Optical Mineralogy in a Nutshell Use of the petrographic microscope in three easy lessons Edited T.S. Hamilton 02/05/2013 Part III © Jane Selverstone, University of New Mexico, 2003

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 We look at minerals in 2D cross sectional slices, depending on the orientation these structures may show or be absent. For thin sections grains might be oriented but are usually more random in orientation. For grain mounts minerals tend to lie on their flat sides controlled by crystal shapes or cleavage directions. Some crystal forms are elongate, prismatic or flat. Habit normally applies to special crystal aggregate forms, radial, foliated, botryoidal, framboidal… Numbers of different cleavage directions and angles between them are important.

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

Cleavage 120° 60° 2 cleavages intersecting at ~90° pyroxene Olivine grains in high relief to left with fractures exhibit a basal parting perpendicular to c/z crystal axis. 60° 120° 2 cleavages intersecting at 60°/120°: amphibole

Cleavage random fractures, no cleavage: olivine

Presence and style of twinning can be diagnostic Plagioclase here on the right in black/grey/white stripes shows mainly albite twinning or combined carlsbad albite twins with 010 as the composition plane. The augite on the left shows one of the cleavage traces and a single twin plane between the bright blue and dark green blue halves. Twins are usually most obvious in XN (upper polarizer in)

Twinning - some examples Clinopyroxene (augite) Simple twin on {100} Plagioclase Other twin laws that cut across albite twins include: acline, ala-a, ala-b, and mannebach. Pericline twins are often dagger shaped rather than parallel lamellae. Calcium rich plagioclases generally have more abundant twinning than do Na rich ones. Baveno twins cut across at an angle and are more common in alkali feldspars Pericline twin on (h01) Polysynthetic albite twins on (010) Simple (Carlsbad) twin on (010)

Extinction angle Extinction behavior is a function of the relationship between indicatrix orientation and crystallographic orientation inclined extinction parallel extinction In Tetragonal, hexagonal or orthorhombic minerals C optical = z crystallographic and elongate grains show parallel extinction. For Monoclinic grains sections show inclined extinction but for the prismatic end on section the angles are symmetric (equal to both sides). Triclinic always shows 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) orthopyroxene XN XN = XPL in Tarks nomenclature, crossed polars as these are polarizing plastic sheets and not nichol’s crystal prisms any more! PPL

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) extinction angle clinopyroxene

Habit or form acicular anhedral/irregular bladed blocky elongate euhedral The upper slide is a gabbro with a cumulate texture, crystals grew together or accumulated and intergrew in a pluton. Here the plagioclase accumulated and the olivine crystallized in between as an intercumulus phase later on as the magma cooled. The plagioclase crystals in the upper right are an anhedral crystal aggregate. The elongate blocky grain is embayed and partially resorbed by a rise in temperature since it first crystallized. The six sided isotropic grains with poililitic (net like) inclusions are either analcite or garnet in an alkaline lava. The microlites or matrix here is aligned or shows flow foliation. fibrous prismatic rounded tabular

Habit or form acicular anhedral/irregular bladed blocky elongate euhedral Any of these textures could also be called foliated. Slide 2 PPL, is a blueschist with glaucophane or riebeckite in blue-violet. The elongate grains are high relief, fractured and might be garnet. In slide 3 a schist in XPL, shows hot pink muscovite, brown biotite and grey quartz and feldspar. fibrous prismatic rounded tabular

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? Cumulus textured gabbro with Plag and Olivine.

Go to Nesse or similar 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 Optic orientation diagrams. Ranges of refractive indices and extinction angles relate to substitution of Fe for Mg. Generallyu Fe addition makes for higher refractive indices, more electrons and higher optical density.

Another example Crystallographic axes: a, b, c Optic axes Indicatrix axes: X, Y, Z or e, w Cleavages Extinction angles Pleochroic formulae depend on both composition and position/orientation of grain (generally more or fewer Fe atoms in the direction of vibration. Then read text re color, pleochroism, habit, cleavage, twinning, distinguishing features, occurrence – make sure properties match your observations. If not, check another mineral…

On to real rocks… …good luck and have fun! All 3 slides XPL. nUpper left Ol and Pl in Gabbro. Upper right Plag and Hornblende phenos in a fine graiend porphyritic volcanic rock. Lower slide has plag and augite with little wedges of pink birefringent biotite, also a gabbro. …good luck and have fun!