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Archive for February, 2011

The first mineral on our tour was quartz.   Most weeks, we’ll only cover one mineral, but there was a very strong push to include magnetite & hematite when I polled the blogosphere, so I chose to add both.

Both magnetite (Fe3O4) & hematite (Fe2O3) are iron oxides.   However, the oxidation state of iron (Fe) is not the same for both minerals.   Hematite contains only the 3+ type of Fe.  Magnetite in contrast contains both 2+ and 3+ Fe.   Why is this important?  Whether an element bonds in a higher or lower oxidation state will depend on how many free anions (usually oxygen) are available while the mineral is forming.   More oxygen = higher oxidation state.   Hematite forms in environments that are more oxygen-rich than magnetite, such as water-rich or geological environments in contact with the atmosphere.

There are several differing ways that oxidation state of an environment might be presented:

  • oxygen fugacity = fO2 = frequently used in igneous petrology and is buffered by mineral reactions such as NNO (Nickel-Nickel-Oxide; lower oxygen) or QFM (Quartz-Fayalite-Magnetite; higher oxygen); metamorphic petrology will also use fO2, though not as common as in igneous fields
  • activity of electrons = Eh = used in as vertical axis on Eh – pH diagrams to depict the relationship between oxidation state (high Eh = higher oxidation state) and the pH of a system; common in soils, sedimentary geology, hydrogeology type fields
  • descriptive as either anoxic (oxygen-poor) or oxic (oxygen-rich) – this seems to occur frequently in paleotonology and coal & oil petrogenesis discussions

Where can we find magnetite? In order to have a combination of 2+ and 3+ Fe present, magnetite requires at least a moderate fO2 value.   At depth, magmas are usually more mafic to ultramafic in composition tend towards lower fO2 values.   Typically, these more primitive magmas are water-poor, which also influences their fO2 values.   The first oxides in ultramafic or mafic magmas are frequently rutile (TiO2) or ilmenite (FeTiO3).  [The iron in ilmenite is 2+]   The iron in the system will preferentially go into olivine or another silicate in 2+ form.   More evolved magmas (intermediate to felsic compositions) tend to be both more water-rich and have a higher fO2 value.   In these rocks, magnetite will form as a minor component of the rock.   Magnetite will also be found in metamorphic rocks in minor amounts.   Magnetite can also be formed biologically (e.g. bird brains, magnetotactic bacteria).   There are some cases where magnetite can be found in more abundant modes, such as in banded iron formations and layered mafic intrusions, serving as iron ores.

Before I move onto hematite, let’s quickly mention about an important property of magnetite–the fact that it will preserve a magnetic field.   Chris Rowan of Highly Allochtonous has discussed the field of paleomagnetism several times in the past four years–I’m going to leave that explanation to him.

Where is hematite found?  Magnetite alters easily to hematite in more oxygen-rich environments, so rocks that once contained magnetite may be altered to contain hematite instead.   Hematite will also form in oxic sedimentary environments, mineral hot springs, as a weathering product within soils, and in some volcanic rocks.   Hematite has also been reported from the surface from Mars, which has implications for the presence of water on the surface at some point in Mars’ past.   Banded iron formations, if altered, may contain significant amounts of hematite and be another source of iron ore.

Ok, so how we recognize magnetite & hematite in the field?   If you carry a Brunton Compass with you, you can use it to determine whether a rock is magnetic or not (just watch out for power lines–they also create magnetic fields that can screw up your readings).   Magnetite is not quite as dense as hematite, but that’s a hard thing to judge in the field.   If you carry a streak plate (I don’t), hematite will have a red-brown streak and magnetite a dark grey – black streak.   Both minerals can either be metallic, submetallic, or non-metallic depending on how much their outer surfaces have weathered, but magnetite tends towards dark silvers to greys & black, which hematite will go from silver to black to reddish-browns.  Magnetite tends to be massive in shape, though you can find octahedral crystals of it occasionally.   Hematite also can be massive, but will occasionally be tabular in shape.   Hematite is the harder of the two minerals (6.5 vs. 5.5-6), but not by much.

weathered grey to reddish hematite; http://www.iron-ore.biz/Hematite-ore.jpg

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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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Before I get to what my students found this week to write about, I’d like to highlight several blogs that basically covered the assignment I gave my students:

Ok, now onto my students:

See you next week!

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In addition to mineralogy, structure is also reading a paper each week for discussion.   Since structure is a more advanced class and the students have experience with reading research papers, instead of assigning papers from Geology, I’m looking for papers that are a bit longer, require more structural-based knowledge, and relate to the topic covered in lecture that week.   The first paper (which I didn’t blog about) was from GSA Bulletin, which is where I’m going to start my weekly search.   This week I branched out to Lithosphere because I couldn’t find a recent rheology-themed paper.   Next week for microstructures, we’ll be back in GSA Bulletin.

Post-lunch, structure discussed a relatively short paper on using Field-based constraints on finite strain and rheology of the lithospheric mantle, Twin Sister, Washington by Tikoff et al.   In lab last week, the students did an analogue materials lab by Dyanna Czech and we’ve been talking about rheology in lecture, so the paper fit well into where the class currently is.

The questions the students considered when reading the paper:

  • what was the motivation for this study?
  • what kinds of data were used?
  • how was the data analyzed?
  • what kind of assumptions were made by the authors?
  • terms you didn’t understand?
  • concepts that were difficult to comprehend?

Unfortunately, there are a few issues with my structure paper discussions that don’t appear in mineralogy:

  1. it’s post-lunch and the students are ready for their naps
  2. there are only three students, so no one can have an “off” day
  3. so far I’ve chosen papers that require the students to actually remember material learned during previous semesters in other classes (e.g. how thermobarometry works; sedimentary basin formation), which I’ve had some backlash to

I feel like I have to lead the discussion more strongly with structure, which is something I want to move away from.   I’m considering assigning the student’s in rotating fashion ownership of the day’s discussion, but with three they’ll have to take a turn fairly frequently.   I’d welcome suggestions.

Full citation (paper is behind a paywall):

Tikoff, B., Larson, C.E., Newman, J., and Little, T., 2011, Field-based constraints on finite strain and rheology of the lithospheric mantle, Twin Sisters, Washington: Lithosphere, v.

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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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While our thoughts and hopes are with the people of New Zealand at the moment (and I’m sure that my students will write about the earthquake at the end of the week), its time for a short glimpse into the news stories from last week that my students discovered:

The Arkansas swarm was chosen by several of my students (who all found different articles) across both classes, so it won the “popularity vote” this week.

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