Geology Word of the Week: P is for Pleochroism

Pleochroism in an andalusite gemstone. Image taken from gemstonebuzz.com here: http://www.gemstonebuzz.com/andalusite.

def. Pleochroism:

1. An optical property found in many minerals in which a crystal is able to absorb different wavelengths of transmitted light depending on the orientation of the crystal.

2. A useful characteristic for identifying minerals in thin section with an optical microscope.

3. A color-shifting mirage that adds extra pizazz and pop to certain gemstones.

Pleochroic crystals are the chameleons of the mineral world. Pleochroic means “many-colored” in ancient Greek, and pleochroic crystals certainly live up to their name, changing colors– often dramatically– when viewed from different directions. For example, the pleochroic mineral zoisite can appear clear or yellow or pink. The mineral tanzanite ranges from rich purple to rich blue, although gem-quality tanzanite is often heat treated to remove pleochroism and increase the brilliant blue hue. Some pleochroic minerals do not change color exactly but rather display different shades of the same mineral. For example, kunzite ranges from light to dark pink.

The color-shifting nature of pleochroic minerals is revealed through transmitted light, not reflective light. So, while many minerals are pleochroic, this pleochroism is only visible if the mineral is thin or clear enough for light to be transmitted. Thus, for many minerals pleochroism is only observable in translucent, gemstone-quality specimens or when a crystal is sliced thinly enough to allow the transmission of light through the crystal.

Geologists often examine pleochrosim in crystal or rock slices viewed under the microscope. The slices are generally 30-40 micrometers thick and are appropriately named “thin sections.” To increase pleochroism and other optical properties of minerals, these thin sections are usually viewed with a polarizing microscope, which orients (polarizes) the light in a very bright bulb placed below the thin section. These microscopes use both plain polarized light and cross-polarized light. I won’t go into too many details about optical mineralogy here. Thick reference guides and much practice under the microscope are needed to fully understand optical mineralogy, which many geology students find intimidating in these days of button-pressing, mechanical mineral identification. However, optical mineralogy still has many uses and is often a quick way to identify minerals. So, I highly recommend taking a course if you have the opportunity. All you need to know for now is that pleochroic minerals display different colors in thin section when rotated under plain polarized light. The pleochroism of a crystal– the colors displayed and the angles at which the colors change– is a very useful characteristic that geologists can use to identify minerals in thin section. Similar-looking minerals can often be distinguished by differences in pleochroism.

To give you a sense of what pleochroism looks like under the microscope, Shawn Wright of the geoblog Vi-Carius kindly sent me some wonderful sets of pictures displaying pleochroism in minerals viewed from different angles in plain polarized light using an optical microscope. In the set of pictures below, the mineral biotite changes color from dark brown to black when the thin section is rotated:

And in the set of pictures below, the mineral hornblende changes from brown to dark gray when the thin section is rotated:

In the hornblende pictures above, the large crystal of hornblende is actually a composite crystal of hornblende which formed at slightly different orientations. One section of the large horblende crystal is brown at the same time that another section has turned dark gray. This “composite pleochroism” is commonly observed in thin section and can sometimes provide useful information about crystal structure and growth.

Callan Bentley of Mountain Beltway has also put together a wonderful set of .gif files which illustrate pleochroism in biotite and riebeckite. He has slow and fast versions in his blog post. Here’s the two slow versions:

Not all minerals display pleochroism, however. Some minerals display none, some minerals display two different colors, and some minerals display three different colors. The type of pleochroism (or lack thereof) displayed by a mineral is determined by crystal structure of that mineral.

When light is transmitted through different axes of a crystal, there are three options: the same color is displayed (no pleochroism), two different colors are displayed (dichroism), or three different colors are displayed (trichroism). Because we live in a 3-dimensional world, crystals have three axes: x, y, and z. Or a, b, and c if you (and my mineralogy text) prefer. Minerals which do not display pleochroism have symmetrical crystal axes. That is, the crystal structure is identical along the a, b, and c axes. Minerals which have identical crystal structures along all three crystal axes are known as “isometric” or “cubic” minerals. Because isometric minerals have the same structure in all directions, changing the angle through which light is transmitted does not change the color of the mineral. Minerals with two identical crystal axes and one distinct crystal axis (trigonal/rhombahedral, tetragonal, or hexagonal crystals) can display two different colors. Minerals with three distinct crystal axes (triclinic, monoclinic, and orthorhombic crystals) can display three different colors.

