Geos 306 Mineralogy Fall 2003

Lab4/10 TS code and provenance

volcanics:

SJA6: as lab1 ex2.1: plag, amph, bt, san, cpx, opq

SJA9: as lab3 ex 4.1 both are andesites w/bt amp cpx plag

text54: from texture: basalt w/ophitic texture, ol, cpx, plag

haway 7: orphan: cpx olivine plag

tex1: texture: ol, cpx, plag, biot,

text53; texture: thinner: ol, cpx

tex2; texture: cpx, ol, plag, bt

igneous:

2pqd: orphan: bt, brown horn, chlorite, plag, microcl, quartz, ortho,

61-53: opx cumulate, plag and cpx intercumulate,opaques, phlogopite, exsolution

7-5-61-3: quitz minerals: cumulate: opx cumulate, opx/cpx exolution cumulate, cpx/opx exolution intercumulate USE FOR EXAME

min105: quitz min: cpx cumulate, plag inter very altered, apatite, biot

min88: min ref: same as 105

min84: cumulite: ol cum, plag inter, as for lab 2, 2.4

text48:, ol cum, py plag inter, as for lab 2, 2.4

7-5-61-10: box3: cumulate: chromite, opx, ol,

20:5W: orphan: thicker: plag, quartz, green horn, bt, opx w/exsolution,, few cpx

metamorphic

k65: same as lab2: ol, phlog, spinel


Lab 4

Pyroxene and olivine

In this lab we are going to study how pyroxene (clinopyroxene - orthopyroxene) and olivine appear under the microscope. We will keep exercising on recognize textures, i.e. distinguish metamorphic, volcanic and igneous rocks. In lab3 we studied amphiboles and phyllosilicates, in lab1, lab 2 and lab3 we have encountered quartz, plagioclase, Kspar. Therefore, in the description of the TS your memory will be tested and will have to ID those minerals, if present. You should also report if opaques are present.

Exercise 1.1

Look at the drawing for pyroxenes in a textbook. Give the orientation of the cleavage planes with respect to the crystallographic axis c, and sketch what you expect to see in TS for a section parallel and perpendicular to the c axis, keeping in mind what you have learned about cleavage in lab1.

Exercise 1.2

With respect to the minerals in ex 1.1, do you expect to see color in TS or not?

Exercise 2.1

TS 2PQD

You worked on TS......

Look at the mineral in the black dotted area (or areas), you will see the same mineral in different orientations. Report observations on the ID sheet indicating the TS #. Look at more than one grain, if possible. This mineral is orthopyroxene; look for a mineral with interference colors (high 1st order/low 2nd order) and straight extinction.

Exercise 2.1a -

you worked on TS.....

Once done with the observation of the mineral under study, try to take a look at the texture of the rock and presence of other minerals. Indicate if any of the minerals we studied in lab3 are present (biotite, muscovite, chlorite, green/brown hornblende, glaucophane)

Furthermore let’s see what you remember about minerals we just looked at in lab 1 and lab 2

Do you see quartz?

How did you recognize it?

Do you see plagioclase or Kspar?

How did you recognize it?

Do you see garnet?

How did you recognize it?

NOTEs on texture

TS 2PQD represents a plutonic rock. Plutonic rocks are characterized by complete crystallinity, therefore no glass is present, absence of shistosity (most of the times) and grain boundary with angles different from 120 degrees. Minerals that crystallize earlier are usually euhedral. Minerals that came late in the Bowen’s series are, on the other hand, usually anhedral. While looking at this TS try to see what could have been the sequence of crystallization. You know already most of the minerals.

Some metamorphic rocks can resemble plutonic ones since they do not show shistosity and do not have very specific mineral assemblages that point towards the metamorphic condition. A very useful trait that gives information about their nature is the presence of equilibrated grain boundaries, called triple junctions. No more than 3 grains are in contact and the angle is at 120 degrees. This is not often seen in plutonic rocks.

Exercise 2.2

TS 61-53 (Down to 1! TS)

You worked on TS......

Look at the mineral in the circled area (or areas), you will see the same mineral as in exercise 2.1. In this section I like you to pay attention to a very particular feature displayed by the circled mineral in particular and by many others in general. Describe what you observed and make a sketch of it.

What you just studied is called exsolution. Exsolution lamellae are different from twinning lamellae. In class we will discuss their nature. List all the properties of the minerals similar to the one in the circled area. Observe as many grains as necessary to remember the look of the texture

Exercise 2.2a -

you worked on TS.....

Once done with the measurement of the mineral under study, try to take a look at the texture of the rock and presence of other minerals.

