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A number of responses occurred over on the MSA-Talk listserv to the discussion about the importance of optical mineralogy since my update yesterday.   There was also a great comment added by a fellow-MN resident Bryan Brandli at the end of my previous post I would encourage everyone to read.

Comments that have been posted since Monday at 10.00 CST (its now Tuesday, 10.00) in the pro column:

  • Miguel Gregorkiewitz (University of Sienna, Italy): “I’d like to stress that polarized light microscopy is useful to much more than petrography.  I started getting involved in materials science as a chemist, and during my PhD, I got eventually stuck for 6 months trying to resolve the crystal structure of a new “hexagonal” phase – until I decided to have a look through the petrographic microscope revealing that my “single” crystal was actually a perfect thrilling of an orthorhombic b=sqrt(3)*a unit cell.   At the time, I was happy to find the necessary instruments and knowlegde in our mineralogy department, and now I include the basics of the polarized light microscope and some applications in mineralogy/petrography in my crystallography courses, mainly as a very easy and rapid supporting tool to determine symmetry.  I can’t understand why such a simple, cheap and conclusive experimental technique should be sacrificed from our curricula, instead, I would offer it also to solid state physics and engineering labs, to strengthen the training in optics.”
  • Volker Gobel (Stephen F. Austin State Univ): “It usually is also an undergraduate deficiency for  incoming graduate students. Some U.S. mining industry is bitterly complaining about the unpreparedness of  new, young mineralogists lacking basic mineralogic knowledge and microscopic skills necessitating extensive in-house training as academia is failing to do what it once did.”
  • Darrell Henry (LSU): “One way in which you can view optical mineralogy (no matter how it is introduced to students) is at a programmatic level or even college/university level. This skill is an excellent way for students to demonstrate critical thinking through hypothesis-testing. After all, when we are doing petrography we are continually generating hypotheses
    about the minerals we are observing and then testing these hypotheses using optical properties or, as a short cut, the rock context (what makes sense in that kind of rock). As the experience level of the student rises the sophistication of the hypotheses are elevated as well.  At a time, when colleges and universities are asked to demonstrate that
    the students are critical thinkers, optical mineralogy can be a vehicle
    to do just that.”
  • Phillippe D’Arco (Universite Pierre et Marie Curie): “In fact fancy techniques have to be applied to decipher some identified problems !  Microprobe does not discriminate sillimanite/andalousite/kyanite, or ….  XRD does not tell you anything about phase relation. Texture is a key for rock interpretation.  Intensity of Raman lines is highly dependent on  orientation, lines can easily disappear.”
  • John Hogan (Missouri University of Science and Technology): “One additional unifying skill for geology and geophysics students to develop is understanding the significance of spatial relationships
    between critical elements of a problem. I present petrography as geologic mapping of the spatial relationships among minerals at the thin section scale. Being able to identify the minerals is one part of making this map, recognizing the spatial relationships among these minerals is even more important part of making this map, and a lot more difficult to
    teach. Tools that identify minerals and mineral compositions, whether you were determining the Fo content of Olivine from the 2V or now using some other weapon of choice (e-probe), these tools will remain a “black box” unless the spatial context of the minerals is utilized to form a testable hypothesis as to how the rock formed.”

I think the following two are more of “this is the way the world currently works” comments:

