Geos 306 Mineralogy Fall 2003
LAB 1
Introduction to the petrographic microscope
Properties in PPL: Color, pleochroism, cleavage, habit
Properties in XPL: Twinning, zoning
Learn these minerals: plagioclase, K-spars polymorphs
(and look a bit at hornblende, pyroxenes and olivine)
Before starting any exercise read the instructions. When asked to sketch something, be neat and do it as well as you can, indicate colors of minerals, the mode of observation (XPL or PPL), and the magnification.
Introduction to the petrographic microscope
About Centering of the microscopes:
All microscopes should be centered. Centering is important to be able to recognize the properties of minerals under the microscope properly. Each of the lenses must be centered with respect to the axis of rotation of the microscope stage. Objective lenses are commonly mounted on a turret that can be rotated, never, never ever rotate the turret by pulling on the lenses.
How to recognize proper centering of the microscope:
Choose a small grain with the lOx objective and place it in the center of your field of view (indicated by the crosshairs in the ocular). It should remain on or very closely around the crosshairs as you carefully turn the stage. Adjustment is done using the Stage Centering Knobs situated at two o' clock and ten o' clock; be careful not to touch them bv by accident when turning the microscope stage. Should the microscope not be centered ask your TA for assistance.
PPL vs. XPL
If light waves are coming at you in one plane only (using only one polarizer) you see “plane polarized light” (PPL). Using two polarizers at 90° to each other, no light can pass throuugh, unless you have something in between to re-scatter the waves. The cross polarizers are called “cross polarized light” (PPLXPL)
How to recognize if the polarizer and the analyzer are perpendicular to each other:
To determine whether the polarizer and analyzer have their vibration directions at right angles, remove the thin section from the stage and insert the analyzer. Use a medium power objective. The field will be black if the nicols are crossed. If not, call for help.
Be careful about focusing!
While it is fine for low magnifications to just look into the ocular as you focus the slide, this procedure can be deadly under high magnification as you run the risk of actually going through the thin section. Thin sections are fragile and costly to replace so please focus carefully. For high magnification always close in on the slide watching from the outside and do the final focusing with the fine focusing knob (inner, more prominent knob).
What is the correct way of turning the objective lenses? (1 Point)
How do you recognize that the microscope is properly centered? (1 Point)
EXERCISE 1: getting to know your microscope
Exercise 1.1: With the analyzer out, look at a blank slide (or no slide). What happens when you put in the analyzer?
Is there any change when you rotate the stage?
Exercise 1.2: Focus on a mineral grain and fill in the table below. Note which way the stage moves when you turn the focusing knob (up or down). Be careful when using the high power lens; make sure the slide is correctly placed on the stage, i.e. with the cover slip on top. Thin sections are fragile and costly to replace so please focus carefully.
Type of microscope:______(Zeiss, or Olympus, or Meji)
Lens Color Code / Objective Magnification / Total Magnification / Diameter of Field / Relative BrightnessRed
Yellow
Green
Blue
OPAQUE VERSUS TRANSPARENT
An opaque mineral -often referred to as an “opaque” -is a mineral that absorbs all the light going through it and therefore it appears completely dark under the microscope in PPL as well as XPL regardless of stage rotation. Opaque minerals are present in almost every TS and they are mostly represented by sulfides or oxides.
Exercise 2.1
name at least two minerals – per group - belonging to the families just mentionedsulfide and oxide families.
Transmitted light microscopy can provide only minimal information about the nature and the identity of these opaque minerals. The technique used for opaque minerals is “reflected light microscopy”.
Exercise 2.2
TS (min92) or min65 or 7-5-61-9
You worked on TS______
Are there any opaques in this TS?
Make a sketch of an opaque mineral both in PPL and XPL
What happens to the opaque mineral as you rotate the stage in XPL?
Are there any transparent minerals in this TS?
RELIEF AND BECKE LINE
Relief is a relative term and refers to how sharply the outlines of a grain “stand out” from the medium surrounding it. If the difference in RI (refractive index) between the grain and the glue/adjacent mineral is great, relief will be high; if small, it will be low. In H.G. Wells’ story, the “invisible man’s invisibility was achieved not only by making the man colorless, but also by causing his refractive index to be equal to that of air. Thus, he had zero relief.
In the following sections we will exercise our eyes to recognize the minerals with high relief from the one with low relief. Afterwards we will introduce the ‘Becke line’ method to test which mineral has higher relief among grains with similar but not equal RI.
Relief and Becke line are best observed in low-medium illumination, PPL (it is very difficult to make any statement about relief in XPL), slightly closed diaphragm.
Exercise 3.1:
What is the mathematical definition of RI?
R.I =
If a mineral A has an R.I.=2 and mineral B has R.I.= 1.5, in which one does the light travel faster?
Exercise 3.2
TS 3-4-59-2 (Three TS)
Set the microscope in PPL, low illumination (two red rectagularsrectangles) and close the upper diaphragm at 0.3. At first observe the TS with the 4X objective.
Scan the TS, how many different types of minerals do you see, not including the opaques?
It looks like there is just one in prismatic and granular habit, but if you insert the second polarizer (put things in XPL) you will see that the empty spaces fill up with other mineral grains that were not as evident in PPL. The reason why you do not see the second mineral is because it has very low RI and therefore its grain boundaries and other features do not make much contrast when the light goes through it. Go back to PPL setting.
Now increase the magnification to 10X and observe the TS again, you will notice that the mineral with higher relief “stands out”. Give yourself some time to be able to see this feature and if necessary use higher magnification. It is like the mineral is resting on top of the TS. Do you agree with this statement?
The mineral with low relief is quartz. Learn how it looks like in both PPL and XPL.
