Geos306 Mineralogy
Fall, 2003
Ts for lab2_part1 and 2, code and provenance
metamorphic
10-20-59-1: orphan: garnet, ky, sillimanite, biotite, muscovite, quartz, plag?
103-102: orhpan: muscovite, granato, biotite, quartzo, plagioclasio:good for bt
min 102: box reference minerals: biotite, sillimanite, granato, cianite, quartz, plag
74-501-25: box reference minerals: staurolite, granato, biotite, quartz
min30: box reference minerals: garnet, + something else.
5-30-61-21, 22, 1-5-59-1 same as for lab 1
min25 and min 31 from min ref calcite
5-31-63-2 from quitx box calcite
19, mineral reference: tormalina, muscovite, biotite, quartz
min 137 and min 140 and min13: from qutiz minerals: tourmaline, plag, quarz
min89: box minquitz: diopside
min 76 e 2b from reference minerals: diopside
min58 min44 : quitz min: ky
min63: ref minerals: ky
igneous
min135: box reference minerals: green spinel
K65: orphan: gray spinel, olivine, biotite; spectacular thin section
3(a) orphan: gabbro, opx-plagioclase
7-5-61-17 orphan: gabbro opx-ol-cpx plagioclase
min114: quitzbox: quartz, ortho
min118: quitzbox: quartz, ortho
min275:2 quitzbox: quartz
min85 & min 84 from minbox: olivine
min 61 & text48 as for lab1: olivine
min104: qt-ortho
volcanic
min4: box reference minerals: leucite, aegirina
min1: box reference minerals: leucite, sodalite, glass
min2: minquitz: leucite, sodalite, aegirine
92:9: petrology lab#7: leucite, sodalite, aegirina
4: orphan : green hornblend sanidine
74-501-31 as for lab1 (there is another section like this)
min49
74-837-13 from lab1
Lab 2 part 2
Page 0
Geos306 Mineralogy
Lab 2 part 2
Fall, 2003
Page 1
Geos306 Mineralogy
Fall, 2003
LAB2- part 2
EXTINCTION ANGLE AND SLOW AND FAST
DIRECTION
More on how to distinguish among different textures
Metamorphic, Volcanic or Plutonic?
Hints on how to distinguish among these different types of rocks:
Two major criteria at first will help you:
1) mineralogical associations: i.e. certain minerals are found exclusively (or close to
exclusively) in specific environments. Examples: glaucophane, lawsonite, Al-silicate,
garnet (with some exemptions) are found ONLY in metamorphic rocks.
2) Texture: i.e. the geometric relations of different grain in the rock are a function of the
environmental conditions at the time of crystallization.
*******************************************************
Parallel, inclined and symmetrical extinction
The thin-section making process produces a slice of rock with 30micron thickness and
planar surfaces (if the job is well done). Where a cleavage plane, twin’s composition
plane, or crystal face of the mineral grain (for euhedral minerals) intersects its planar
upper or lower surface, a line of intersection, called its trace, may be visible. The angle
between this trace and the crystal extinction position constitutes the crystal’s extinction
angle.
How do we measure extinction angles?
1) align the trace to the crosshair (usually with the N-S direction)
2) read the angle on the stage (M1)
3) bring the crystal to extinction by rotating the stage
4) read the angle on the stage (M2)
5) M1-M2 = extinction angle
6) always measure more than one grain, since the extinction angle is a function of the
orientation of the grain itself.
Remember: by measuring the extinction angle, you measure the angle between a
vibration direction and a specific trace. The most useful trace is that of a cleavage plane.
Sometimes the quality of the extinction can be very revealing: wavy extinction or patchy
extinction often indicates that the crystal is quartz; “birds eye” extinction is a
characteristic of biotite (more on later labs) or phyllosilicates in general.
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
There are three types of extinction:
1) parallel or straight when the angle of extinction is zero or close to it
2) inclined or oblique when the angle of extinction is anywhere in-between zero an ninety
degrees
3) symmetrical when two different traces are visible and the crosshair, when the mineral
is in extinction position,- bisects the angle between the traces.
Slow and fast direction
If you are wondering whether or not is possible to discriminate between the fast and slow
direction on vibration, the answer is yes.
In order to accomplish this task we need to use ‘accessory plates’. The one mounted on
our microscopes is called gypsum plate or first-order red or 550 plate. Look in your
microscopes in XPL with nothing on the stage and push the gypsum plate in, what do you
see?
Take the 550 plate off, what happens now?
Can you explain your observations?
The slot for the ‘accessory plates” is set up such that an angle of 45 degrees is made
between the plate and the polarizers. Furthermore the direction of the privileged
directions of vibration of the mineral used for the plate are always reported, for the 550
plate the slow direction is parallel to the NE-SW direction of our field of view. Whenever
the gypsum plate is inserted 550 nanometers of retardation are always added to the path
of the rays travelling towards the analyzer.
Locating a grain’s slow vibration
To follow this exercise try to have an interference color chart in front of you.
Assume that a grain exhibits 2nd order blue (retardation = 650 nm). To locate its slow
direction we need to rotate the grain 45o off extinction.
