Lab_13_15_optics_qz-feldspar_amph_px_2023.docx
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Dec 6, 2023
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13
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Labs 13 – 15 : Optics: Quartz and Feldspars, Pyroxenes and Amphiboles
Due Monday October 30 by noon on bCourses
At the end of these labs, you should understand the following aspects of optics:
i)
Refractive index, and how it is important for determining relief (including + and
–)
ii)
Interference colors and how birefringence is used in mineral identification
iii)
How to obtain and analyze an interference figure. How to determine + sign and –
sign
iv)
How to use the type of interference figure and its sign to identify minerals.
Important information:
Isotropic:
cubic
Uniaxial:
tetragonal, trigonal, hexagonal
Biaxial:
orthorhombic, monoclinic, or triclinic
When asked to describe optical properties of a mineral, include:
●
Crystal morphology (general description, e.g., equidimensional, rod shaped, etc.)
●
Relief (high or low, positive or negative)
●
Cleavage, and any relation of cleavage to morphology (crystal shape) if the
morphology well defined
●
Is it isotropic or anisotropic? If isotropic, all crystals in all orientations remain
dark
under
cross-polarized
light
as
the
stage
is
rotated.
If
not
isotropic
(anisotropic), then crystals will go black as you rotate the stage under plane
polarized light.
●
Uniaxial or Biaxial?
o
Distinguish between these based on an interference figure (see below)
o
If biaxial, estimate 2V (see below)
●
What is the maximum birefringence?
(1)
Thin section
104-397
containing garnet
i) Is garnet isotropic or anisotropic? To answer this you need to view a selection of
crystals under cross-polarized light and rotate the stage.
ii) What is the crystal system of this mineral?
1
iv) What is the shape of the indicatrix? (The shape that describes how RI (refractive
index) varies with direction in the crystal):
v) Does it have a high or low RI value? To answer this, compare the relief to that of
epoxy or quartz. If possible, state positive or negative relief relative to quartz/epoxy.
2) The interference figure is used to constrain the crystal system and provide
information useful for identification. To obtain an interference figure
: Survey the
thin section: identify a grain very with low birefringence (one viewed down an optic axis)
– it should stay dark as your stage is rotated (not isotropic, has just been cut down the
optic axis).
a)
Focus your view. Now CAREFULLY insert the 40x objective (rotate the turret,
don’t touch the objective) and CAREFULLY focus using the inner focus knob
(turn it gently!).
b)
Insert the
condenser
lens (below the stage) and make sure this lens is as close to
the thin section as possible without touching by using the knob on the left of the
stage to adjust. As shown below, this will condense light into a spot on the thin
section.
c)
The 2
nd
polarizer should already be inserted
d)
Insert the
Bertrand lens
(round knob between the eye pieces, labeled “B”).
e)
Look. You should see an illuminated circle with a dark band or a cross.
If ‘
uniaxial
’ you should see isogyres (dark lines in the interference figure) that form a
cross, as shown in (A). The center of the cross may move around as you rotate the stage
but there should always be a cross. After inserting the
gypsum plate
: for a uniaxial
positive mineral you will see yellow in the first (top left) and third (counting clockwise)
quadrants, blue in the second and fourth quadrants, as shown in (B); and for a uniaxial
negative mineral you will see the opposite color pattern, shown in (C)
2
If
“biaxial”
you should EITHER see something similar to (A), but as you rotate the stage
the isogyres that make the cross will separate and rejoin (small 2V as shown in (D)) OR
you will just see one curved isogryre (large 2V as shown in E). Insert the gypsum plate
and use the same information as for B and C to determine if biaxial positive or negative.
You can estimate the “2V” of biaxial minerals from the curvature of the isogyres using
this chart
Framework silicates: quartz and the feldspars
(1)
A commonly occurring silica-rich mineral is
quartz.
The transition from
α
to
β
quartz upon heating is reversible and rapid, so
β
-quartz is never found below the transition
temperature.
3
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Examine the hand samples of
quartz.
Note that color is not a good way of telling quartz
from other minerals. The lack of cleavage (conchoidal fracture) is a very useful
characteristic for distinguishing quartz from feldspar.
