow ray (n) =
LENGTH SLOW (HZ +)
= PRISMATIC CRYSTAL
Long dimension of mineral is parallel to the slow ray (n) =
LENGTH SLOW (HZ +)
= TABULAR CRYSTAL
Sillimanite (+)
Muscovite (-)
Optical character and HZ
Exsolution (XN)
Exsolution lamellae of orthopyroxene in augite
Exsolution lamellae albite in K-feldspar
(perthite)
Twinning (XN)
simple (K-feldspar) polysynthetic (plagioclase)
cross-hatched or ‘tartan‘ (microcline) sector (cordierite)
Mineral
Polarisedlight (E_W)
Fast wavewith vf
(lower nf)Slow wave with vs
(higher ns)
Polariser(E-W)
= retardation
d
Retardation (Gangunterschied)
After time, t, when the slow ray is about to emerge from the mineral:• The slow ray has travelled distance
d…..• The fast ray has travelled the
distance d + …..
Slow wave: t = d/vs
Fast wave: t = d/vf + /vair
…and so d/vs = d/vf + /vair
= d(vair/vs - vair/vf)
= d(ns - nf)
= d ∙ Δn
Retardation, = d ∙ Δn (in nm)
Interference
Polariser forces light to vibrate E–W Light split into two perpendicular rays Analyser forces rays to vibrate in the N-
S plane and interfere.
Destructive interference (extinction):
= k∙k = 0, 1, 2, 3, …
Constructive interference (maximum intensity):
= (2k+1) ∙ /2k = 0, 1, 2, 3, …
Retardation, 550 550 550 550 550 550Wavelength, 400 440 489 550 629 733
13/8 l 11/4 l 11/8 l 1 l 7/8 l 3/4 l
No green (eliminated) red + violet purple interference colour
Fig 7-7 Bloss, Optical Crystallography, MSA
Retardation, 800 800 800 800 800 800 800Wavelength, 400 426 457 550 581 711 800
2 l 17/8 l 13/4 l 11/2 l 13/8 l 1 1/8 l 1 l
No red or violet(eliminated) green interference colour
Fig 7-7 Bloss, Optical Crystallography, MSA
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)