Test 2 Flashcards
What are the benefits of using a petrologic microscope as opposed to a non-petrologic microscope?
By using a petrologic microscope, we can identify minerals, determine rock type, determine crystallization sequence, document deformation history, observe frozen-in reactions, constrain pressure/temperature history, note weathering/alterations
List the different parts and functions of a petrologic microscope.
Ocular
Eyepiece; part of the microscope that you look through.
Bertrand lens
An accessory lens that can be inserted to bring interference figures into focus.
Analyzer
Upper polarizer; reorients light as it passes through the scope (on our scopes, reorients light in the north-south direction)
Accessory plate
Additional plates that can be inserted to change the way a thin section is used. We most commonly use a gypsum accessory plate, but have also used a quartz wedge.
Objective
The major magnifying lens of the microscope–our microscopes include 4x, 10x, and 40x.
Stage
Where the thin section is placed.
Focus knob
Knob to the rear of the microscope that allows the user to adjust focus (changes height of the stage–must be used carefully when using 40x objective)
Polarizer
Lower polarizer; only light moving in one direction can pass through (light moving in east-west in our microscopes)
Iris diaphragm
Controls the amount of light passing from the light source to the thin section
Condenser
Concentrates light from the light source so that it passes through the thin section.
Light source
Bulb on the base of the microscope from which light is emitted.
What is the difference between the orthoscopic versis the conoscopic mode of microscope observation?
Orthoscopic mode – usual mode used, unfocused beam travels though sample up microscope tube
Conoscopic – special condenser lens is inserted to produce conoscopic illumination, causes light to focus on a specific point
What is the difference between an interference figure and interference colors?
Interference figures are used to determine whether a mineral is uniaxial or biaxial, and the optic sign of a mineral. Interference colors are colors that are visible when a thin section is viewed in XPL.
Birefringence
Determining the range of refractive indices by using interference colors and the Michel-Lévy chart.
Isogyre
Black “cross” of an interference figure.
Melatope
Dark spot (center) of an interference figure that marks the location of the optic axis or axes. Uniaxial have one, biaxial have two–the distance bewteen the two indicates the 2V angle.
2V
Angle between optic axes.
Optix axis
Direction in which a ray of light suffers no birefringence.
Optic plane
The plane containing the optic axes of a biaxial crystal.
Amplitude & Wavelength
What happens to an incident ray of light upon entering a crystal of calcite?
The ray of light is doubly-refracted.
What is plane poloarized light?
Light that has only passed throught the lower polarizer of the microscope (only reoriented in the east-west direction on our scopes).
Explain how interference colors are produced in a petrologic microscope.
Interference colors are visible when a thin section is viewed with both polarizers engaged (in XPL). These colors are visble as you rotate the stage, but go to black at 4 points along the rotation (every 90 degrees)–these points are called “extinction”.
What is the angular relationship between the Ordinary and Extraordinary rays?
Ordinary ray = stationary
Extraordinary ray = rotating
Sketch and describe the Michel-Lévy chart.
Using the Michel-Lévy chart, you can use interference colors to calculate the birefringence of a mineral.
Draw an organizational chart that classifies minerals on the basis of their optical characteristics.
Opaque (isotropic) vs. Non-opaque (anisotropic)
IF anisotropic, can be uniaxial vs. biaxial
Can be positive or negative
What is the difference bewteen the polarizer and the analyzer?
The polarizer is the “lower” polarizer, ours reorients light along the east-west direction. The analyzer is the “upper” analyzer, ours reorients light along the north-south direction.
Explain how extinction occurs.
When viewing an anisotropic mineral in XPL to view intereference colors, the mineral will appear to “black out” every 90 degrees.
How do you choose a good grain for optaining an interference figure?
Find a grain that stays gray/dark as the stage is rotated.
Pleochroism
Colors that are observed in PPL.
Corundum–yellow, brown, orange. Al2O3
Apatite–blue, yellow, colorless.Ca5(PO4)3(F,Cl,OH)
Difference between pleochroic colors and interference colors
Pleochroic colors are viewed in PPL, interference colors are viewed in XPL
How would you differentiate between a pyroxene and an amphibole mineral optically?
Look for the 90 degree cleavage angles of pyroxene.
How many RIs are associated with isotropic, uniaxial, and biaxial minerals?
Isotropic – one RI
Uniaxial – two RIs
Biaxial – three RIs
How is a Refractive Index (RI) measured?
RI = n
n = veolcity in air/velocity in mineral
Describe the difference between First Order, Second Order, and Third Order interference colors.
First order – blacks, grays, oranges, reds
Second order – bright, “Pepto” colors
Third order – “pastel” lighter colors
What are the three types of extinction? Provide examples and chemical formulae for each.
Parallel extiction – orthopyroxene
Inclined extinction – most micas
Symmetrical extinction – calcite
What is the most expensive material in the universe?
Antimatter
$1750 trillion / ounce
