STUDY SESSION TOPICS- I made this study guide and handed it out at the study session, if you couldn’t
make it. It has most every general subject on the slides, handouts, and classroom notes. Not all
definitions are included so you’ll have to look them up! Hope this helps.
- Alea Propster
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Crystalographic features
o Cleavage Planes- tendency of crystals to split along definite crystallographic along which
bonding is weaker than in other planes.
o idiomorphic features- crystal shape
o color
Effects/Interactions b/t crystals and light:
o crystal system
o identify the mineral
o establish composition (maybe)
o growth history-petrology
Features of Light. eg: Polarized light vs. Nonpolarized
Snell’s Law:
The Index of Refraction: a measure of how much light is bent
o n= Vair/Vmedium
o A light beam passing from a less dense medium to a more dense medium is bent toward
the vertical
o A light beam passing from a more dense medium to a less dense medium is bent away
from the vertical
The Critical Angle: angles at which total reflection occurs
Dispersion: n decreases as λ increases
Petrographic Microscope
o Polarizing filters
 Plane Polarized
 Crossed Polars
o Accessory Plates:
 Single λ (550nm)- turns the black field purple
 Quarter λ (125 nm) – turns the black field white
 RULES:
 fast on fast= colors add (rise)
 slow on slow = colors add (rise)
 fast on slow= colors subtract (fall)
Opaque Minerals: USE MORPHOLOGY
Relief
Sketch the Cross Section of a thin section
Quartz Plate
Michel-Levy Birefringence Chart
INDEX OF REFRACTION IS THE INVERSE OF VELOCITY
Isotropic
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Isometric (Cubic) Minerals
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For identifying Isotropic substances, we can:
o Color
o relief is high or low
o relief is + or –
o crystal system
n is the same in any direction, so the indicatrix is a sphere
Interference Figures.
Anisotropic
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All crystal systems except isometric (cubic)
Color
Pleochrioism
Uniaxial (end in “al”):
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Hexagonal and Tetragonal:
n for c is distinct from n for a1 through a3.
Indicatrix: prolate spheroid e>o (+) or oblate spherioid (– ) e<o
Incident Light
o transmitted parallel to the c-axis: one index of refraction in all vibration directions
o transmitted perpendicular to c-axis: separated into two rays, 1 vibrating parallel to c and
has ne, the other perpendicular to c and has no.
 PARALLEL TO C-AXIS = e ray: rotates radially (IF)
 PERPEDICULAR TO C- AXIS= o ray: rotates tangently (IF)
o oblique section to the c-axis: resolved rays follow two paths, e and e’ (MOST COMMON)
Δ=d |ns- nf|
o d=thickness of slide
o |ns- nf|= absolute value of birefringence
o units are in μm, or 10-3 mm
Interference Figures:
o isogyres create a melatope (centered or off-centered)
o BURP (blue upper right)(+)/ YURN (yellow upper right)(-)
Optic Sign
o Uniaxial (+) = the LARGEST index of refraction
o Uniaxial (-) = the SMALLEST index of refraction
Biaxial: (“2 optic axes”)
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OPTIC PLANE: Indicatrix is a triaxial ellipsoid (optic axis, trace of circular section, 2V)
OPTIC AXES:
o Bxa, acute bisectrix, axis bisecting the acute angle between the optic axes
o Bxo, obtuse bisectrix, axis bisecting the obtuse angle between the optic axes
o 2V= the angle between the optic axes.
OPTIC SIGN:
o Biaxial (+)- optic axes are close to Z (the large index) (Bxa=Z):
 circular section is closer to X than to Z.
o Biaxial (–)- optic axes are close to X (the small index) (Bxa=X):
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 circular section is closer to Z than to X.
o 2V=90° means that it is neither (+) or (-)
EXTINCTION
o Orthorhombic minerals have parallel extinction
o Monoclinic will have inclined extinction, except when looking perpedicular to b
 one principle index must equal the b crystallographic axis
o Triclinic have inclined extinction in ALL orientations
INTERFERENCE FIGURES
o Vibration directions form an elliptical pattern
o Isogyres pull apart and come together as the slide is rotated
 The larger the 2V of a mineral, the further apart the isogyres pull upon rotation
o BURP and YURN
o The point of maximum curvature is the melatope.
o TYPES OF BIAXIAL INTERFERENCE FIGURES:
 Bxa Figure
 Isogyres don’t leave field of view or leave only with large degrees of
rotation
 Bxo Figure
 Isogyres leave field of view with rotation of 5°-15°
 Flash Figure
 Isogyres leave field of view with rotation of < 5°
 Optic Axis Figure
 Single isogyre rotates in the field of view.
 Random Figure
 A single curved isogypre is present but the melatope cannot be found
Geologic Phase Diagrams