Mineral color and Mineral color and pleochroismpleochroism

ColorColor True colorTrue color Not interference colorsNot interference colors Observed in plane polarized lightObserved in plane polarized light Not crossed nicolsNot crossed nicols

Most minerals are colorlessMost minerals are colorless

PleochroismPleochroism

Property of having two or more true Property of having two or more true colorscolors Occurs only in anisotropic mineralsOccurs only in anisotropic minerals Each principal vibration direction has a Each principal vibration direction has a

unique colorunique color Preferentially absorbs selected Preferentially absorbs selected

wavelengths of lightwavelengths of light

Pleochroism – Glaucophane – Glaucophane (amphibole – Na, Mg, Fe – (amphibole – Na, Mg, Fe –

Silicate)Silicate)Pleochroism – orthopyroxene – – orthopyroxene –

(Ca, Fe, Mg – Silicate)(Ca, Fe, Mg – Silicate)

Color depends on which vibration Color depends on which vibration direction parallels polarized directiondirection parallels polarized direction Slow ray has one colorSlow ray has one color Fast ray another colorFast ray another color Color intermediate if neither direction Color intermediate if neither direction

parallel to polarized directionparallel to polarized direction Pleochroic formulaPleochroic formula

Relationship of color to index of refraction Relationship of color to index of refraction (() that shows the color) that shows the color

Pleochroic formulaPleochroic formula

Multiple types of formulas:Multiple types of formulas:1.1. Color of Color of , , oror rays rays

2.2. Greater absorbance e.g. Greater absorbance e.g. > > or or > > 3.3. ““strongly” or “weakly” pleochroicstrongly” or “weakly” pleochroic

Determination of formula – Determination of formula – uniaxial mineralsuniaxial minerals

Find grain with Find grain with = 0 = 0 This is value of This is value of color color

Find grain with maximum Find grain with maximum This has both This has both and and Already know Already know , so other color must be , so other color must be

Fig. 7-Fig. 7-3030

•1st grain (not shown) – complete extinction•Viewed in plane polarized light gives color•2nd grain (shown) – provides and colors• Since know already, the other color is •Determine fast and slow with accessory plate

Grains seen in plane polarized light (not crossed nicols)

Direction of polarized light

Biaxial PleochroismBiaxial Pleochroism

Biaxial minerals Biaxial minerals maymay have three colors: have three colors: One for One for andand

Procedure similar to uniaxial minerals, Procedure similar to uniaxial minerals, but more complexbut more complex Find extinct section – Find extinct section – color color Find maximum Find maximum – this grain has – this grain has and and

colorscolors Determine fast and slow direction with Determine fast and slow direction with

accessory plateaccessory plate

Vibration directions Vibration directions parallel to accessory parallel to accessory plateplate

If addition, color If addition, color associated with nassociated with n

If subtraction, color If subtraction, color associated with nassociated with n

Remember – check Remember – check color without analyzer color without analyzer inin

Fig. 7-31

nn

ExtinctionExtinction

Four Categories:Four Categories: Parallel extinctionParallel extinction – feature (usually – feature (usually

cleavage) parallel to cross hairs at extinctioncleavage) parallel to cross hairs at extinction Inclined extinctionInclined extinction – extinction when feature – extinction when feature

is at an angle to cross hairsis at an angle to cross hairs Symmetrical extinctionSymmetrical extinction – occurs in minerals – occurs in minerals

with two cleavages: bisect cleavagewith two cleavages: bisect cleavage No extinction angleNo extinction angle – minerals with no – minerals with no

elongation or cleavageelongation or cleavage

Parallel

No extinction angle

Inclined

Symmetrical

Fig. 7.32

Extinction may not be Extinction may not be uniformuniform

Physically deformed mineralsPhysically deformed minerals Minerals with variable chemical Minerals with variable chemical

composition (chemically zoned)composition (chemically zoned) Undulatory Extinction

Zoned Extinction

Extinction angleExtinction angle

Inclined extinction - angle betweenInclined extinction - angle between long axis of mineral grainlong axis of mineral grain prominent cleavageprominent cleavage TwinsTwins Other Other crystallographiccrystallographic feature feature

Fig. 7-31Fig. 7-31

Long direction, also Long direction, also parallel to cleavageparallel to cleavage

Extinction angle

•Rotate stage until crystallographic Rotate stage until crystallographic feature is parallel to cross hairsfeature is parallel to cross hairs•Record angle on goniometerRecord angle on goniometer•Rotate stage until mineral is extinctRotate stage until mineral is extinct•Now mineral vibration direction is Now mineral vibration direction is parallel to polarized light directionparallel to polarized light direction•Amount of rotation is extinction angleAmount of rotation is extinction angle

Possible to determine chemical Possible to determine chemical composition from extinction anglecomposition from extinction angle Michel-Levy techniqueMichel-Levy technique

Characteristics:•Sharp boundaries between twins•Twin lamellae have same interference colors

Section cut perpendicular to {010}Albite twin lamellae

High plagioclase = volcanicLow plagioclase = plutonic

Michel-Levy Technique

Cut of mineral must be with {010} plane vertical, b crystallographic axis horizontal

Fig. 12.15 & 12.17

NaAlSi3O8 CaAl2Si2O8

a

c

b

b b

p. 245p. 245

Feldspars - Triclinic minerals:•Two cleavages•Many types of twins

Extinction angles show relationship between X-Y-Z axes (indicatrix axes) and a-b-c axes (crystallographic axes)

An0 to An10 An30 to An50

An90 to An100An50 to An70

b

b

b

b

Z

ZZ

Z

AlbiteNa-feldsparNaAlSi3O8

AnorthiteCa-feldsparCaAl2Si2O8

X

X

XX

Sign of ElongationSign of Elongation

Length fastLength fast: elongate direction of : elongate direction of mineral parallels fast vibration directionmineral parallels fast vibration direction Also called Also called negative elongationnegative elongation

Length slowLength slow: elongate direction of : elongate direction of mineral parallels slow vibration directionmineral parallels slow vibration direction Also called Also called positive elongationpositive elongation

Length fast and length slow depends on Length fast and length slow depends on cut of graincut of grain

DeterminationDetermination

Orient grain with vibration direction Orient grain with vibration direction and length about 45º to polarized and length about 45º to polarized directiondirection

Use accessory plate to determine Use accessory plate to determine addition or subtraction of retardationaddition or subtraction of retardation Determines if fast or slow rayDetermines if fast or slow ray

Vibration directions Vibration directions parallel to accessory parallel to accessory plateplate

If addition, length If addition, length slow (positive slow (positive elongation)elongation)

If subtraction, length If subtraction, length fast (negative fast (negative elongation)elongation)

Fig. 7-31

Length slowLength fast

Orthorhombic Orthorhombic Minerals Minerals (biaxial)(biaxial)

Always length slow

Always length fast

Either length slow or length fast

n = elongate n = elongate n = elongate