1. Optical Mineralogy in a Nutshell Use of the petrographic microscope in three easy lessons Part III © Jane Selverstone, University of New Mexico, 2003 Used and Modified with Permission

2. A few new properties, and then some review… Cleavage – number and orientation of cleavage planes Twinning – type of twinning, orientation Extinction angle – parallel or inclined? Angle? Habit – characteristic form of mineral

3. Cleavage Most easily observed in Plain Polarized Light (PPL) (upper polarizer out), but visible in crossed nicols (XN) as well No cleavages:quartz, olivine 1 good cleavage:micas 2 good cleavages:pyroxenes, amphiboles

4. Cleavage 2 cleavages intersecting at ~90° pyroxene 60° 120° 2 cleavages intersecting at 60°/120°: amphibole

5. Cleavage random fractures, no cleavage: olivine

6. Twinning Presence and style of twinning can be diagnostic Twins are usually most obvious in XN (upper polarizer in) Carlsbad Twin in Pyroxene Polysynthetic Twins in Plagioclase

7. Crystal FORMS A crystal form is a set of crystal faces that are related to each other by symmetry. To designate a crystal form (which could imply many faces) we use the Miller Index, enclosing the indices in curly braces, i.e. {101} or {111} A form refers to a face or set of faces that have the same arrangement of atoms. Thus, the number of faces in a form depends on the symmetry of the crystal. Figure: Three pyramid faces related by a three-fold rotational axis

8. Crystal ZONES A zone is defined as a group of crystal faces that intersect in parallel edges. The edges will all be parallel to a line. A zone symbol implies a line that is perpendicular to the face with the same index. In other words, [001] (note the square brackets) is a line perpendicular to the face (001). It can thus be used as a symbol for a line. In this case, the line is the c crystallographic axis.

9. Combination There are also the forms {110} and {100} Note the Zone [001]. All the prism faces intersect in lines parallel to the c-axis. The face (001) is perpendicular to the c-axis line.

10. Twinning - some examples Clinopyroxene (augite) Plagioclase Simple twin on {100} Polysynthetic twins on {010}

11. Carlsbad Twins Carlsbad twins, unlike polysynthetic twins, are not multiple, repeating twins, and instead have a single twin plane that splits the crystal into two domains with different orientations.

12. Extinction angle Extinction behavior is a function of the relationship between indicatrix orientation and crystallographic orientation parallel extinction inclined extinction

13. Extinction angle – parallel extinction All uniaxial minerals show parallel extinction Orthorhombic minerals show parallel extinction (this is because xtl axes and indicatrix axes coincide) PPL XN orthopyroxene

14. Extinction angle - inclined extinction Monoclinic and triclinic minerals: indicatrix axes do not coincide with crystallographic axes These minerals have inclined extinction (and extinction angle helps to identify them) clinopyroxene extinction angle

15. Habit or form blocky prismatic anhedral/irregular elongate fibrous tabular euhedral

16. Habit or form blocky prismatic anhedral/irregular elongate fibrous tabular euhedral

17. Review – techniques for identifying unknown minerals Start in PPL: Color/pleochroism Relief Cleavages Habit Then go to XN: Birefringence Twinning Extinction angle Uniaxial or biaxial? 2V if biaxial Positive or negative?

18. Go to an optical mineralogy book: Chemical formula Symmetry Uni or biaxial, (+) or (-) RIs: lengths of indicatrix axes Birefringence 2V if biaxial Diagrams: * Crystallographic axes * Indicatrix axes * Optic axes * Cleavages * Extinction angles

19. Another example Then read text for color, pleochroism, habit, cleavage, twinning, distinguishing features, occurrence – making sure the properties match your observations. If not, check another mineral… Crystallographic axes: a, b, c Indicatrix axes: X, Y, Z or  Optic axes Cleavages Extinction angles

20. Good luck and have fun.