The 7 Different Crystal Lattice Groups. Image taken from molecularsciences.org here: http://www.molecularsciences.org/book/export/html/125

Pyrite is a cubic (isometric) mineral and thus does not display pleochroism. Photo credit: JJ Harrison. Image taken from Wikipedia Commons here: http://en.wikipedia.org/wiki/File:Pyrite_from_Ampliaci%C3%B3n_a_Victoria_Mine,_Navaj%C3%BAn,_La_Rioja,_Spain_2.jpg.

Andalusite is an orthorhombic mineral and dislays pleochroism, which is often highlighted in gemstones (see images at top of post and below). Photo credit: Didier Descouens. Image taken from Wikipedia Commons here: http://en.wikipedia.org/wiki/File:AndalousiteTyrol.jpg

Pleochroism in an andalusite gemstone. Image taken from Rocks & Co. website here: http://www.rocksandco.com/?task=rocksBookSecond&action=gemstoneSpecEffects.

Dichroic minerals can be a little bit tricky to identify in thin section. Because thin sections essentially represent a 2D slice of a 3D mineral, dichroic minerals may or may not display pleochroism in thin section. If the thin section displays two crystal axes which are distinct, then the mineral will display pleochroism. However, if the thin section displays two crystal axes which are symmetric, then the dichroic mineral will not display pleochroism. Similarly, trichroic minerals may display different pleochroism, depending on the orientation of crystals viewed in thin section. Thus, pleochroism is a useful tool not only for identifying minerals but also for identifying which crystal axes are visible in thin section.

Here’s another great set of pictures from Shawn Wright, this time showing tourmaline in thin section. Tourmaline is a trigonal mineral, which means that it is dichroic. When oriented so that two distinct crystal axes are displayed (the first set of pictures), pleochroism is visible, and the mineral color changes from gray to dark blue when the thin section is rotated. However, when oriented so that two symmetrical crystal axes are displayed (the second set of pictures), no pleochroism occurs when the thin section is rotated.

Pleochroism is found in many gemstones, and gemstones are often cut to either display or hide pleochroism. Here’s a great website with many images of pleochroism in gemstones.

If you want to learn more about microscopes and thin sections, here’s a teaching website with more information on pleochroism and also on optical mineralogy in general.

Reference:

“pleochroism, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 15 September 2011.

***Thanks to Ryan Brown for suggesting this week’s word. Thanks to Shawn Wright for providing the wonderful microscope images and to Callan Bentley for creating the neat .gif animations.***

Geology Word of the Week: O is for Ooid

Ooid sand from the shores of the Great Salt Lake, Utah. Photo courtesy of Matt Kuchta.

def. Ooid:
A small (generally less than 2 mm), spherical or ellipsoidal concretion of calcium carbonate (CaCO3) that has generally formed around a “nucleus” such as a shell fragment or a quartz grain. The word ooid is derived from an ancient Greek word meaning egg-shaped. According to the Oxford English dictionary, the name came about because ooids resemble roe (fish eggs).

Some geology words I love just because they’re just so much fun. “Ooid” is one of those words. “Ooid” is a great word because it looks and sounds like the geological entity it represents. The word is oval (O) and round (o) just like ooids themselves. The word also rolls off the tongue in a squishy way that makes me think of marine ooze, which might be found in the vicinity of ooids.

Here are a few bonus, related words:
def. Oolite:
A sedimentary rock composed of lithified (“made into rock”) ooids.

def. Oolith:
A synonym for ooid, often used to refer to a single grain.

A synonym for oolite is roestone— literally, fish egg stone!

Other related oo- words are ooidal, oolithic, oolitic, and oolitiferous. That last word sounds somewhat fake, but I found it in the trusty Oxford English Dictionary!

For more scientific information about ooids, here is a good article titled “Ooid Formation” (on a wonderfully-named website called Geology Rocks) that describes ooids far better than I could.

Here are oodles of ooid and oolite pictures:

Modern ooids from the Bahamas. Photo courtesy of Callan Bentley. Note scale in top left corner.

More modern ooids from the Bahamas. Photo courtesy of Callan Bentley. Note scale in top left corner.

Oolitic limestone from the Rierdon Formation, a Jurassic unit from Montana. Photo courtesy of Callan Bentley. Note scale in top left corner.

A jar of ooids! Photo courtesy of Paul Glasser.

