1. 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

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
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.

3. 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

4. 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

5. Cleavage
random fractures, no cleavage:
olivine

6. 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)

7. 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)

8. 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.

9. 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

10. 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

11. 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

12. 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

13. 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.

14. 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.

15. 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…

16. 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!