Petrography Flashcards

refractive index
Terms Definitions
Opt (-)
perfect rhombohedral cleavage {10-11}
Extream Birefringenece
Polysynthetic twinning with {01-12} (extinction obtuse to twinning)
Opt (+)
fibrous plates or fibrous scally aggregates
Fair relief
moderate birefringence
parallel extinction
length fast orientation in the scally aggregates
Opt (-)
euhedral short prismatic hexagonal crystals
Imperfect cleavage parallel to {10-10}
very low relief
weak birefringence
interference figure without any rings
Halite, Fluorite
Monazite, Apatite, Lazulite
Sphalerite, Pyrite, Calcopyrite
Calcite, Dolomite, Magmesite
six-sided euhedral crystals
fair relief
no birefringence
Idocrase (Vesuvianite)
Opt (-)
euhedral crystals
high relief
weak birefringence
Triclinic Minerals:
Plagioclase, Microcline, Wollastonite
Retardation is measured in:
black with submetallic luster - translucent to brown on the edges
no cleavage
concodial fracture
high relief
Crystal Shape:
Euhedral, Subhedral, Anhedral
Triclinic System
No axes of rotation
Retardation is manifested by?
interference colors
An ellipsoid which represents geometrically the different vibration directions in a mineral and illustrates conceptually the optical features of a crystal. The origin is a point lying at the centre of the ellipsoid and the axes of the ellipsoid are proportional in length to the refractive indices of beams of light vibrating at right angles along them.
Isotropic Minerals
Isometric, Tetragonal, & Hexagonal
Crystal Systems
Cubic, Tetragonal, Hexagonal, Monoclinic, Orthorhombic, & Triclinic
Quartz, Chalcedony, Opal, Garnet, Olivine, Zircon
Fear of Fire: Pyro=fire; xene=fear
Optic Sign = (+) (P for positive)
Tetragonal Minerals:
Rutile, anatase, cassiterite, vesuvianite, calcopyrite, wulfenite
Hexagonal minerals are Isotropic or Anisotropic?
Monoclinic minerals are Isotropic or Anisotropic?
Polychromatic Light
White Light. Contains many wavelengths.
Monochromatic Light
Light of a single wavelength.
Hexagonal Minerals:
Illmenite, tourmaline, calcite group, corundum, brucite, apatite, beryl
Refractive Index of common mounting medium
Isometric minerals are Isotropic or Anisotropic?
Orthorhombic System
3 twofold axes of rotation
Triclinic minerals are Isotropic or Anisotropic?
colorless to green - olive green - brown
euhedral or subhedral
imperfect octahedral cleavage
high relief
no birefringence
distinguished from garnet by its octahedral form.
colorless to gray - pale blue - bluish green to deep blue
euhedral to anhedral octahedrons and dodecahedrons
low relief
refractive index is higher than sodalite.
Vibrations direcrtions of the mineral are exactly coincidence with vibration directions of the microscope (Black Cross or parobola)
Important Positive Uniaxial minerals:
Quartz, Brucite, Zircon, Rutile
The degree of visibility of a transparent particle in a medium under the microscope.
Uniaxial substances:
"Substances are those that crystallize in the tetragonal and hexagonal systems.
Biaxial Indicatrix
Illustrates the refractive index for vibration directions of light in 3 principal planes.
3 Biaxial Refractive Indices:
nα, nβ, & nγ
Is an optical phenomenon in which mineral grains within a rock appear to be different colors when observed at different angles under crossed-polarizing
Miller indices
Way of describing planes. a1=∞ a2=∞ a3=1 (111), (100)
How may light waves interfere?
Constructively or destructively
Uniaxial indicatrix
Illustrates the refractive index for vibration directions of light in 2 principal planes.
Refractive index of the ordinary ray in a uniaxial system.
Organization of mineralogy
System - Class - Group
Light is measured in __________.
Millimicrons 10-9 m
Amphi = anti or negative
Optic Sign = (-)
Isochromes must be a ______ in a uniaxial mineral.
In passing from a medium of lower index to a medium of higher index, a light ray is bent toward the _______.