Ooooo so many ooids! Photo courtesy of Paul Glasser.

Ooo000ooids! Photo courtesy of Paul Glasser.

Ooids in a petri dish. Photo courtesy of Paul Glasser.

Oolitic limestone deskcrop. Photo courtesy of Ron Schott.

Oolitic Portland Limestone, commonly used as a building stone. Photo courtesy of Ian Stimpson.

References:

“ooid, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 11 September 2011.

Other words such as roestone, oolith, etc. were also looked up in the OED.

***Thanks to Christie Wilcox for suggesting this week’s word. Thanks to Matt Kuchta, Callan Bentley, Paul Glasser, and Ian Stimpson for providing pictures. Thanks especially to Paul Glasser for naming his picture folder “oodles of ooids,” a delightful phrase which I promptly stole for this blog post.***

Geology Word of the Week: N is for Nummulite

Nummulite fossils. The small ones were collected near Notre Dame, France. Photo courtesy of Callan Bentley.

def. Nummulite:
1. A fossil or living foraminiferan of the Nummulites genus (or a related genus) that has a disc-like, spiral, calcareous skeleton. Fossil nummulites range up to several inches in size, making them quite impressive protozoa (single-celled, eukaryotic organisms). Nummulite fossils are common in Tertiary rocks, particularly in the Mediterranean area. The term nummulite originates from the Latin word “nummulus,” which means coin.
2. The unwitting star of a very strange and scientifically bunk, yet somehow delightful, book titled “The Nummulosphere: An Account of the Organic Origin of So-called Igneous Rocks and Abyssal Red Clays” by Randolph Kirkpatrick.

Nummulites are beautiful and very distinctive fossils that are relatively easy to recognize in the field– they look like little coins set into the rocks. Because nummulites have calcium carbonate skeletons, they are generally found in limestone rocks. Nummulite fossils are even found in some of the limestone blocks used to construct Egyptian pyramids!

Ian Stimpson of the blog Hypo-theses sent me this beautiful photograph of nummulite fossils in limestone:

This Nummulitic limestone is from the Tertiary of the Spanish Pyrenees. The nummulites are up to 1cm across in this sample. Photo courtesy of Ian Stimpson.

Nummulites can be very small (microfossils) but can also range up to several inches (or centimeters, to use the more-scientific metric system) in size, such as the ~2 cm example in the top image. What is impressive about the size of these macro-nummulites is that all nummulites are protozoa, which means that they are single-celled organisms. I’m not much of a biologist, but those large nummulite fossils look like pretty big cells to me!

Lorraine Casazza of the University of California Museum of Paleontology does know a thing or two (or many things!) about biology and also about nummulites, which she studies. I highly recommend reading Casazza’s description of her research on Egyptian nummulites. Casazza has some great discussion on how and why single-celled nummulites became so large. One reason that nummulites may have become so large is because of an interesting symbiosis with algae. Again, I’m not much of a biologist, but according to this abstract (thanks to Lockwood DeWitt for finding it), all modern nummulites house symbiotic algae.

Kirkpatrick's 1912 "Nummulosphere" book. Image taken from Wikipedia. The book is now in the public domain.

Nummulites are fascinating and important foraminifera, but they aren’t quite as important as indicated by Randolph Kirkpatrick in his self-published 1912 book “The Nummulosphere: An Account of the Organic Origin of So-called Igneous Rocks and Abyssal Red Clays.” In this book, Kirkpatrick claims that all rocks– including the “so-called igneous rocks”– actually formed through the accumulation of foraminifera such as nummulites. The book has a catchy and clever title, but alas the book is mostly pseudoscience and, fortunately, was not taken seriously by many scientists when it was published. In fact, Kirkpatrick’s crazy ideas about “The Nummulosphere” tarnished his scientific reputation. Kirkpatrick actually was a good scientist when it came to certain aspects of his work. Kirkpatrick had kooky– and very wrong– ideas about how rocks formed, but he was very good at studying the biology of sponges. However, much of his good scientific work on sponges was probably overlooked by his contemporaries because of his crazy ideas about how rocks formed. Not until a decade or so after his death was his work on sponges truly recognized.

