The assignment background:
This assignment will give you some prac5cal familiarity with quartz and the other silica
minerals covered in lecture 18c.
Some ques5ons will relate to mineral physical proper5es. Of course, you would nominally
make such observa5ons on real mineral specimens, but as that may be a bit imprac5cal at
the moment, you’ll be making your observa5ons on images and videos of minerals instead,
and on data from mindat and from other online resources.
PLEASE KEEP THIS IN MIND: this ac5vity is part of your lab for week 11. In a non-Covid
semester, you would be spending a 3-hour block of 5me working on this in class (and the
combined ac5vi5es for this week should hopefully take you less than three hours to
complete). Please treat this ac5vity as you would treat a tradi5onal lab. In fact, if you are in
Tucson, feel free to come to your actual in-person lab to work on it, if that would help you
with your 5me management! Please remember, if you don’t turn in these lab ac5vi5es, you
are essen5ally skipping lab. And that can really hurt your grade!
The assignment:
The following slides will ask you ques5ons about the minerals and mineral groups we’re
covering in lecture this week.
Although some ques5ons here may relate to specific proper5es (for example, “how does
the density of ________ compare to ________?”, or, “what is the typical luster of
________?”), keep in mind that you are expected to independently learn some basic facts
about each week’s blue box minerals (depending on their emphasis [whether bold text or
plain text], this may include knowing names, formulas, a few key proper5es, something
about their occurrence, and +/- an ability to visually iden5fy a good example from a photo
or video). You may use the photos and descrip5ve text available at mindat to help you (or
beWer yet, if you’re in Tucson, come in to lab and check them out for yourself!)
Other ques5ons here will focus on strengthening your understanding of broader
mineralogic or petrologic concepts (for example, mineral-specific ques5ons about bonding,
structure, or rela5onships to other minerals on a phase diagram).
The assignment:
To gather the necessary data to answer these ques5ons, you have mul5ple resources available.
For quesCons on physical properCes, you may look up the mineral on mindat or on one of the other mineral
databases. Although some physical proper5es may be ascertained from the descrip5ve text, in many cases, you’ll
greatly benefit from viewing the mineral photos in the photo gallery or photo search (refer back to lecture 1c2). If
you are in Tucson and wish to see the minerals in-person (for those which we have examples available), you are of
course encouraged to do so during the op5onal lab periods. For less commonly measured physical proper5es (e.g.
mel5ng point, solubility, radioac5vity, odor, etc.), Wikipedia or even Google searches may be useful.
For quesCons related to opCcal properCes, the thin sec5on scans and op5cal videos on rockPTX.com are the
recommended resource (refer back to lecture 1c1), and this content can be supplemented with the op5cal data
listed on mindat. We also have two microscopes set up in lab for in-person viewing of minerals in thin sec5on.
For quesCons related to mineral chemistry, solid soluCon series & mineral groups, mindat is the best resource.
For quesCons about mineral structures, you’ll definitely want to use VESTA (refer back to lecture 6d2), although
again we do have a very limited number of physical structural models in lab that are available for you to examine.
For quesCons related to crystal external morphology and forms, you’ll want to supplement mindat images (or
actual lab specimens, although not many of ours are great examples) with the probably much beWer smorf models
(refer back to lecture 4a2).
QuesCon #1: Are you more likely to find
tridymite in a rhyolite (felsic volcanic rock)
or in a granite (felsic plutonic rock). Explain
your reasoning.
QuesCon #2: Will the tridymite you find in
this igneous rock be the hexagonal hightridymite (β-tridymite) polymorph, or will it
be one of the orthorhombic/monoclinic
low-tridymite (α-tridymite) polymorphs?
Explain your reasoning.
Retrieve and view the structural model for the mineral berlinite (AlPO4) in VESTA, and answer the
following ques5ons. Please also submit a screen capture of your VESTA image (or use VESTA’s “export
raster image” under the file menu) and include it with your assignment submission. In order to beWer
visualize the model, you should extend the X, Y and Z boundaries from the default 0 to 1, to a more
expansive -1 to 2. Op5onally, you may also wish to change the color of the Al (or P) polyhedra (for
example, you can change one of the RGBA set of numbers to a different value, or simply select a
different color from the color menu).
QuesCon #3: What is the coordina5on number for Al in berlinite?
QuesCon #4: What is the coordina5on number for P in berlinite?
QuesCon #5: Which polymorph of SiO2 does the structure of berlinite most resemble? What criteria
did you use to make your decision. You may view the SiO2 polymorph models using VESTA (good
prac5ce, but a bit 5me-intensive), or you can simply review the images already included in video 18c.
Qtz
QuesCon #6: Recall these images of coesite from lecture 18c. Note that the matrix SiO2 is all quartz (far lej image),
and that there is also a rim of quartz surrounding the coesite (center magnified view). Also no5ce the radial cracks
in the garnet hos5ng the coesite inclusion.
Assume the rock starts in the coesite stability field, and during exhuma5on follows the P-T path outlined below in
red. Incorporate all of the observa5ons noted above (and which are shown in the thin sec5on images below) to
offer a detailed hypothesis of what happened to the sample as the rock crossed the [coesite]-[α-quartz] phase
boundary. Is this phase transi5on displacive or reconstruc5ve? Suggest reasons for the textures you see now (for
example, the quartz rim around relict coesite; the radial cracks in the garnet; the SiO2 por5on of the matrix now
being all quartz), and how/why these features evolved over the course of the exhuma5on process.
QuesCon #7: Based on the image below (this is from lecture 18c), use Snell’s Law and Bragg’s Law
(lecture 5b1) to determine the diameter of the opal-AG spheres in a sample of precious opal.
Snell’s Law is: sin θ2 n1 where n1 = 1.000 (the refrac5ve index of air) and
n2 = 1.460 (the refrac5ve index of opal)
If the angle of ini5al incidence (θ1) is 5° and the wavelength (λ) of the flashes of diffracted light is 550
nm (green), what is the diameter of the SiO2
.
nH2O spheres? Hint: for the Bragg’s Law por5on of the
calcula5on, keep in mind that the “sin θ” you should use will be “sin (90°- θ2)” (that is, the angle
measured from the horizontal, not measured from the ver5cal).
sin θ1
=
n2
helpful hints:
Please submit your answers in a separate
document (either as a text file, a Word
document, or an Excel spreadsheet). You
may convert your document into a PDF if
you’d like, but this isn’t necessary. The
ques5ons are sequen5ally numbered for
your convenience.
For your accompanying VESTA image
submission, you may embed them in
your Word or Excel document (this
method is preferred), or you may include
it as a separate submission. But in either
case, please label what is what.
helpful hints:
As usual, feel free to work with other
students in your “quaran5ne bubble”
or via online means, but remember
that you should turn in original work
in your own words and reflecCng
your own thinking process.
If you need addi5onal help or
assistance, don’t hesitate to contact
your TAs or instructor.
This is due by 18:00 hrs on Monday,
09 November 2020; please turn this in
on 5me via email or upload to D2L.