GEOS306 MINERALOGY FALL 2003 TS FOR LAB2PART1 AND 2

EXTINCTION ANGLE AND SLOW AND FAST DIRECTION

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

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:

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.
Texture: i.e. the geometric relations of different grain in the rock are a function of the environmental conditions at the time of crystallization.

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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.

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 45^o 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:

Retardation of gypsum plate

original interference color and its retardation

for slow direction parallel to slow of plate, final color =

for fast direction parallel to slow of plate, final color =

550 nanometers

2nd order blue

650 nanometers

650nm + 550 nm =

1200 nm

3rd order blue

650nm - 550 nm =

100 nm

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.

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

the type of extinction
the angle of extinction
the position of the slow and fast ray, and where applicable determine if the crystal is length slow or fast.
The max interference color (color and order).
the thickness of the slide by comparison with the standard interference colors for quartz and plagioclase.
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:

the color
if the mineral is pleochroic give the pleochroic scheme
relief
alteration
zoning
cleavage (give angles if you see more then one direction of cleavage)
fractures
habit

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.

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.

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.

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:

Look at least at 4 TS; try different minerals if possible
I will give you hints about the mineral I want you to look at
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 120^o 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

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?

more biotite than hornblende
more hornblende than biotite
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: 1^st order gray/white interference color

Locate the mineral and enter your observations in the given table

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: 1^st 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?

quartz
microcline
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 1^st 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?

Min81 – plutonic

Mineral to describe: olivine

Hint: serpentine alteration along the fractures; 3^rd 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, 3^rd 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:

less then 5
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 Page 12

Tags: geos306 mineralogy, geos306, mineralogy, lab2part1