Kirkpatrick is an intriguing example of a smart and capable scientist who fell victim to pseudoscience. Many scientists– myself included at times– fall victim to pseudoscience. Just because scientists are smart and educated doesn’t mean that they can’t fool themselves, even in their own research. For example, Linus Pauling won not one but two Nobel prizes but had some very strange (and now largely discredited) ideas about how taking large quantities of vitamins could make you live longer. Physicists Russell Targ and Harold Putoff convinced themselves that Uri Geller has “genuine” paranormal powers, even though it has been demonstrated repeatedly that Geller is likely using simple magic tricks. In my own family there is an excellent example of a very smart person believing in pseudoscience. Upton Sinclair (I was named after Upton’s cousin, my great-grandmother Evelyn Sinclair) wasn’t a scientist, but he was a brilliant writer, journalist, and political activist. However, my Uncle Upton (as I like to call him) also wrote a book called “Mental Radio” in which he described his belief that his second wife had telepathic abilities. I’m sorry, Uncle Upton, but your psychic experiments were not carried out in a proper scientific environment and, really, most long-married husband-wife pairs develop non-verbal communication that may seem telepathic at times. In my own scientific encounters, I’ve met many a scientist who is mostly rational and reasonable but who also believes in one or more flavors of pseudoscience: homeopathic medicine, talking to the dead, chiropractics, and so on.

I guess the main point I want to make is that scientists are smart, but they aren’t smart about everything. Just because someone is a smart and accomplished scientist does not mean that that person is always right. PhD or not, Nobel Prize or not, scientists are not always right. The great thing about science, though, is that (eventually) data and evidence always trump scientific reputation. For example, just because Linus Pauling had a PhD and two Nobel Prizes didn’t mean other scientists weren’t critical of views on vitamins. Perhaps his scientific prestige helped him push the vitamin idea at first, but eventually concrete data largely dismissed his pseudoscientific idea. Similarly, just because a scientist has one crazy or scientifically wrong idea does not mean that the scientist’s entire body of work should be dismissed. For example, Kirkpatrick’s work on sponges should not have been dismissed just because he didn’t understand rocks very well. Kirkpatrick is an extreme example. However, too often a scientist publishes a paper with an idea that is later dismissed, and then this scientist receives a “bad reputation,” and other scientists become critical of all of this scientist’s ideas. The whole point of science is putting ideas– hypotheses– out there. Just because one of a scientist’s hypotheses turns out to be wrong does not mean that all of this scientist’s hypotheses will be incorrect. We must remember that science is a process, not a popularity contest. Reputations should not matter where evidence and good (or bad) data abound. Of course, I do simplify. Some scientists have good (or bad) reputations for good reasons. Regardless, we must never let prestige or reputation blind our science– we scientists must strive to be as neutral as possible.

A final thought: be cautious when listening to a scientist talk about something that is clearly outside of that scientist’s field. For instance, I’m a geologist with specialties in marine geology, geochronology, and isotope geology. When I’m talking about one of those three specialties, you can probably trust what I say. However, if I’m talking about something else, you better make sure I’ve done my homework and actually know what I’m talking about. When I step outside of my scientific specialties, it is very important for me to talk to other scientists and develop collaborations. As I mentioned above, I don’t know very much about biology. So, if I were to take on a research project involving some biology (for example, a study of biological influences on rock weathering), it would be important for me to work with some biologists. Kirkpatrick was a biologist, not a geologist. Perhaps if he had worked with some geologists and had better understood geology, he would never have written his Nummulosphere book. That would have been a shame, though. Nummulosphere is such a wonderful-yet-terrible little volume.

Reference:

“nummulite, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 4 September 2011.

***Thanks to Etienne Médard for suggesting this week’s word. Thanks to Callan Bentley and Ian Stimpson for providing pictures. Thanks to Lockwood DeWitt and Callan Bentley for some information and interesting discussion of nummulites on twitter.***

Mystery Rock #3

Jennifer & Jeff's beautiful granite countertop.

Back in May I posted about a Mystery Rock and then a Mystery Rock #2. Pictures of these two mystery rocks were sent in by blog readers. I had a great time thinking about these two mystery rocks and helping with their identification, and there was some great discussion among geologists and non-geologists in the comments.

For awhile I’ve been meaning to post Mystery Rock #3. A couple of months ago, my good friends Jennifer & Jeff, who live in Chicago, sent me some pictures of their kitchen countertop. The mystery of this rock actually isn’t the identification of the rock. The countertop is granite, probably a true granite. I have a little bit of trouble estimating the mineral proportions from the pictures (of polished rock! the horror… all the distinctive cleavage is gone!), but the countertop is definitely a true granite or a close granitoid relative.