Indicate if any of the minerals we studied in lab 3 are present (biotite, muscovite, chlorite, green/brown hornblende, glaucophane)

Furthermore let’s see what you remember about minerals we just looked at in lab 1 and 2

Do you see quartz?

How did you recognize it?

Do you see plagioclase or Kspar?

How did you recognize it?

Do you see garnet?

How did you recognize it?

NOTEs on texture

TS 61-23 is a plutonic rock. The texture of the TS indicates that the rock is a cumulate. We have euhedral-subheuhedral mineral, the one you have been studying (which is.........) and chromite (that appears as.......................). Then the interstices are filled with plagioclase that is completely anhedral and forms the intercumulate phase. This texture indicates something very interesting about the order of crystallization for the rock. If you have already done exercise 2.1 then you can answer the following question. Otherwise wait until you have done the previous exercise and then answer.

Which mineral crystallized first and why?

Make a sketch of cumulate texture


Exercise 3.1

TS min88 or min105

You worked on TS......

This rock is composed mainly of two minerals: the one you are going to study, and one that is heavily altered by ..................... Look for a little marking on the TS, you might confuse it with opaques so if you do not find it call me. All the grains with IC higher than gray are the same mineral; this mineral is clinopyroxene; the max extinction angle for cpx is 45o. Report observations in the ID sheet indicating the TS #

Exercise 3.1a -

you worked on TS.....

Once done with the observation of the mineral under study, try to take a look at the texture of the rock and presence of other minerals.

Indicate if any of the minerals we studied in lab 3 are present (biotite, muscovite, chlorite, green/brown hornblende, glaucophane)

Furthermore let’s see what you remember about minerals we just looked at in lab 1 and 2

Do you see quartz?

How did you recognize it?

Do you see plagioclase or Kspar?

How did you recognize it?

Do you see garnet?

How did you recognize it?

NOTEs on texture

TS min88 and min105 are plutonic rocks. They are very altered as shown by the presence of hydrous minerals that you already know (................ and ..............) and are cumulates. This is shown by the texture of the rocks. The euhedral mineral (the one you have studied) was the first to crystallize and the altered mineral with low IC (...........................) crystallized afterwards as intercumulate phase.

Exercise 4.1

TS SJA-9 or SJA-6

You worked on TS......

Look at the black dotted area (or areas). You will see a pale green mineral in different orientations; this mineral is clinopyroxene; report observations on the ID sheet indicating the TS #. Look at more than one grain, if possible.

Exercise 4.1a -

you worked on TS.....

Once done with the observation of the mineral under study, try to take a look at the texture of the rock and presence of other minerals.

Indicate if any of the minerals we studied in lab 3 are present (biotite, muscovite, chlorite, green/brown hornblende, glaucophane)

Furthermore let’s see what you remember about minerals we just looked at in lab1 and 2

Do you see quartz?

How did you recognize it?

Do you see plagioclase or Kspar?

How did you recognize it?

Do you see garnet?

How did you recognize it?

Exercise 4.1b: is this a plutonic, metamorphic or volcanic rock? why?

Exercise 5.1

TS text54 or haway7

You worked on TS......

Look at the black circled or dotted area (or areas). You will see the same mineral in different orientations. This mineral is olivine; you will recognize it by its high IC. Report observations on the ID sheet indicating the TS #. Look at more than one grain, if possible.

Exercise 5.1a -

you worked on TS.....

Once done with the observation of the mineral under study, try to take a look at the texture of the rock and presence of other minerals.

Indicate if any of the minerals we studied in lab 3 are present (biotite, muscovite, chlorite, green/brown hornblende, glaucophane)

Furthermore let’s see what you remember about minerals we just looked at in lab1 and 2

Do you see quartz?

How did you recognize it?

Do you see plagioclase or Kspar?

How did you recognize it?

Do you see garnet?

How did you recognize it?

Exercise 5.1b

Text 54 and hawaii 7 are the same type of rock. Are they metamorphic, volcanic or plutonic, and why?

Exercise 6.1

Look at TS text3 or text4 or text54 (previous exercise): the texture you see is called ophitic. Make a sketch of it.

Exercise 7.1

TS k65 or text48 or min84

You worked on TS......

Look at the black circled area (or areas). You will see a mineral with high relief in different orientations. This mineral is olivine. Report observations on the ID sheet indicating the TS #. Look at more than one grain, if possible.

Exercise 7.1a -

you worked on TS.....

Once done with the observation of the mineral under study, try to take a look at the texture of the rock and presence of other minerals.

