Mineral color and pleochroism True color Not interference colors Observed in plane polarized light Not crossed nicols Most minerals are colorless

Pleochroism Property of having two or more true colors Occurs only in anisotropic minerals Each principal vibration direction has a unique color Preferentially absorbs selected wavelengths of light

Pleochroism – Glaucophane (amphibole – Na, Mg, Fe – Silicate) Pleochroism – orthopyroxene – (Ca, Fe, Mg – Silicate)

Color depends on which vibration direction parallels polarized direction Slow ray has one color Fast ray another color Color intermediate if neither direction parallel to polarized direction Pleochroic formula Relationship of color to index of refraction (a, b, g, e, w) that shows the color

Pleochroic formula Multiple types of formulas: Color of e, w, a, b, or g rays Greater absorbance e.g. w > e or e > w “strongly” or “weakly” pleochroic

Determination of formula – uniaxial minerals Find grain with d = 0 This is value of w color Find grain with maximum d This has both e and w Already know w, so other color must be e

Grains seen in plane polarized light (not crossed nicols) Direction of polarized light Grains seen in plane polarized light (not crossed nicols) 1st grain (not shown) – complete extinction Viewed in plane polarized light gives w color 2nd grain (shown) – provides w and e colors Since know w already, the other color is e Determine fast and slow with accessory plate Fig. 7-30

Biaxial Pleochroism Biaxial minerals may have three colors: One for a, b, and g Procedure similar to uniaxial minerals, but more complex Find extinct section – b color Find maximum d – this grain has a and g colors Determine fast and slow direction with accessory plate

Vibration directions parallel to accessory plate If addition, color associated with ng If subtraction, color associated with na Remember – check color without analyzer in na ng Fig. 7-31

Extinction Four Categories: Parallel extinction – feature (usually cleavage) parallel to cross hairs at extinction Inclined extinction – extinction when feature is at an angle to cross hairs Symmetrical extinction – occurs in minerals with two cleavages: bisect cleavage No extinction angle – minerals with no elongation or cleavage

Parallel Inclined No extinction angle Symmetrical Fig. 7.32

Extinction may not be uniform Physically deformed minerals Minerals with variable chemical composition (chemically zoned) Undulatory Extinction Zoned Extinction

Extinction angle Inclined extinction - angle between long axis of mineral grain prominent cleavage Twins Other crystallographic feature

Long direction, also parallel to cleavage Extinction angle Long direction, also parallel to cleavage Rotate stage until crystallographic feature is parallel to cross hairs Record angle on goniometer Rotate stage until mineral is extinct Now mineral vibration direction is parallel to polarized light direction Amount of rotation is extinction angle Fig. 7-31

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

Michel-Levy Technique Section cut perpendicular to {010} Albite twin lamellae Cut of mineral must be with {010} plane vertical, b crystallographic axis horizontal b a b b NaAlSi3O8 CaAl2Si2O8 Characteristics: Sharp boundaries between twins Twin lamellae have same interference colors High plagioclase = volcanic Low plagioclase = plutonic Fig. 12.15 & 12.17

Feldspars - Triclinic minerals: Two cleavages Many types of twins Albite Na-feldspar NaAlSi3O8 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) Z b X b Z X An0 to An10 An30 to An50 Z Z b b X X Anorthite Ca-feldspar CaAl2Si2O8 An50 to An70 An90 to An100 p. 245

Sign of Elongation Length fast: elongate direction of mineral parallels fast vibration direction Also called negative elongation Length slow: elongate direction of mineral parallels slow vibration direction Also called positive elongation Length fast and length slow depends on cut of grain

Determination Orient grain with vibration direction and length about 45º to polarized direction Use accessory plate to determine addition or subtraction of retardation Determines if fast or slow ray

Vibration directions parallel to accessory plate If addition, length slow (positive elongation) If subtraction, length fast (negative elongation) Length fast Length slow Fig. 7-31

Orthorhombic Minerals (biaxial) ng = elongate na = elongate nb = elongate Either length slow or length fast Always length slow Always length fast