Examine well-formed crystals of quartz. How many “forms” do the crystals display?
Thin section
104-299
(quartz-rich rock called quartzite)
Obtain an
interference figure
for quartz to determine whether quartz is uniaxial or
biaxial, positive or negative. Follow the instructions above.
Sketch the interference figure before and after inserting the gypsum plate (annotate your
sketch, and using color may help):
Thin section
102-54
(granite rock). Identify quartz based on its lack of color, low relief,
low birefringence and lack of twinning. Find a grain that remains dark as you rotate the
stage under cross-polarized light and obtain an interference figure (this is a grain viewed
down the optic axis). Sketch the interference figure. Is it the same as for
104-299
?
Examine the models for
α
- (low) and
β
- (high) quartz
, the low and moderately high
temperature polymorphs, respectively. Note how the SiO
2
tetrahedra are arranged in each.
For each, sketch one tetrahedron and all the tetrahedra to which it is linked (showing the
connections).
4
α
- (low) quartz
β
- (high) quartz,
Examine the model for a high-pressure form of SiO
2
,
coesite.
Identify the rings
consisting of 4-tetrahedra (two pointing up, two down) in chains. Sketch how they are
connected to form chains of 4-membered rings.
(2) Examine a
phase diagram
for SiO
2
(lecture notes or internet). What would you
conclude about the conditions experienced by a rock if it contains coesite?
Two examples of processes that might create these conditions are:
What polymorphs would you expect to find in volcanic lavas (very hot, low pressure
rocks)?
5
Which polymorph would you expect to be the most dense?
(3) Feldspars
Feldspars are the most common rock-forming minerals in the crust and their
compositions
reflect elements that are very abundant in the crust.
Feldspars differ from quartz in that a subset of the tetrahedral sites normally occupied by
Si
4+
(in quartz) are occupied by Al
3+
. Charge balance is accomplished by incorporation of
cations (Ca, Na, K) into the cavities within the structure.
Complete these exchange reactions using either Ca or K, adding the valences of ions:
Si + Na <-> Al + ?
Si + Na <-> Si + ?
Si + K <-> Si + ?
Compare any of the feldspar models to the coesite model. Focus on the arrangements of
Si and Al tetrahedra. Note that these structures are similar.
Sketch the pattern in the microcline (a low temperature, ordered feldspar), indicating
whether the tetrahedra are occupied by Al (grey) or Si (white).
Note the locations of the large (non-tetrahedrally coordinated) cations in the structure,
describe where they are located.
6
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Alkali feldspar hand samples:
Describe the alkali feldspar hand specimens and pay
special attention to cleavage, which distinguishes feldspars from quartz.
Alkali feldspar in thin section
102-59
. Distinguish
Orthoclase
KAlSi
3
O
8
from quartz by
the dusty alteration evident in plane polarized light. Describe the optical properties of
orthoclase (listed on page 1), including whether it is biaxial or uniaxial (positive or
negative) and estimate the 2V if biaxial:
Alkali feldspar in thin section
102-55.
Identify
Microcline
KAlSi
3
O
8
, the low
temperature form of K-feldspar in which the cations are highly ordered. Cation ordering
gives rise to ‘cross hatch’ twinning, which you should clearly see in the thin section.
Sketch the cross-hatch twinning.
Plagioclase
(NaAlSi
3
O
8
-CaAl
2
Si
2
O
8
): Thin section
102-65
: describe plagioclase feldspar
features, noting the crystal shape, cleavage, twinning, relief, pleochroism maximum
birefringence etc.
7
Examine plagioclase feldspar
hand specimens and describe what you see:
Unmixed (exsolved) feldspar: perthite in thin-section
102-56
. Examine at high
magnification and sketch what you see. Can you identify unmixing features in the
perthite hand specimens?
Pyroxenes and Amphiboles
Pyroxenes you should be familiar with
Amphiboles you should be familiar with
enstatite
tremolite
hypersthene
anthophyllite
diopside
actinolite
augite
hornblende
hedenbergite
And their compositions, especially as it relates to the quadrilateral diagram shown in
lecture.