Before going on with the exercise try to understand why we need to perform this rotation
from the extinction position, and when an answer is reached write it here
Lets assume that the slow vibration direction is NE-SW, or if you want parallel to the
slow direction of vibration of our gypsum plate.
What happens to our rays is as follow: the light coming out of our crystal has to go
through the gypsum plate before reaching the analyzer. The slow ray enters undisturbed
(why? ) the gypsum plate but since it vibrates along the slow direction of the plate, its
retardation compared to the fast ray, will be increased. Therefore upon inserting the
gypsum plate, if the slow direction of vibration for our sample parallels the one for the
plate, the IC of our sample will increase, since the retardation between the two rays
increases as well.
Lets prove this to ourselves with some small calculations:
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
Retardation of
gypsum plate
original interference
color and its
retardation
550 nanometers
2nd order blue
650 nanometers
for slow direction
parallel to slow of
plate, final color =
650nm + 550 nm =
1200 nm
or
3rd order blue
for fast direction
parallel to slow of
plate, final color =
650nm - 550 nm =
100 nm
or
1rd order gray
Therefore depending on the color we will see, it will be possible to identify the slow or
the fast ray, and as a consequence find the other one.
When minerals are euhedral and have elongated shapes (like prism) is possible to
associate the direction of vibration with the direction of elongation of the crystal, being
this a very important information useful in the identification of minerals.
If the slow direction of vibration is parallel to the direction of elongation of the mineral
we say that the mineral is ‘length slow”; if the fast direction of vibration is parallel to the
direction of elongation of the mineral we say that the mineral is ‘length fast”.
Remember, is always possible to find out the orientation of the slow and fast ray, but only
when grains are elongated we can say whether they are length fast or length slow.
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
READ THE INSTRUCTIONS FOR THE NEXT GROUP OF EXERCISES
NOTE:
In the next exercises you will practice the concepts developed in this lab and in LAB8
part 1 and in LAB7. For each group of TS you will have to determine
1) the type of extinction
2) the angle of extinction
3) the position of the slow and fast ray, and where applicable determine if the crystal is
length slow or fast.
4) The max interference color (color and order).
5) the thickness of the slide by comparison with the standard interference colors for
quartz and plagioclase.
6) Do the measurements on more than one grain.
NOTE: Every time you determine the slow and fast direction of vibration you have to
show beforehand a table as the one just developed in this section and the position of fast
and slow ray on your sketch.
FURTHERMORE: Once done with the first part of the exercise that is carried out in XPL
you need to describe the mineral in PPL. Indicate:
1) the color
2) if the mineral is pleochroic give the pleochroic scheme
3) relief
4) alteration
5) zoning
6) cleavage (give angles if you see more then one direction of cleavage)
7) fractures
8) habit
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
Exercise 3.1
TS min13 or min140 or min (137)or (19)
You worked on TS.............................
In these TS the mineral we are going to study shows olive-green/pink color in PPL.
For at least three grains do the tests mentioned above and for each crystal draw a sketch
with the crosshairs and the position of the mineral as in extinction.
Once done with the first part, proceed with the observation in PPL and report the
information asked.
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
Exercise 3.2
TS min89 or min76 or 2(b)
You worked on TS.............................
In these TS the mineral we are going to study shows pale green color in PPL.
For at least three grains do the tests mentioned above and for each crystal draw a sketch
with the crosshairs and the position of the mineral as in extinction.
Once done with the first part, proceed with the observation in PPL and report the
information asked.
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
Exercise 3.3
TS min44 or min58 or min63
These slides are often with the petrology lab: “Intro to metamorphic
rocks”
You worked on TS.............................
In these TS the mineral we are going to study shows tan color and high relief in PPL.
For at least three grains do the tests mentioned above and for each crystal draw a sketch
with the crosshairs and the position of the mineral as in extinction.
Once done with the first part, proceed with the observation in PPL and report the
information asked.
Lab 2 part 2
Page 8
Geos306 Mineralogy
Fall, 2003
Exercise 3.4
TS: 1(a), 3, 9-11-69-4, 74501-31, R15, text29, 7-5-61-2, 172, 5-30-61-18, min81
Here is the list of the major minerals in the above mentioned TS:
1(a): glaucophane, lawsonite, garnet, chlorite
3: bitotie, sanidine, amphibole, opaques, alteration sausserite
9-11-69-4: orthoclase, plagioclase, chlorite, biotie, quartz, sphene, opaques
74501-31: plagioclase, amphibole (brown hornblende)
R15: quartz, microcline, biotite, opaque
text29: opaques, orthopyroxene, plagioclase
7-5-61-2: orthopyroxene, plagioclase, clinopyroxene (augite)
min81: clinopyroxene (augite), olivine, amphibole, opaques, plagioclase
172: garnet, muscovite, chlorite, biotite, quartz
5-30-61-18: : Staurolite, garnet, chlorite, biotite, muscovite, quartz
The exercise will proceed in the following manner:
1) Look at least at 4 TS; try different minerals if possible
2) I will give you hints about the mineral I want you to look at
3) Once you have identified it with the help of the hints given to you (you might use the
book if you like) you will have to answer the questions and fill in the identification
table provided at the end of the lab
TS 1(a) – metamorphic
Mineral to describe: glaucophane
Hints: this mineral belongs to the ‘amphibole group’. One of its distinctive
features represented by the presence of cleavage traces. Two planes at about 120o
for sections cut perpendicular to the c axis and 1 plane for sections cut parallel to
the c axis.
