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

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

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

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

Cleavage random fractures, no cleavage: olivine

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

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

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.

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.

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

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.

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

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

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

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

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

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?

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

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

Good luck and have fun.