(1) 103-472 Peridotite rock
(compare to hand specimen) contains
pyroxene
, olivine,
minor magnetite (opaque)
Distinguish pyroxene from olivine based on cleavage. Olivine should show no consistent
planar cleavages, Pyroxene should have two good cleavages.
Then answer:
Olivine
optical characteristics, including maximum birefringence?
8
Unixial or biaxial, if biaxial, estimated 2V? Sketch the interference figure:
Uni/bi axial positive or negative?
Pyroxene
: optical characteristics, including maximum birefringence?
Unixial or biaxial, if biaxial, estimated 2V? Sketch what you see:
Uni/bi axial positive or negative?
(2) Examine the
pyroxene
and
amphibole models.
Is pyroxene a single or double chain silicate?
9
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Sketch the basic configurations of pyroxene using an I-beam representation, including
how the I-beams are arranged. Mark onto your diagram the cleavages. Be sure to note
that the angle between them is around 90°.
Is amphibole a single or double chain silicate?
Sketch
the
basic
configurations
of
amphibole
and
pyroxene
using
an
I-beam
representation, including how the I-beams are arranged. Mark onto your diagram the
cleavages. Be sure to note that the angle between them is around 120°/60°.
For amphibole, show on your diagram where a big cation (e.g., K
+
) would go? Check the
model for
hornblende
to see if you got this right!
From the model: what is the coordination number of K in this structure (or Ca, if it goes
into that site):
Both pyroxene and amphibole are
subdivided into monoclinic and orthorhombic
types
.
The monoclinic vs. orthorhombic structure results from the stacking sequence of the
I-beams.
Use “+” to designate octahedral pointed one way and “–” for the other and
indicate the sequence in:
Clinopyroxene:
Clinoamphibole:
10
Orthopyroxene:
Orthoamphibole:
You should see how the stacking sequence (orientation of the I-beams) results in a
monoclinic vs. orthorhombic structure. If not, ask us to explain.
Examine and briefly describe the hand specimens of the pyroxene minerals
Enstatite
Mg
2
Si
2
0
6
Diopside
CaMgSi206,
Hypersthene
(Mg,Fe)
2
Si
2
0
6
,
Augite
Ca(Mg,Fe,Al)(Si,Al)206 and
Hedenbergite
CaFeSi2O6
Note that the minerals become darker brown with increasing Fe content.
Examine the thin sections
102-43
and
103-517
and describe the relief, pleochroism,
cleavage and estimate the maximum birefringence of the
pyroxenes
you identify. Include
information obtained from the interference figure:
11
Examine the hand specimens of
amphiboles
– these are primarily identified in hand
specimen by the presence of 2 good cleavages that meet at an angle of about 120°/60°.
These cleavages cause amphiboles to break with a prismatic or even fibrous appearance.
If the minerals are in a sealed box, do not open the box!
These are dangerous
“asbestiform” minerals. Just note the general appearance.
Hand specimens of
Tremolite
Ca
2
Mg
5
Si
8
O
22
(OH)
2
Hand specimens of
Actinolite
Actinolite: Ca
2
(Mg,Fe)
5
Si
8
O
22
(OH)
2
Examine thin section
104-395
: focus on smaller, well-defined crystals and note all
features. Pay particular attention to crystals with well-developed morphology and two
well-defined cleavages at ~ 120°/60°: Describe the relief, pleochroism, cleavage and
estimate maximum birefringence.
12
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Describe general features of the hand specimens of
Hornblende:
usually darker green to
black with increasing Fe content. Hornblendes usually have very complex composition.
Amphiboles also tend to grow as nice crystals that are sometimes liberated from granitic
rock by weathering. Examine the collection of such crystals and sketch their morphology.
Thin section
102-39.
The large green mineral is hornblende. Describe the optical
properties of hornblende:
Cleavage (if so, approximate angle)?
Pleochroic or not?
Isotropic or anisotropic?
If anisotropic, uniaxial or biaxial?
2V?
Maximum birefringence?
Sketch the shape of a grain in an orientation where you can measure the 2V; include
cleavages (sketch when the grain is at extinction).
Remember
: Find crystals with low
birefringence (and good cleavage) to check the interference figure and 2V.
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