Wooster’s Fossil (and Mineral) of the Week: Marcasite worm burrows from Bolivia

June 9th, 2013

MarcasiteBurrowsBoliviaFixedHere’s a type of fossil I’ve never seen: worm burrow casts made entirely of the mineral marcasite. These come from the George Chambers (’79) gift collection, so we know only that they were found in Bolivia. Despite the lack of information about them, they’re curious enough to be featured in our series.

As best as I can figure out, these started as tunnels burrowed into a muddy substrate by worms of some kind. Iron sulfide, in the form of the mineral marcasite, precipitated within the abandoned tunnels, eventually filling them completely. Later the mud matrix was eroded away, leaving these intertwined tubes of silvery marcasite.

The tunnel walls were probably coated with an organic slime from the original worms or later bacteria. Sulfate-reducing bacteria may have then colonized the organic material, precipitating the pyrite in a manner described by Schieber (2002) and Virtasalo et al. (2010). The marcasite would ordinarily have converted to the more common (and stable) form of iron sulfide, pyrite (see Murowchick, 1992), but for some reason this did not happen here.

A cool combination of mineralogy and paleontology!


Murowchick, J.B. 1992. Marcasite inversion and the petrographic determination of pyrite ancestry. Economic Geology 87: 1141–1152.

Schieber, J. 2002. The role of an organic slime matrix in the formation of pyritized burrow trails and pyrite concretions. Palaios 17: 104–109.

Virtasalo, J.J., Löwemark, L., Papunen, H., Kotilainen, A.T. and Whitehouse, M.J. 2010. Pyritic and baritic burrows and microbial filaments in postglacial lacustrine clays in the northern Baltic Sea. Journal of the Geological Society 167: 1185-1198.

Wooster’s Pseudofossil of the Week: Manganese dendrites from Germany

January 20th, 2013

We haven’t had a pseudofossil in this space for awhile. A pseudofossil is an object that is often mistaken for a fossil but is actually inorganic. The above may look like  fossil fern, but it is instead a set of beautiful manganese dendrites in the Solnhofen Limestone (Jurassic) of Germany (scale in millimeters). I put this photo on Wikipedia a long time ago as manganese dendrites. That didn’t stop one website from still using it as an example of a fossil.

Manganese dendrites are thin, branching crystals that grew over a surface in a rock or mineral. Often they are found in cracks or along bedding planes (as in the above example). These dendrites are usually some variety of manganese oxide. The minerals represented can include hollandite, coronadite, and cryptomelane. Apparently they are never pyrolusite, despite what you may see in textbooks. It is also impossible to tell the mineralogy from the shape of the dendrites alone.

How can you tell this is not a fossil plant? For one, the branches are too perfect: none overlap or are folded over or broken as you would expect in a buried three-dimensional plant. Next you’ll notice that all the branches extend from a line at the bottom of the image rather than from a single branching point. Finally, there is no distinction between branch, stem or leaf; instead it is a fractal-like distribution of tiny sharp-edged crystals.

As a bonus, check out this benefit you get from having manganese dendrites:

“Metaphysically, stones with dendrites resonate with blood vessels and nerves. They help heal the nervous system and conditions such as neuralgia. Dendrites can help with skeletal disorders, reverse capillary degeneration and stimulate the circulatory system. It is the stone of plenitude; it also helps create a peaceful environment and encourage the enjoyment of each moment. Dendrites deepen your connection to the earth and can bring stability in times of strife or confusion.”

The Stone of Plenitude! (I hope you do see my sarcasm here …)

This post, by the way, marks the completion of the second year of Wooster’s Fossils of the Week. So far we’ve had 104 posts. Check out our very first edition about a sweet auloporid coral!


Potter, R.M. and Rossman, G.R. 1979. The mineralogy of manganese dendrites and coatings. American Mineralogist 64: 1219-1226.

Post, J.E. and McKeown, D.A. 2001. Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy. American Mineralogist 86: 701-713.

Early Morning Powdering Session

October 2nd, 2012

WOOSTER, OH – Our dedicated Mineralogy students appeared in the lab bright and early this morning for an optional sample prep session.

Wide-eyed and alert in the early morning hours, the Mineralogy students are diligently powdering their samples for XRD analysis.