For those of you who are not familiar with the classification of granites and closely-related rocks, igneous rocks with less than 90% mafic minerals (such as olivine and pyroxene) are classified using the QAPF Diagram. The letters in the acronym stand for Quartz, Alkali Feldspar, Plagioclase, and Feldspathoid, which are minerals or groups of minerals. For igneous rocks with large crystals that can easily be seen in hand sample (these are called plutonic rocks), the plutonic QAPF diagram is used to determine the rock name:

Plutonic QAPF Diagram. Image downloaded from Wikipedia Commons. The original diagram is from Igneous Rocks: A Classification and Glossary of Terms, 2nd Edition; 2002; R.W. Le Maitre editor; Cambridge University Press.

Granites and granitoids plot in the top triangle on the diamond-shaped diagram. A true granite contains 20-60% quartz and about equal proportions of plagioclase and K-feldspar. If a granitoid rock contains more plagioclase than K-feldspar, then it is called a “monzogranite.” Similarly, if a granite contains more K-feldspar than plagioclase, then it is called a “syenogranite.” Granitoid rocks that contain more than 65% plagioclase are technically “granodiorites” while granitoids that contain more than 90% K-feldspar are technically “Alkali feldspar granites.” In addition to quartz, plagioclase, and K-feldspar, granites also usually contain mica (biotite or muscovite or sometimes both) and hornblende. Apatite, zircon, titanite, and magnetite are commonly present in small amounts.

If you want to learn more about the use of classification diagrams for igneous rocks, here’s a great website.

Here’s a picture of Jennifer & Jeff’s countertop, with some minerals labeled:

Jennifer & Jeff's granite countertop with some minerals labeled.

An aside before I go on to the mystery: many kitchen countertops are actually not granite. For example, countertops are often made of rocks such as granodiorite, quartzite, rhyolite, travertine, marble, soapstone, or gneiss. Many countertops are manmade– they are made of pieces of rocks, often quartz-rich, that are put together in a manner designed to be pretty and also nonporous. As a geologist, I sometimes run into trouble when I visit someone’s house and compliment the rock countertops. Fortunately, many of my friends are fellow geologists or scientists (or are poor students who can’t afford nice countertops), but sometimes the conversation goes something like this:

Me: Wow! Those are some beautiful kitchen countertops you have.

Host: Thanks! I’ve always wanted granite countertops. When we built this house, we decided we just had to have them, even though it was quite expensive.

Me: Granite? These countertops aren’t granite. But they’re gneiss.

Host, slightly taken aback: What do you mean they aren’t granite? They must be granite.

Me: Oh, they’re not granite. But they are gneiss, which is…

Host: Nice? Just nice? Henry, did you hear this? She says our countertops aren’t real granite!

Host’s Husband, Henry: Well, of course they’re granite. I bloody well paid enough for them.

Me: Please, let me explain. I mean the rock type gneiss, which I actually think can be prettier for countertops than granite.

Henry: Ha ha! Oh, I’ve heard of gneiss. You geologists have gneiss schist, don’t you know?

….awkward digression into geology puns…

Me, desperately attempting to change the conversation: And you have such beautiful cabinets, too! I love the cherry color.

Host: Yes, but you scientists are probably going to tell me they’re not real wood…

I bet many a geologist has had a variation of the above conversation. Anyway, Jennifer & Jeff do have a real granite countertop– quite a pretty one as well!

Here’s a few more pictures of the countertop:

Another picture of the countertop from above.

And here’s a picture illustrating the rock mystery:

A small magnet is attracted to the edge of the countertop!

Jennifer & Jeff noticed that a small magnet was attracted to the edge of their countertop– but only to the dark portions of the rock. The dark portions are likely hornblende and/or biotite (it’s a little difficult for me to tell from the photographs of the polished surface). These two minerals are generally not magnetic, so my guess is that these dark minerals contain inclusions of a mangetic minerals such as magnetite, which is commonly found in granites in trace amounts.

Do any other geologists have insight into this countertop mystery? Does anyone else have a magnetic granite countertop?