Locate the mineral and enter your observations in the given table
Mineral to describe: garnet
Hints: this mineral is isotropic
Locate the mineral and enter your observations in the given table
Answer the following question:
Which mineral, glaucophane or garnet, is the prominent mineral in the TS?
TS 3 – volcanic
Mineral to describe: biotite (phyllosilicates)
Hint: when the cleavage traces for this mineral are parallel to the E-W polarizer,
you see a very strong pleochroism. Pleochroic scheme: black, light brown.
Locate the mineral and enter your observations in the given table
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
Mineral to describe: hornblende
Hint: this mineral belongs to the ‘amphibole group’ (remember the cleavage). The
highest interference colors for hornblende in this TS are lower second order.
Locate the mineral and enter your observations in the given table
Answer the following question:
What is the relative amount of these two minerals (biotite to hornblende) in the TS?
a) more biotite than hornblende
b) more hornblende than biotite
c) about the same
In a general sense, how much water was there in the rock? Explain your reasoning. (Let
me know if this question doesn’t make sense to you. I’ll explain it.)
TS 9-11-69-4 – plutonic
Mineral to describe: chlorite (phyllosilicates); widespread mineral that can be primary or
secondary in origin. I this TS chlorite is secondary in origin, i.e. forms from alterations of
amphiboles and biotite. As a consequence its habit might recall the one of the mineral
that chlorite is substituting for, a phenomena called “pseudomorphism’.
Hint: green pleochroism and weak birefringence
Locate the mineral and enter your observations in the given table
Mineral to describe: quartz
Hint: quartz is recognized by its low relieve, lack of twinning or alterations
Locate the mineral and enter your observations in the given table
Answer the following questions:
1) How abundant is plagioclase compared to quartz?
2) What is the grain size for quartz compared to the other tectosilicates present in the thin
section?
What does the grain size tell you about the minerals and the rock (nucleation rates of
crystals, cooling rates, etc.)?
TS 74501-31 – volcanic
Mineral to describe: hornblende (amphibole)
Hint: its pleochroic scheme is : dark black green / yellow green
Locate the mineral and enter your observations in the given table
Mineral to describe: plagioclase
Hint: 1st order gray/white interference color
Locate the mineral and enter your observations in the given table
Lab 2 part 2
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Geos306 Mineralogy
Fall, 2003
Answer the following questions:
what % of the TS is made up of plagioclase?
Is it more or less abundant then the amphibole?
What does this tell you about the composition of the rock?
TS R15 – plutonic
Mineral to describe: microcline
Hint: cross-hatched twinning.
Locate the mineral and enter your observations in the given table
Mineral to describe: quartz
Hint: 1st order yellow white interference colors
Locate the mineral and enter your observations in the given table
Answer the following question: which of the minerals identified above is the most
dominant species?
a) quartz
b) microcline
c) about same amount
What does this tell you about the composition of the rock?
text29 – plutonic
Mineral to describe: plagioclase
Hint: polysynthetic twinning.
Locate the mineral and enter your observations in the given table
Mineral to describe: orthopyroxene
Hint: highest interference color is 1st order yellow
Locate the mineral and enter your observations in the given table
Answer the following questions:
Which mineral displays the most euhedral habit?
What does this tell you about this mineral?
Lab 2 part 2
Page 11
Geos306 Mineralogy
Fall, 2003
Min81 – plutonic
Mineral to describe: olivine
Hint: serpentine alteration along the fractures; 3rd order interference colors
Locate the mineral and enter your observations in the given table
Mineral to describe: opaque
Locate the mineral and enter your observations in the given table
Answer the following question:
How much olivine compared to the opaque minerals is present in the TS?
What does this tell you about the composition of this rock?
172 - metamorphic
Mineral to describe: garnet
Hint: isotropic
Locate the mineral and enter your observations in the given table
Mineral to describe: muscovite
Hint: not pleochroic, high relieve, 3rd order interference colors
Locate the mineral and enter your observations in the given table
Answer the following questions:
Compare the size of garnet and muscovite. Which mineral species on average displays
the bigger size?
5-30-61-18 - metamorphic
Mineral to describe: staurolite
Hint: pleochroic mineral in the shades of yellow; sieved texture
Locate the mineral and enter your observations in the given table
Mineral to describe: quartz
Hint: low birefringence, low relife
Locate the mineral and enter your observations in the given table
Answer the following question: How many garnets do you see in the TS:
a) less then 5
b) between 5 and 10
c) more then 10
What does this tell you about the protolith (pre-metamorphose “version”) of this rock?
Lab 2 part 2
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