Becke line test: this test allows you to identify minerals by comparing the RI’s of adjacent grains. The Becke line moves towards the mineral with higher RI as the distance between the objective lens and the stage is increased by lowering the stage. In order to observe the Becke line you have to focus and defocus slightly by using the small focusing knobs. Call your TA if not able to see any line.
Perform this test on the given TS (3-4-59-2) using as reference the same mineral with high relief as before, and draw a sketch of what you observed.
Exercise 3.3
You must have finished ex. 4.2 before starting this exercise
TS 223-1,2,3 (pick one of these and indicate the full code before answering the questions):
TS 223-......
Examinelook in the circled area, in PPL. Do you see grains with different relief?
How many different minerals can you see just by using their relief to discriminateas discriminant?
Draw a sketch of what you see, code the minerals using numbers, and indicate the relief of the minerals grains by using modifiers such as high relief, low relief...(you can check the TS in XPL to help you identify the different grains)
Pick two adjacent mineral grains and perform a Becke line test. Which grain has the highest RI? Indicate the grains you picked by using the code number.
TWINNING
Exercise 4.1
TS text33 or min46
TS text33
This section is composed of one single grain of plagioclase, and is thicker than the usual 30 microns.
TS min46
Observe the circled area in XPL.
If you observe the TS in XPL, you will see that the grain displays two distinct bands of different colors. As you rotate the stage you see an alternation of yellow and dark zones in the mineral; then you are observing twinning. When twinning is observed, the mineral grain under the microscope doesn’t represent a single grain, but is made of two individual grains that share one atomic plane. To understand this phenomenaphenomenon, you might think of an apartment complex where different units share one or more walls. There are many different laws that govern twinning. What you see here is called ‘polysynthetic twinning’ and is very common in plagioclase. Another type of twinning, called cross-hatched twinning, is usually enough to indicate that a mineral is microcline. Look for cross-hatched twinning in the next TS’s, and record it in your notes. Look in your textbook and find the optical description for K-spars. Learn how to distinguish plagioclase, K-spar and quartz!
Exercise 4.2
TS text55 or TS (text56) or (LV-1)
You worked on TS ______
Before this exercise make sure you have seen TS text33 or TS min46
TS text55 or TS LV-1: Observe the TS in PPL, you see white and green crystals in a dark matrix. Choose a white mineral and observe it in XPL, you will immediately notice that the whole look of the TS has changed. Try to stay focused on the minerals that display a gray/black /white/pale-yellow color. Rotate the stage. Do you recognize any particular feature? Name it and make a sketch of it.
TS text56: under PPL this TS displays prismatic and tabular green/brown minerals, some black spots, and white patches with no definition or particular features. Switch to XPL, what happens to the ‘white filling?
Observe these minerals by looking around the thin section do you see any feature you know? Name it and make a sketch of it.
ZONING
Exercise 5.1
TS SJA6 (2 TS)
Look in the area circled with solid line, center the white mineral grain and go in XPL. If you rotate the stage you observe what is known as zoning. This feature is a consequence of changes in composition from the core to the rim of the mineral grain.
Take a look around the TS and see if there are other grains showing the same feature. Many times zoning is displayed by twinned minerals (see exercise 1.1 and 1.2).
Chose a grain that you like and draw how zoning appears in XPL.
COLOR AND PLEOCHROISM
Exercise 6.1
TS 5-30-61-21 or TS 1-5-59-1 or TS 5-30-61-22 or 74837-13 (These slides are shared by Lab 2, exercise 2.3)
You worked on TS ______
Before starting the exercise make sure your microscope is set in PPL and you are using the objective 4X and that the diaphragm is open.
Chose one of the TS listed for this exercise and place it on the stage. Do not rotate the stage but observe your TS, feeling free to explore as much of it as you please. As you have already noticed some minerals display color, while some others do not, just like inas for hand specimens. There is one feature that you can’t observe in hand specimens but is very useful in microscopy and will appear under your eyes as you rotate the stage. (Please rotate the stage). What is happening to your colored minerals?
What you have just observed is called pleochroism, which means change of color, as you observed for yourself. Pleochroism is a very distinctive characteristic of some minerals. The minerals in these TS display very strong pleochroism, and therefore it is easy to observe it. Sometimes the change in color is very subtle and is necessary to train the eye in order to be able to detect it.
In these TS there are two or three different minerals with different pleochroic schemes (different color change). Look in the circled area in PPL and make a sketch of the grain you like the best indicating its prominent colors (indicate the whole pleochroic scheme).
Rotate the stage paying attention to the range of colors displayed by your grain and describe it in the following space.
As you may have already noticed, using a range of color in describing pleochroism might be a little confusing; therefore pleochroic colors are given with respect to specific direction. Here is the procedure for to determine what is known as pleochroic scheme:
1. Center the crosshair on the grain you like the best and if necessary change to higher magnification
2. Switch to XPL.
3. Rotate the stage until the grain that you are looking at becomes completely black. In this position the mineral is said to be extinct.
4. Switch back to PPL (do not move the stage)
5. Write down the color you observe
6. Rotate the stage of an angle equal to 90 degrees (you can read the angles on the stage)
7. Write down the color observed in this position
8. Optional: go back to XPL and make sure the crystal is extinct in this position.
Note: You will understand more about the logic of this procedure during class and in the next labs
Now repeat the measurements on the mineral you chose before following the points from 1 to 7 and report the two colors.
CLEAVAGE and FRACTURE
Before looking at the TS please take a look at the hand specimens of minerals listed for this exercise: biotite, plagioclase, microcline, amphibole, quartz and olivine in peridotite.
Answering to these few questions should not take more than a couple of minutes.
Do they have cleavage?