Biaxial Interference Figures:
Centered Optic Axis Figures, and Centered Bxa Figures
"The decomposition of a ray of light into two rays when it passes through certain types of material. This effect can occur only if the structure of the material is anisotropic. If the material has a single axis of anisotropy or optical axis, (i.e. it is uniaxial) birefringence can be formalized by assigning two different refractive indices to the material for different polarizations. The birefringence magnitude is then defined by Δn=nω-nε. For biaxial nα,nβ,nγ
Isotropic minerals and mineraloids
Volcanic glass, fluorite, sodalite, analcime, hauyne, garnet, spinel, chromite, perovskite. Leucite is often considered isotropic, although it is weakly anisotropic.
Interference Figures Give:
Optic sign, 2V, Extinction angel, birefringenence
Describe High-order white
The interference colors at orders greater than order 6, become a blend of colors that is referred to as high order white.
Equation for Biaxial Optic Sign for positive crystals:
(nβ-nα) < (nγ-nβ)
The bisector of the obtuse angle between the the optic axes is termed:
Obtuse Bisectrix (Bxo)
Relief is measured with respect to:
The mounting medium n=1.54
Centered Optic Axis Figures:
Obtained on sections of lowest birefringence.
Relief is expresses qualitatively as:
low, moderate, high, and extreme
3 Principal Optic Direction of Biaxial Substances:
X, Y, & Z
Flash Figure:
If you look at a uniaxial or biaxial mineral perpendicular to the optic axes, a flash figure results. Grains with this orientation have the maximum interference color for the material
What are Light Waves?
Light waves are transverse waves that vibrate perpendicular to the line of propagation
Biaxial minerals are characterized by:
Extinction angles & 2V
Relief, Crystal shape, Color, Cleavage, Associated Minerals, Optic Sign, Birefringence, Pleochroism, and sign of elongation
Snell's Law
n2/n1=sin i/sin r
where i and r are the angles of incidence and of refraction, respectively.
If 2V is small:
You will see both isogyres in Bxa Figure
When the Polarizers are crossed....
no light passes through the optical column
Triclinic Crystals Biaxial Optical Properties
None of the optical direction or indices coincide with crystallographic direction
How do you tell the difference between Beryl, Apatite, Corundum, and Vesuvianite?
Beryl=Large prizmatic crystals.
Apatite=cleavage parallel to the length of the crystal. Corundum=High releif, twinning lamellae. Vesuvianite= Tetragonal, high releif
The polarizer of a microscope provides.......
plane polarized light with which to view minerals.
Do the highest orders have white?
No, because white light is the combination of all light. As you move above first order there are always destructive waves blocking some color
Given the three principal refractive indices, how can you estimate 2V?
Using the Extinction Angle.
If EA < 30° = Small (Curved isogyres, may look like Uniaxial Cross)
If EA is 30° - 60° = Intermediate (Noticeably curved isogyre)
If EA > 60° = Large (Single isogyre)
The Becke Line:
is a thin bright line of light at the interface between a mineral fragment and an immersion medium.
If the optic axial direction X is the Bxa
The mineral is Negative (-)
Orthorhombic Crystals Biaxial Optical Properties
X = a,b, or c
Y is perpendicular to a,b, or c
Z is perpendicular to a,b, or c
Is there a first order blue?
No, blue is a shorter wavelength and doesn't show up until there's destructive wavelenghts blocking the red. Red is the longer wavelenght
Monoclinic Crystals Biaxial Optical Properties
One of the X, Y, Z direction is parallel to the b crystalographic axis and the other two don't coincide with crystalographic directions
In convergent light, a centred optic axis figure is obtained on sections that appear isotropic under crossed polarizers. Why?
These are sections cut normal to the c-axis of the crystal.
On insertion of the accessory plate (550 μm), colors rise in the NE quadrant if the mineral is positive, but fall in the NE quadrent if the mineral is negative.
Using a centered Bxa figure of a biaxial mineral estimate 2V.
Looking at the curvatures of the isogyre.Then measure the angle from the crosshairs to one leg of the isogyre.
Using a centered optic axis figure of a biaxial mineral measure optic sign.
Using the accessory plate to determine if the mineral is (+) or (-)
Slow + Slow = Sign (+ or -) & Blue or Yellow
Positive (+) Blue
"The refractive index of a substance is defined as ....
the velocity of light in a vacuum (v) divided by the velocity of light in the substance (V).
The acute angel between the optic axes is refferd to as what?
Normally taken as the acute angle unless 2V is very high.
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