Geology Word of the Week: M is for Migmatite

Typical migmatite rock. Photo courtesy of Etienne Médard.

def. Migmatite:
A heterogeneous silicate rock with properties of both igneous and metamorphic rocks. Typically, the rock contains alternating lighter layers (leucosomes, comprised of light-colored minerals such as quartz, feldspar, and muscovite) and darker layers (melanosomes, comprised of dark-colored minerals such as amphibole and biotite). The heterogeneous nature of the rock results from partial melting (called anatexis) that occurs when a precursor rock is exposed to high pressures and temperatures. The light-colored layers originate from the partial melting and have igneous characteristics– that is, their appearance indicates that they have been crystallized from a melt. The dark-colored layers have experienced metamorphism, but they do not have igneous characteristics. The distinctive light-and-dark banding (similar to that seen in gneiss) as well as the folding commonly found in this rock results from the partial melting as well as from high-grade metamorphism and deformation. The heterogeneous nature of this rock occurs on a wide range of scales, from microscopic (seen on thin sections) to intermediate (within a hand sample) to very large (only observable on a large outcrop). The word “migmatite” was first introduced into the geologic literature in 1907 by Jakob Johannes Sederholm and literally means “mixed rock,” originating from ancient Greek.

Migmatites are truly extraordinary rocks. They are very beautiful rocks to observe in the field or in hand sample. They usually consist of dramatic alternating black-and-white layers, and I like to call them “Zebra rocks.” These alternating layers usually undulate in a serpentine fashion, often containing beautiful, sinuous, ptygmatic folds.

Tight folds in a migmatite boulder (called "Bob's Rock") on display in the Grassy Hallow Visitor's Center, Angeles National Forest, California. Photo courtesy of Tisha Irwin.

Another view of the "Bob's Rock" migmatite boulder. Photo courtesy of Tisha Irwin.

As a quick aside, the word “ptygmatic” was also introduced into the geological literature by Jakob Johannes Sederholm in 1907 and originates from the ancient Greek word for “fold” or “anything folded.” In a way, using the term “ptygmatic fold” is somewhat redundant– like saying “a folded fold.” However, the term “ptygmatic” in the geologic literature generally refers to tight folds that form when the folded material has a greater viscosity (resistance to flow) than the surrounding medium. In migmatites, ptygmatic folds often form in the more-viscous lighter layers.

Pink (K-feldspar-rich) ptygmatic folds. Photo taken from Wikipedia Commons here: http://en.wikipedia.org/wiki/File:Ptigmatite.jpg. Photo credit: Siim Sepp.

Ptygamtic folds, a screenshot from part of the migmatite Gigapan by Callan Bentley. See end of post for a link to the Gigapan.

Beauty aside, migmaties are also extraordinary rocks because they form at the boundary between the metamorphic and igneous worlds. As a quick reminder for those of you who are a little rusty on Geology 101, metamorphic rocks are rocks which deform at very high pressures and temperatures and which may recrystallize but which have not formed through melting. Igneous rocks, on the other hand, are rocks which form by cooling from completely molten material. Migmatites are hybrid rocks: the dark layers (most often composed of biotite and amphibole) experienced metamorphic changes, but did not melt. The light layers (most often granitic in composition; as a reminder, granite consists of the minerals quartz, feldspar, and muscovite), on the other hand, crystallized from partial melts of the precursor rock.

A quick look at Bowen’s Reaction Series, which (in a very general way) organizes the order in which minerals melt (coldest to hottest) or crystallize (follow the diagram the opposite way, from hottest to coldest), gives a sense of why dark-colored minerals such as amphibole and biotite are more resistant to melting than light-colored minerals such as quartz, k-feldspar (often light pink in color), and muscovite. Generally, the light-colored minerals melt at lower temperatures than the dark-colored minerals.

Bowen's Reaction Series. Image taken from here: http://www.eoearth.org/article/Igneous_rock.

Here’s a nice figure, courtesy of fellow AGU blogger Callan Bentley, illustrating partial melting:

Partial melting of a rock to form a felsic (generally, light-colored) melt and a mafic (generally, dark-colored) residue. Image courtesy of Callan Bentley.

The above figure refers to “felsic” melt and “mafic” residue. Felsic melts are silica-rich and form from silica-rich, generally light-colored mineral such as quartz and k-feldspar. Mafic melts are silica-poor (relative to felsic melts) and form from minerals with lower silicon contents such as olivine, pyroxene, and amphibole. Mafic minerals are generally darker in color, often black, brown, or dark green.