Each student has an unknown mineral that they are studying in a semester-long research project. Most of the work involves using modern analytical techniques for identification. Not all samples are well-suited for every technique, though. All of the students have been able to describe the physical properties of their sample, but only some can observe the crystallography. Some students are starting to use optical methods while others are using the XRD. Eventually, a few students will get to analyze the chemistry of their samples by XRF.

As with any research project, some students are struggling with uncertainty: “how am I supposed to be sure about the identification of my mineral?” That’s a fantastic question, and one that we are constantly engaging in our I.S. program. The answer is that the best identification is the one that is supported by the evidence. By using a variety of techniques, the students should become more confident in their identifications. Instantly, students see the advantages of some methods. In fact, one student asked if it was “cheating” to use the XRD to identify his/her mineral. No, it’s not cheating. It’s called data.

Remember those wooden crystal models?

September 4th, 2012

WOOSTER, OH – I’m always awed by the beautiful and perfect symmetry of crystals. I can think of no better way to teach external symmetry than with wooden crystal models. The wooden crystal models are a common experience in geology, across generations and continents, although it seems they may be on the endangered list. I’ve chosen to continue using the models in Mineralogy because they allow students to see and “feel” the symmetry operations and our structural geologist thinks the blocks help students with their spatial reasoning skills.

Behold, the beautiful and mysterious crystal model.

Most people develop a love-hate relationship with the models, but I have to admit I’ve always been infatuated. They’re like logic puzzles with a million different secrets all wrapped up in what seems like a simple wooden block. Spend some time with the block and it will reveal a wealth of information.

This block has 5 mirrors, 1 four-fold rotation axis, and 4 two-fold rotation axes.

This combination of symmetry elements belongs to the point group (or crystal class) 4/m 2/m 2/m. In Hermann-Mauguin notation, 4/m refers to a four-fold rotation axis perpendicular to a mirror. The second and third 2/m terms refer to two-fold rotation axes that are perpendicular to mirrors, one set of axes that exits the crystal in the middle of the faces and another set of axes that exits the crystal on the edges.

The axes (a1, a2, and c) of the tetragonal crystal system align with symmetry elements. Some of the faces have been labeled with their Miller Indices.

The 4/m 2/m 2/m point group belongs to the tetragonal crystal system, which has three mutually perpendicular crystallographic axes. The two horizontal axes (a1 and a2) are equal in length and coincide with the two-fold rotation axes. The vertical axis (c) is longer than the horizontal axes and coincides with the four-fold rotation axis.

Once the crystallographic axes have been determined, we can describe the orientation of the crystal faces using Miller Indices. In short, Miller Indices consist of three numbers (four in the case of hexagonal crystals) that are derived from the intercepts of crystal faces. The crystal face that intersects the a1 axis but parallels the a2 and c axes is assigned a Miller Index of 100. The crystal face that never intersects a1 but cuts both a2 and c is assigned a Miller Index of 011.

We could keep going with this…describing forms, measuring angles, plotting on stereonets, but we won’t. Making it through Miller Indices this week will be enough for the Mineralogy students. Here’s the big secret: this is one of crystals we’re working on in class. I guess we’ll find out which students read the blog on a regular basis!

Teaching Mineralogy, Petrology, and Geochemistry

August 10th, 2011

MINNEAPOLIS, MN – If you haven’t been following my twitter feed (http://twitter.com/meagenpollock), you may not know that I’ve been at the 2011 Cutting Edge Workshop on Teaching Mineralogy, Petrology, and Geochemistry (twitter hashtag #sercMPG). Expert educators have gathered to discuss teaching strategies, develop course materials, and exchange ideas about issues in MPG. Fortunately, the Cutting Edge runs several workshops across the geoscience disciplines (look for the updates in September), which have generated an invaluable collection of teaching resources. You’ll be able to find all of the materials from this workshop on the web and we’ll be adding more teaching activities soon after the workshop ends. Here’s a quick (and incomplete) list of resources that I’ve discovered:

Please comment here or add to the Cutting Edge collection if you have useful MPG teaching resources!

According to the Cutting Edge: "Core of Ely Greenstone outside Pillsbury Hall, home of the Geology & Geophysics Department at the University of Minnesota. Metamorphosed Archean basalt pillows are visible in the core, which is approximately 2 meters tall. Photo by Sharon Kressler." Aka: What I want for my next birthday!