  • Frank Dudas (MIT): “It’s not cutting edge, it’s not “sexy science.”  Progress consists of using novel methods and new toys or tools.  There is no question that, successively, XRD, the electron microprobe, and other increasingly sophisticated gadgets, most of them black boxes, have offered us the opportunity to gather more precise, accurate, detailed and comprehensive information than a technology that developed in the 19th century, and still depends on that poorest of all observation tools, the human animal.  To be “relevant,” we have make sure our students have a 21st century education, and know how to push buttons on black boxes, even though they may not understand how the boxes work.  As some poet said, “Progress is a comfortable disease/There’s a hell of a good universe next door/Let’s go.””
  • Winton Cornell (Univ of Tulsa): “Now that we’ve been up-and-down the list of reasons/applications/needs of PLM to/in teaching, and in the University environ as a whole (use by non-GEO Departments), I’d like to ask the question re: it’s use by GEO students after they leave academe, this to see/sample what its extension is beyond the classroom these days.   That asked, I do already have in hand some info re: – at least – local Independent Oil Companies here in Tulsa/Oklahoma/Texas = where about 1/2 of our students end up working after finishing up GEO here in our mid-continent setting, To quote our Chairman, who has contact with many of these students, and the companies themselves: Winton – “Very little, if any, PLM is done. The larger research labs still do microscopy, but I do not know of any Independents that still do microscopy. Even simple, reflected light microscopy is dying out. And, with the log-while-you-drill technology that’s available, even mud loggers are going away, and as such there is even little well-site geology done these days.” Mind you, he’s not a pessimist, he’s just telling it like it is around here for companies of that scale. We, ourselves, acquired our current CAMECA microprobe from a ‘Major’ that no longer had need for it as a tool. The company did hold on to its light microscopes, though to what end I don’t know.   So what is in the ‘sedimentary mineralogy/petrology’ arsenal of oil companies these days? To this info about logging (above), I can add that the ‘hot’ thing around here when it comes to looking into reservoir rocks with microscopy is to do so with dual-beam instruments = SEM-FIB (now that’s a leap!; we have one here and a ‘major’ independent in OK City/at OU has one), with the emphasis on reconstructing/examining pore volume and permeability in shales = the ‘hot’ rocks for natural gas these days. Rendered volumes for SEM-FIB are micrometer-scaled, so for core-scale rendering they use CT scans. Holy cow! Backing all this up is the ‘mineralogy’, it determined via X-ray powder diffraction and modeled via whole pattern fitting (Rietveld). Rocks are thus simple, physical entities evaluated as media that hold oil and gas.   Certainly the ‘mining geologists’ touch and examine the rocks? Please say it’s so.”

The last comment occurred yesterday at just after 16.00 CST, so this may be the end of the discussion.   If not, I may update again!

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This is the third in my series on how the petrographic microscope is used in geology.   First we had the behavior of light, then minerals in PPL.   Today’s topic: crossed polars or XPL.

If you remember, we orient the light coming out of the lower polarizer E-W.   Unless the light split within the mineral, it will continue to be oriented E-W and when it reaches the inserted N-S polarizer (also referred to as the “analyzer”), nothing will pass through and all the petrographer will see is black.   There are two cases when this will happen:

  1. isotropic minerals, where light is not split into two rays because of high symmetry
  2. circular sections of anisotropic minerals, which are a special case; in uniaxial minerals, there is one circular section that is perpendicular to the c-axis, and in biaxial minerals, there are two circular sections perpendicular to the two optic axes; we’ll get back how to distinguish between between uniaxial & biaxial minerals and why circular sections are important in next week’s installment on interference figures

when the light path isn't changed within the mineral; http://www.tulane.edu/~sanelson/eens211/proplight.htm

within isotropic minerals, the refractive index is the same in all directions; http://www.tulane.edu/~sanelson/eens211/proplight.htm

uniaxial representation of how light behaves in different mineral orientations; http://www.tulane.edu/~sanelson/eens211/uniaxial_minerals.htm

how light behaves in different orientations in a biaxial mineral; http://www.tulane.edu/~sanelson/eens211/biaxial.htm

Isotropic minerals are not the most fascinating things to look at in crossed polars, which is why this blog is entitled “Life in Plane Light” and not… “The analyzer is in”?  “Light Interference”?  “Maximum birefringence”?

garnet schist from the NE Kingdom in Vermont in PPL (picture from my BA thesis)

XPL of the same thin section; garnet is an isotropic mineral

Anisotropic minerals, in contrast, are quite interesting in XPL.   When light enters most orientations of an anisotropic mineral, its split into two rays that have different speeds.   When the light exits the crystal, it has to condense back down one ray.   So, let’s talk for a second about light addition / subtraction.   In 2D, we simply take the two rays and add them together:

There are two special cases to consider: when the two light rays are exactly in sync with each other they will add to give the maximum value (“in phase”); when 1/2 a length “out of phase” from each other, they will cancel out each other to give the minimum value.

In 3D, we have to do a bit of vector math in order to add the two waves together.   Since light is divided into two waves mutually perpendicular to each other, the following is true:

As the crystal is rotated, the orientation of the crystal varies and the light entering will vary in speed, which means that the sum of light will vary as you rotate the stage.   There will be two end-members: one oriented perpendicular to the upper polarizer (minimum light) & one parallel to the analyzer (maximum light).

Turns out that every 90 degrees rotation of the stage, every mineral will go through both the maximum and minimum.   We call the minimum “extinction.”

