Optical Mineralogy Lab Final
2006
NAME:
1)
R15
The grain at the center of the green circle has a form of twinning we’ve discussed, but
have not focused on in thin section – look for the grain with a crosshatched pattern in
XPL. Describe the optical properties of the mineral and identify it. There is a lot of this
mineral in the thin section, so feel free to examine other grains as well. The circled grain
is just a particularly nice example of this sort of twinning.
Crosshatched twinning in microcline
2)
74501-31
Examine the two large crystals at the center of the black circle. These two crystals are of
the same mineral, but they look different. Explain why.
Examine the grain with parallel extinction. What are the orientations of the vibration
directions in this grain, with respect to cleavage.
Describe the optical properties of this mineral and identify it.
The grain size distribution in this rock is bimodal (the crystals are either large or very
small, with nothing in between). What kind of rock is this (plutonic, volcanic,
metamorphic) and why?
2 orientations of hornblende, in a volcanic rock
3)
SJA-6
Look at the large crystal circled in green (the one with the arrow pointing to it, if there
are two). What is up with the funky patterns this mineral has when you rotate it under
XPL?
Describe this mineral and identify it.
Twinning and zoning in plagioclase
Using the gypsum plate, determine if this mineral is length fast or length slow.
4)
Min16
Examine the area of the thin section circled in green. There are three dominant minerals
in this area – two phylosilicates and another mineral we’ve seen many times before in
class. Describe all three minerals and identify them.
How can you distinguish between the two phylosilicates?
Really nice biotites & muscovites + qtz
Look at the large, strongly pleochroic grain at the center of the circled area. Define the
absorbtion properties of the two vibration planes of this crystal, and describe the
procedure.
5)
LQ1
Examine this thin section. The sample is made up almost entirely of two minerals. There
some blobby looking crystals with bright interference colors, but most of the section
comprises the other mineral, which has more washed out interference colors. Describe
this second mineral and identify it. (Hint: Make sure to look at a number of different
crystals of this mineral, and check out what happens when you try and bring the crystals
to extinction.)
What kind of extinction does this mineral exhibit?
Mostly Calcite
6)
247
Examine the large crystal circled by green dots. Describe this mineral and identify it.
(Feel free to also examine a few of the other grains of this mineral present in this slide,
some of them have very nice – and indicative – crystal form.)
DOUBLE OPTICAL MINERALOGY BONUS QUESTION!
There is some strange, strange stuff going on with this slide, and most of the minerals are
difficult to identify (apart from the one I asked you to look at, of course). For example,
look around the edges of some of the crystals showing super bright interference colors.
See the repeating rainbow colors? You can see the entire range of interference colors
extremely clearly, which isn’t usually this easy in normal sections. The problem
becomes clear when you realize there is quartz in this slide with up to first order magenta
interference colors. What is wrong with this slide??? (Be quantitative!)
Garnet – section is ~0.05 nm thick.
7)
Min84
Look at the mineral in the center of the black dots. This thin section is getting old, but
where the epoxy is still good you’ll see that this mineral is fairly ubiquitous. Describe
this mineral and identify it.
Olivine
8)
TLH-1
Before putting this slide in your microscope, hold it up to the light or lay it on a piece of
white paper. Notice the majority of the slide appears bluish-grey, with some streaks of
green, and it has pink spots. Now look at the slide under thin section. The mineral that
gives the streaks a green color is Epidote, and we haven’t look at it yet. Describe and
identify the mineral that gives the slide a bluish color, and the mineral that makes up the
‘spots’.
Epidote garnet blueshist. (gla + qtz + grt + epi)
9)
87
This sample is made up almost entirely of one green mineral. Describe it and identify it.
DOUBLE OPTICAL MINERALOGY BONUS QUESTION!
Besides the omnipresent green mineral in this section, there is one more mineral that
makes up almost everything that isn’t green. Look for the transparent mineral that fills in
the veins between masses of green crystals. Describe this mineral and identify it.
Chlorite with veins of Calcite
10)
20:SM
Locate the area of this slide circled in green. There are a lot of different minerals in this
circled area: A green mineral, a mineral with high interference colors, an opaque mineral
(maybe hematite), a mineral with twinning, and a mineral with none of these properties.
Measure the extinction angle of the mineral with high interference colors.
Determine the refractive index of the mineral with twinning relative to the mineral with
high interference colors.
Now, describe the mineral with high interference colors and identify it. You may want to
examine other grains of the same mineral elsewhere in the slide.
Clinopyroxene, Green hornblende, plagioclase, quartz. (Sketch)
You don’t need thin sections for questions 11-16.
11)
Do you observe the following properties in PPL or XPL or both?
Circle both PPL and XPL if you can see the property in either, or both if you must use
both plane and cross polarized light to see these properties.
Extinction angle
Opacity
Twinning
Zoning
Pleochroic scheme
Color
Cleavage
Isotropic vs. opaque
12)
13)
14)
PPL
PPL
PPL
PPL
PPL
PPL
PPL
PPL
XPL
XPL
XPL
XPL
XPL
XPL
XPL
XPL
BOTH
BOTH
BOTH
BOTH
BOTH
BOTH
BOTH
BOTH
How much retardation does the gypsum plate provide?
What are two optical properties that can be determined by using the gypsum plate?
What is the difference between isotropic and opaque minerals?