Mineral color and pleochroism n Color True color

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

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

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

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

Pleochroic formula n Multiple types of formulas: 1. 2. 3. 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 n Find grain with d = 0 n n This is value of w color Find grain with maximum d This has both e and w n Already know w, so other color must be e n

Direction of polarized light Grains seen in plane polarized light (not crossed nicols) • 1 st grain (not shown) – complete extinction • Viewed in plane polarized light gives w color • 2 nd 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 n Biaxial minerals may have three colors: n n One for a, b, and g Procedure similar to uniaxial minerals, but more complex Find extinct section – b color n Find maximum d – this grain has a and g colors n Determine fast and slow direction with accessory plate n

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

Parallel Symmetrical Inclined No extinction angle Fig. 7. 32

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

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

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

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

Michel-Levy Technique Section cut perpendicular to {010} Albite twin lamellae b c b a Cut of mineral must be with {010} plane vertical, b crystallographic axis horizontal b Na. Al. Si 3 O 8 Ca. Al 2 Si 2 O 8 Characteristics: High plagioclase = volcanic • Sharp boundaries between twins Low plagioclase = plutonic • Twin lamellae have same interference colors Fig. 12. 15 & 12. 17

Albite Na-feldspar Na. Al. Si 3 O 8 Z b X b Z X An 0 to An 10 An 30 to An 50 Z Z b X An 50 to An 70 b X An 90 to An 100 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) Anorthite Ca-feldspar Ca. Al 2 Si 2 O 8 p. 245

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

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

n n n 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 Always length slow na = elongate nb = elongate Always length fast Either length slow or length fast