We can have a variety of types of extinction (EA):

  • parallel: when the crystal / cleavage is oriented N-S or E-W, the mineral is extinct
  • inclined: when the crystal / cleavage is NOT N-S or E-W when extinct; we record this including the acute angle between when the mineral is extinct & oriented N-S
  • symmetrical: mineral goes extinct at the same angle to two sides of a mineral / cleavage planes
  • other: if the mineral isn’t elongate or has cleavage, you will not be able to record an EA; minerals that have undergone deformation may have undulatory extinction

The maximum color will depend on the thickness of the crystal (it can vary somewhat depending on how well your thin section is made) as well as what the difference between the refractive indices of the two rays.   The color we see in crossed polars, is called the “interference color” and is varies through a sequence of ever more pastel sequence of rainbow colors on a “Michel-Levy chart.”   If you look at the photomicrograph above, you can see that the non-garnet minerals vary from quartzes & plagioclases that are 1st order greys to white to muscovite & biotite that are 2nd to 3rd order.

A Michel Levy chart; lowest order to the left & increase to the right; http://www.olympusmicro.com/primer/techniques/polarized/images/michellevy.jpg

to find the birefringence, we determine the interference color of the mineral at the maximum light (45 degrees from extinction) and go up to the thickness of the mineral (usually 0.3 mm).   Follow the inclined line up to the top or to the right side to read off what the birefringence is.

Another property we can specify for elongate minerals is the sign of elongation (stop sniggering).   The property depends on whether the fast or slow light ray is parallel to the elongate direction of the crystal.   However, to test for this one, we need to use one more component on the microscope: the “quartz wedge” or “accessory plate” or “gypsum plate” or “the psychedelic color thingy” (my students occasionally get creative with their naming).

staurolite schist with the accessory plate in; https://people.ok.ubc.ca/jdgreeno/PictureFiles/film1r.htm

basalt (elongate minerals are plagioclase) with the accessory plate in; https://people.ok.ubc.ca/jdgreeno/PictureFiles/film1r.htm

The accessory plate is labeled with the slow vibration direction & the wave length of light that it will add.   The vibration direction is usually oriented SW-NE.   When the slow ray of the mineral is oriented in the same direction, we get addition and the interference color will increase by whatever the specified wave length is.   If the fast direction is NE-SW, the interference color will decrease by the wave length.

If the slow light path is parallel to the elongate direction of the mineral, the mineral will be called “length slow” or “positive elongation.”   The opposite will be “length fast” or “negative elongation.”

top: addition, so 1st greys up to 2nd blues; bottom: subtraction, so 1st greys down to 1st yellows; http://www.tulane.edu/~sanelson/eens211/interference_of_light.htm

The last property we’re going to talk about is the more formal pleochroic scheme (we did informal last week).   In plane light, the absorbed color of the mineral will vary depending on the orientation of the crystal.   In the formal scheme, we assign specific colors to specific orientations.

  • For uniaxial minerals, there are two end-member colors: the “omega” or “w” or “ordinary” direction, which is the only light direction in the circular section; the “epsilon” or “E” or “extraordinary” direction, which is the maximum difference in refractive indices from the ordinary direction & is perpendicular to the circular section (also called the “principal section”).    The ordinary direction will always be present when the crystal is oriented with the circular section parallel to the stage (e.g. when the mineral acts isotropicly).
  • For biaxial minerals, we need to assign three colors: “alpha” or “X” & “beta” or “Y” and “omega” or “Z” which correspond to the smallest, moderate, and largest refractive indices; you can only see a maximum of two of the colors for any given orientation of the mineral, so you’ll need 2 different cuts (circular & principal section) through the mineral to describe the whole pleochroic scheme; beta can be seen in the circular section, while alpha & omega will be found in the principal section (use the accessory plate to determine which one is the alpha = fast and omega = slow directions).

Dave Hirsch has some great pleochroism movies on his website.

Next week, we’re going to go to structure for a post & discuss Mohr circles, but then we’ll be back to mineralogy & optic axis figures.

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This morning, my mineralogy had a discussion about a January 2011 Geology paper by Paasche & Lovlie on the “Synchronized postglacial colonization by magnetotactic bacteria.”   In general, the paper was voted ‘more enjoyable’ than last week’s possible impact structure paper.

The questions I asked the students to consider and submit answers to:

  • what are MTBs?
  • what is the primary question the authors are trying to address?
  • what kind of data did the authors use?
  • would this kind of study work in MN? How would you choose your lakes?
  • to support the “bird repopulated the MTB” theory, what kind of study do you think might help?
  • terms you didn’t understand?
  • concepts that were hard to grasp?