Minerals in My Toothpaste

April 16th, 2011

WOOSTER, OH – I can’t think of a more exciting thing to do on a Saturday morning than play with minerals and X-rays! Wooster’s Geology Department and the Expanding Your Horizons Program girls explored how minerals are used on a daily basis.  First, we tested the physical properties of minerals and made educated guesses about which minerals are used in common household products, like cleaners and toothpaste. Then we analyzed the products on the new X-ray diffractometer (XRD) to see whether our guesses were correct. Finally, we made our own mineral toothpaste. I don’t think we’ll be going into the toothpaste business any time soon, but the lab now smells minty fresh!

One of the EYH girls prepares a sample of powdered drywall for the XRD.

An EYH student places a prepared sample in the XRD.

After the sample is secured, an EYH student starts the run.

The XRD bombards the sample with X-rays, which diffract at specific angles. Meanwhile, the detector circles the sample and measures the intensity of X-rays at different angles. Each mineral has its own unique spectrum, sort of like an X-ray fingerprint.

Once the girls have their spectrum, they compare their sample to the spectra of known minerals to determine which minerals are in which products.

Mrs. Robertson helps the EYH girls make their own mineral toothpaste. Mmmm!

Melissa Torma ('13) and Ana Wallace ('12) volunteered to help the EYH girls and even had a chance to make their own toothpaste. (Stephanie Jarvis '11 helped, too, but had to lead the EYH girls to their next workshop before I could snap her picture).

The EYH girls search through our collection of polished stones for a souvenir. Thanks for a wonderful time, girls!

Structural geology and mineralogy at Calico

March 16th, 2011

ZZYZX, CALIFORNIA–It was surprisingly cool this morning in Zzyzx as we left for our day of fieldwork, but we were not surprised by the wind at our first stop, Calico Ghost Town outside Barstow. Every time we’ve been there it has been blustery. Calico is an old silver mining site with a complex geological structure complimented by hydrothermal mineralization driven by dacite intrusions roughly 17 million years ago. Shelley Judge showed us how strike-slip faulting (ultimately a result of movement on the San Andreas Fault to the west) produced compressional folds in the Barstow Formation. Meagen Pollock talked about how the silver rich veins were formed by thermal alterations of the sedimentary rocks on the flanks of the Calico Mountains. We then spent a little time in the town itself eating our packed lunches and enjoying cold sarsparilla!

Memories of warmer days…

December 2nd, 2010

Now that the semester is winding down and the cold weather has set in, I find my mind wandering back to the beginning of the academic year. It seems like it was years ago, not months, that our Mineralogy class visited Zollinger’s quarry.

2010 Mineralogy class at Zollinger's Quarry in late September.

It didn’t take long for students to discover the beautifully formed gypsum crystals that littered the ground.

From left to right, Will Cary, Matt Peppers, and Kevin Silver caught in the act of discovering the gypsum.

Truly, these are beautiful gypsum crystals.

In fact, next week  we’re using some of the crystals that we collected in our discussion of the thermodynamics of crystal nucleation and growth.

Of course, the minerals weren’t the only stars of the show. The students were excited to find these incredible mud cracks with preserved rain drops -  comparable to these mud cracks that a fellow geologist at Mountain Beltway observed in Turkey.

Mud cracks and preserved rain drops in Zollinger's Quarry.

Gifts for generations of geology students

October 24th, 2010

WOOSTER, OHIO — The Geology Department at Wooster has received many donations of rocks, minerals and fossils over the years. Collectors are always passionate about their specimens, so when they decide to donate their treasures they want them to go where they will be most useful. What better place than a college? We put collections to work right away in our teaching labs and display cases. Because rocks are so durable, these are gifts which serve for decades.

About a third of the rock, mineral and fossil collection recently donated to the Geology Department at Wooster. Here they are in their original home.

Today Meagen Pollock and I visited the Ohio family of a geology alumna and began the process of transferring their donated specimens to Wooster: gorgeous crystals, an amazing diversity of fossil shells, and spectacular dinosaur bones. There are so many boxes that we will have to make a second trip in a cargo van to transport the rest. It is the largest donation we have ever been given.

An exceptionally complete collection of fluorescent and phosphorescent minerals is part of the donation.

Our teaching will be improved by access to these new specimens, and they will stimulate the imaginations of generations of students. We hope to post later on how this collection is being used in our labs and hallways.

Lunar Ilmenite

January 6th, 2010

Lunar Ilmenite: Next Gen Fuel Source Part 1 and Part 2 presented by Andrew Retzler ’11.

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