Indicate if any of the minerals we studied in lab 3 are present (biotite, muscovite, chlorite, green/brown hornblende, glaucophane)

Furthermore let’s see what you remember about minerals we just looked at in lab1 and 2

Do you see quartz?

How did you recognize it?

Do you see plagioclase or Kspar?

How did you recognize it?

Do you see garnet?

How did you recognize it?

NOTEs on texture

TS min85 and min84 are plutonic rocks. The texture is characteristic of cumulates, where the mineral that first crystallizes develops euhedral crystal that sink into the bottom (if heavy) or top (if light) of the magma chamber, while trapped liquid develops the intercumulate phases. Here the mineral studied represents the cumulate phase in min85 and the intercumulate phase in min84.

Exercise 8

In your textbook you find the optical description for the following minerals:

orthopyroxene, clinopyroxene and olivine. Compare your observations with the reported description.

Exercise 9 Uniaxial Minerals

Uniaxial Interference Figures

An interference figure allows you to determine (i) whether an anisotropic mineral is uniaxial or biaxial and (ii) the optic sign. The following procedure is used to obtain an interference figure. (Your microscope must be perfectly centered)

1. Cross the polars

2. Find a grain with very low birefringence

3. Focus on the mineral grain with the high-power objective.

4. Flip in the condenser

5. Insert the Bertrand lens.

If the optic axis (c-axis) is oriented perpendicular to the stage, the interference figure will look like Figure 2.6 in your lab manual.

Isogyres are formed where the vibration directions in the interference figure correspond to the vibration directions of the polarizer and analyzer, respectively. If the optic axis is perfectly vertical, the optic axis figure (OA figure, shown on p.15) will not move or change when the stage is rotated.

Isochromes are bands of interference colors. Their number depends on the birefringence of the mineral and on the thickness of the thin section.

Exercise 3: (2 green envelopes) In order to familiarize yourself with possible uniaxial interference figures, obtain different types of interference figures using sections of quartz mounted in various orientations, and draw a sketch of the different figures. Ask your TA for assistance!

9.1 OA figure

(perpendicular to c-axis)

9.2 Off center OA figure (quartz F)

9.3 Flash figure (parallel to c-axis)

Exercise 10 Biaxial Minerals

Biaxial minerals are somewhat more complex than uniaxial minerals. Where uniaxial minerals have two unique crystallographic axes, Biaxial minerals have three crystallographic unique axes: a ≠ b ≠ c and, therefore, three optical axes (not optic axes): X < Y < Z, and three refractive indices:  <  < . The refractive indices can be represented by a biaxial indicatrix, which is an ellipsoid with three unequal principal radii. The term biaxial refers to the two optic axes oriented perpendicular to the two circular sections contained within the ellipsoid. The angle between the two optic axes is designated 2V.

It is important to distinguish crystal axes from optical axes. Crystal axes -- a, b and c -- represent the physical dimensions of a mineral; they are measured in angstroms and are not necessarily perpendicular to one another. By contrast, optical axes -- X, Y and Z -- represent vibration directions of light traveling through a mineral; they are proportional to the refractive indices and are always perpendicular to one another. X is the fastest direction and represents (the smallest RI). Z is the slowest direction and represents (the largest RI). The direction Y represents (the intermediate RI) and corresponds to the intersection of the two circular sections. In orthorhombic minerals, the optical directions are parallel to the crystal axes, but in any order -- i.e., in some minerals, a is parallel to X (the fastest direction), but in other minerals b or c may be parallel to X. In monoclinic minerals, the plane a-c contains the plane defined by any two optical directions, but neither a nor c is necessarily parallel to those directions; b is parallel to the third optical direction. In triclinic minerals, a, b and c are not necessarily parallel to any of the optical directions.

The biaxial indicatrix.

Two methods for measuring 2V:

Olivine forms a solid solution with complete substitution between the end members forsterite and fayalite. Measuring 2V is one of the easiest ways of estimating olivine composition: pure fayalite is biaxial (-) and has a 2V of ~46°, while pure forsterite is biaxial (+) and has a 2V of ~82°. 2V varies linearly and continuously between the two end-members. The angle 2V can be estimated visually from the curvature of an isogyre from an optic axis figure as shown below:


Muscovite has a moderate 2V, and is a good example for another technique of measuring 2V angles of less than about 55°. 2V is measured by using an acute bisectrix (BxA) figure which is obtained by looking down the Z-axis (for positive minerals; see Figure 1) or X-axis (for negative minerals). When the stage is rotated, the isogyres form a crude cross and then separate. The maximum distance between the two isogyres is a measure of 2V as shown below. BEWARE: the N.A. of your objective significantly affects the apparent separation you observe (see chart on cabinet).