My students believe that in order to test the “bird repopulation” theory proposed by the authors two experiments should be run: 1) force-feed magnetotactic bacteria to birds & see if the bacteria can be recognized in the bird poop and 2) track appropriate bird migration patterns near receding glaciers.

A number of the students highlighted the ARM / SIRM ratio as a term / concept that they could not either understand or find understandable material on when they searched the web.   I will admit that the paper just assumed the readers knew the difference between anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM).   And the best site I could find searching quickly this morning was much more scientific than students without geophysics were going to grasp.   These are the days I wish Chris had written a post that I could simply point the students towards…

The other question that came up that I couldn’t answer was about how to magnetotactic bacteria actually produce magnetite.   If anyone has a good link, I would appreciate it–my biology background is limited to 9th grade & one semester of paleo in German!

Full citation for the paper (which is behind a paywall):

Paasche, O. and Lovlie, R., 2011, Synchronized postglacial colonization by magnetotactic bacteria: Geology, v. 39, p. 75-78.

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I’m going to aim for getting this done while its still Tuesday locally, but things around here have been a bit crazy due to the rather high snow amounts that fell Sunday / Monday.

This week is also a bit different in that we’re going to be looking at two minerals instead of one and both minerals are opaques.   Opaque minerals absorb all of the light that is coming up from the light source below and appear black both in PPL and XPL.   Sometimes, we can tell what they are (or at least have a good guess) based on crystal habit.   But usually, its not obvious and we need to use either reflected light microscopy or some more serious analytical technique (SEM, EMP, etc.).

I haven’t gotten to reflected light microscopy yet in my series on how to use the microscope (how light behaves & PPL are up), but its coming eventually.   If you’re not up on your reflected light techniques, don’t worry – you’re not alone.   Reflected light is frequently not even taught in normal mineralogy courses (sorry to out you Kim, but I don’t remember it when I took min & no professor I’ve worked with as a TA taught it).  So unless your prof had a love of economic mineralogy, you may not have spent time looking at opaques to discern what they are.

Magnetite in reflected light: pale yellow to light-brown to grey in color (color variation is very minute in reflected light).

 

Chalcopyrite (yellow, centre) is intergrown with euhedral pyrite (light yellow-white, high reflectance, bottom left) and magnetite (brown-grey, top centre) which carries chalcopyrite inclusions. Curved laths of molybdenite (right) show strong bireflectance and reflection pleochroism (light grey to dark brown-grey) and also carry chalcopyrite inclusions (top right). http://www.smenet.org/opaque-ore/15f.jpg

A euhedral chromite (green-grey, centre) is fractured (black), the fractures are less well-developed in the magnetite (light brown) overgrowth. http://www.smenet.org/opaque-ore/07f.jpg

Hematite in reflected light: white to white-blue to blue in color.   Difficult to differentiate from magnetite.   Frequently present as an alteration product, so present as rims or within fractures that may be narrow & difficult to ID.

 

The ilmenite host (pink-brown) has fine- and coarse-grained haematite (white, bottom left) exsolution bodies oriented along the (0001) direction of the ilmenite. The coarse-grained ilmenohaematite has fine ilmenite exsolution within it. Rutile (lilac-grey, left) is oriented perpendicularly to the crystallographic orientation of the main exsolution bodies. http://www.smenet.org/opaque-ore/08f.jpg

Tabular haematite (blue) and subhedral pyrite (pale yellow-white, centre bottom) are intergrown with chalcopyrite (yellow). The relief difference between the harder haematite and softer chalcopyrite can be seen clearly (centre right). http://www.smenet.org/opaque-ore/21b.jpg

Two detrital grains of ilmenohaematite (centre, right centre) comprise exsolution bodies of ilmenite (brown) in a haematite host (white). The right-hand ilmenohaematite has an ilmenite-free haematite overgrowth (left margin). Very poorly crystalline haematite (green-white, top centre) is poorly polished and is a pseudomorph after an original ferromagnesium mineral, whereas a single crystal of haematite (top left) is well polished.http://www.smenet.org/opaque-ore/42f.jpg

Magnetite & hematite together:

 

An andesite contains subhedral to euhedral magnetite crystals (light brown, centre) that are partially oxidized to haematite (blue-white, centre left). The alteration proceeds along crystal edges and along curved fractures. (Very subtle color difference.) http://www.smenet.org/opaque-ore/14a.jpg
Magnetite (pink-brown, right) is coarse-grained and is intergrown with haematite (white, centre), which encloses an euhedral basal section of quartz (dark grey, centre bottom). http://www.smenet.org/opaque-ore/64d.jpg

I’ll hit the other properties of hematite and magnetite later this week, which will include the environments that we tend to find them in.

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Last week, we had a basic intro to how light behaves.   This week, we’re going to step it up and talk about what we can observe in plane light or PPL.

What kind of things are we going to talk about this week?

  • Becke Line
  • relief
  • cleavage
  • color / pleochroism

There are few ways to look at minerals, such as immersed in an oil of a specific refractive index.   (A few of the images I’m going to use are going to refer to oil immersion.)   However, today the most common way to examine minerals today is in thin section.   I’ll give a few arguments for why in a few posts.   But let’s quickly talk about what a thin section actually is & how you make one.   First, you need to collect a rock & cut it into a domino with a diamond embedded rock saw.   Second, you “glue” the domino to a glass slide with epoxy.   Third, you cut off the majority of the domino and then grind down until you only have 30 microns of rock left on the glass slide.   At this point, you have a choice: attach a cover slip or continue to polish the slide down till its smooth enough for SEM or EMP analysis.   The example below & these instructions (with pictures for each stage!) are for a covered thin section (from Dave Hirsch).

The epoxy has a specific refractive index (1.54 – 1.56 usually), which is important when talking about the Becke Line.

The Becke Line depends on how light refracts when it intersects a mineral.   Remember that the item that has the refractive index will cause light to bend inwards towards the normal.   With minerals, we use a substance that has a known refractive index (either an oil or the epoxy at the edge of the thin section) and then test for which way the light refracts with it.   Focus the microscope while looking at the mineral boundary.   You may find this works better if you lower the amount of light going through the microscope.   While looking at the mineral boundary, lower the stage of the microscope.   This will increase the focal length and cause the brighter of the two boundary lines to move either inwards or outwards.   Inwards = mineral has a higher refractive index = positive result and outwards = mineral has a lower refractive index = negative result.

Relief is a more casual way to describe how light is refracted by a given mineral.   It is used to describe how well (or poorly) a minerals grains boundaries stand out from its neighbors.   If the refractive index is close to the neighboring mineral’s value, then the light will not be bent much either in or out and the boundary between the minerals will be difficult to distinguish.   If the refractive indices are very different, then the light will bend quite a bit and the boundary between the minerals will be very distinct.   We qualitatively describe relief as “low” or “moderate” or “high” or “very high” and then, if possible, adding whether its “positive” or “negative” relative to the epoxy (positive = higher value, negative = lower value).

garnet (central mineral) has very high relief; quartz & plagioclase (next to garnet) have low relief; muscovite (on the left side--bladed) has moderate relief; http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/garnet.UX.html

If you remember correctly, all anisotropic minerals have two refractive indices depending on how you orient them.   As we talked about last week, most of the time the two values aren’t really that far apart from each other and you won’t see the differences in plane light.   However, those few cases where double refraction is easy to see in hand sample, we can also see it in plane light as you rotate the stage.   For instance, when you watch a calcite grain in plane light as you rotate the stage around, it will vary from a moderate negative relief to a high positive relief.

Minerals can break in a variety of ways depending on how their internal structure is organized.   Though, we can’t always see the cleavage or fracture in thin section, when it does appear its as regular semi-parallel lines that are lighter in weight than the mineral boundaries.   When we describe a minerals cleavage, we describe the quality (perfect, good, poor), how many planes (lines that are parallel to each other) and at what angle they are to each other.   A few examples:

Finally, let’s talk about what color a mineral will be in thin section.   If you look at the photomicrographs above in this post (pictures taken with the optical microscope), you’ll notice that there was quite a bit of variation in the color.   Color in PPL is due to the absorption of light into the mineral.   The light that is not absorbed, is the color we see.

in this example, all but yellow-green is absorbed; http://www.tulane.edu/~sanelson/eens211/proplight.htm

In an isotropic mineral, light is treated the same in all orientations of the mineral, so there will only be one color transmitted.   Orientation and how we rotate the stage will still not impact the color.

In anisotropic minerals, how light reacts will depend on the orientation of the mineral.   In PPL, we can see this represented as a variation between two colors as you rotate the stage, which is called pleochroism.   We can casually describe this as the variation from one color to another (e.g. yellow to brown like the biotite above; blue to purple like the glaucophane below) or we can formally describe the color scheme.   To formally define things, we actually have to know more about the orientation of the minerals & be able to use things in crossed polars.   Therefore, I’m going to cover the longer pleochroism color scheme in the next segment.   See you next week for crossed polars!

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My goal with the papers is to find something that isn’t too hard for the students to understand, though most have only had intro level classes so far.   My goal was not to have the students understand everything in the paper, but to start them on the path of learning how to read papers.

For the first week, we read the paper by Amor et al. (2008) paper entitled “A Precambrian proximal ejecta blanket from Scotland.”   Why this paper?   Meteorite impacts are “sexy” and therefore have a wider appeal to the students in general.   Though we haven’t started discussing crystallographic orientations, they are referred to in this paper in a fairly easy to digest manner that hopefully will serve as a “why the heck to we have to learn about Miller indices?” answer.   The paper also demonstrates why carefully looking at data and occasionally re-evaluating someone else’s data may be important in determining how the rocks formed.

Questions I asked the students to consider:

  • make a list of the characteristics of an impact crater vs. a volcanic unit
  • what kinds of data did the authors use to support the impact hypothesis for the Stac Fada Member?
  • why is quartz important for this study?
  • terms you didn’t understand?
  • concepts that were hard to grasp?

I was really impressed with the quality of the answers I got to the questions and the students had intelligent comments and queries during our discussion last Friday.   All in all, I think week #1 went well — not that the peanut butter chocolate chip bars didn’t help loosen their tongues 🙂

Amor, K., Hesselbo, S.P., Porcelli, D., Thackrey, S., and Parnell, J., 2008, A Precambrian proximal ejecta blanket from Scotland: Geology, v. 36, p. 303-306.

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(I’m behind by a week – my goal is to catch up this week)

As I mentioned earlier, I have a mineral for each week of the semester during mineralogy.   Within the class, we talk about the mineral a bit and read a Geology paper in which that mineral plays a staring role.   I originally had the minerals organized from most important to slightly less important, but swapped things up to match the minerals with the lecture material for that week.   Well, at least as far as I could manage.   But the week #1 mineral is also the most important mineral in the Earth’s crust: quartz.

What is quartz?   The formula for quartz is SiO2, which is rather simple compared to a number of the other minerals my class is going to learn about.   Each silica is bonded to four oxygens (called a silica tetrahedra):

each oxygen is then bonded to two different silica ions:

In itself, the structure is rather simple.   Not easy to visualize in two dimensions, but simple.

The bonds between silica and quartz are strong because of two things:

  • the bonds between silica and quartz are about 50-50 mix of covalent & ionic characteristics due to the electronegativity (ability of an atom to attract electrons to itself) of both elements; ionic bonds are moderately strong, but can be broken up dissolving them in a polar solvent; covalent bonds are just ridiculously strong; the combination of the two types of bonds makes Si-O a strong bond
  • the charge of silica is +4 and the charge on oxygen is -2; to balance charge in the structure, each of the four oxygens surrounding the silica contributes -1 to the central silica (which balances the +4) and has a -1 to share with some other cation; we call this a mesodemic bond according to Pauling rule #2 because its equal charge to balance both cations; in the case of quartz, that other cation is also a silica, so the positive – negative pull on the oxygen is the same towards both cations

Where do we find quartz?  The answer to this is just about everywhere within the crust & on the surface of the Earth.   The reasons for this is multifold:

  • there may be 92 elements found naturally on Earth, but >98% of the crust of our planet is made of just eight (!) elements; top 2?  oxygen & silica

  • the strong bond between silica & oxygen means that quartz itself is a fairly hard mineral and its difficult to weather
  • because the bonds are equal in all directions, quartz doesn’t always break along the same planes (cleavage), but rather randomly (fracture); this also makes quartz less likely to weather

Rock-wise, quartz can be found in:

  • the majority of igneous rocks (the exception are silica-undersaturated rocks)
  • most types of sedimentary rocks either as a principle component (e.g. arenites, shales) or as an accessory phase (e.g. limestones)
  • and because metamorphics were once either igneous or sedimentary, also most metamorphic rocks contain quartz

Loose material on the surface:

  • component within soils
  • as a principal component in clastic sediments
  • as an accessory component in precipitated sediments

Since its found in some many different types of rocks & environments, what is the variation?   Quartz can come in just about any color and in a huge variety of sizes & shapes.   The color is due to impurities within the structure of the mineral (e.g. amethyst is due to iron, rose quartz due to Mn / Ti / Fe, citrine due to Fe).   The size & shape of the mineral is due to how much space the mineral had to grow, quick or slow growth, amount of water present, and whether or not the mineral was deformed post-growth.

I talked about quartz in thin section yesterday and will address the paper we read separately.

Next week?   Hematite & magnetite.

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