Migmatites actually look very similar to a related rock: gneiss. Gneisses also contain alternating light and dark layers which result under high-pressure and high-temperature conditions. However, in a strict definition gneisses are metamorphic rocks, which means that the light bands form through recrystallization alone; the light layers did not form by cooling from a melt. Distinguishing between gneisses and migmatites can be slightly challenging to do in the field. The two rock types are certainly relatives, so to speak. If a gneiss experiences just slightly higher temperatures, it may partially melt and become a migmatite. Most migmatites probably were gneisses on their way to becoming true hybrid metamorphic-igneous (metagneous? ignamorphic?) rocks.

Because migmatites are hybrid metamorphic-igneous rocks , they are important rocks for geologists to study in order to better understand how rocks melt and how these melts migrate and eventually become igneous rocks. Because many migmatites are silica-rich, some geologists have tried studying migmatites as a way to understand another silica-rich rock: igneous granite. For a long time, the origin of granitic rocks was debated by geologists. Between the 1920s and the 1960s, many geologists argued that granites could form from sedimentary or other non-granitic rocks through chemical alteration caused by fluids. This theory has now been largely abandoned, and geologists now believe that granites crystallize from melts of high-grade metamorphic rocks. The precursors of these high-grade metamorphic rocks may be either igneous (I-Type Granite) or sedimentary (S-Type Granite). Geologists now understand that granitic melts can form through a variety of melting processes. The presence of migmatites was one line of evidence that geologists examined to determine that granites probably formed through melting processes, not through chemical alteration processes. The exact relationship– if any– between migmatites and large bodies of melt is ambiguous and still debated amongst geologists, but migmatites do provide clear evidence that granitic melts (and also other types of melts) can be produced through partial melting of metamorphic rocks. Some migmatites are even found in close proximity to granite, such as in the photo below.

Adjacent granite and migmatite. Photo courtesy of Callan Bentley.

Below are a few more pictures of gorgeous migmatite rocks. Enjoy!

Close view of sinuous migmatite layers. Photo courtesy of Etienne Médard.

Light-colored melt layers have aggregated in the center of this migmatite. Photo courtesy of Etienne Médard.

Fine bands of light and dark minerals in migmatite. Photo courtesy of Etienne Médard.

Sharp folds in migmagtite. Photo courtesy of Etienne Médard.

More migmatite layers and folds. Photo courtesy of Etienne Médard.

Gorgeous migmatite observed along the Billy Goat Trail, Maryland. Photo courtesy of Callan Bentley.

Another migmatite observed along the Billy Goat Trail. Photo courtesy of Callan Bentley.

Tight folding in migmatite. Photo courtesy of Callan Bentley.

Gorgeous migmatite hand sample photographed in the lab. Photo courtesy of Callan Bentley.

Migmatite boulder from the Skykomish River near Gold Bar, Washington State. Photo courtesy of Dana Hunter.

A closer view of the above migmatite boulder. Photo courtesy of Dana Hunter.

Another migmatite boulder from the Skykomish River. Photo courtesy of Dana Hunter.

Closer view of the above migmatite boulder. Photo courtesy of Dana Hunter.

Finally, here’s a link to a fantastic migmatite gigapan, courtesy of Callan Bentley.

References:

1. Hall, Anthony. Igneous Petrology. New York: John Wiley & Sons, Inc. : 1995.

2. “migmatite, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 28 August 2011.

3. “ptygmatic, adj.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 28 August 2011.

***Thanks to Erik Klemetti for enthusiastically recommending the word migmatite. Thanks to Etienne Médard, Tisha Irwin, Callan Bentley, and Dana Hunter for providing pictures.***

Nuy Valley in Pictures

Nuy Valley stream with folded sandsontes in the distance.

Sorry posting has been a bit slow recently… I’ve been very busy re-locating to South Africa and trying to make good progress on writing up my thesis. Actually, the next few months may be somewhat lighter with posting as I actually write up this thesis of mine!

To tide over my blog readers, I thought I’d post a few pictures from my new home here in South Africa. Last weekend my fiance and I spent the weekend in Robertson, South Africa to visit family and take care of some arrangements for our upcoming wedding in October. The Robertson area is beautiful and also geological– the landscape around Nuy Valley, the wine farm where we are having our wedding, is dominated by folded and faulted sandstones hills and mountains.

Below are a few pictures from the Nuy Valley Wine Farm. The skies are a bit overcast in some of the pictures because of winter rain, but even in winter the landscape is gorgeous. Enjoy!

A palm tree! And more sandstones in the distance.

Rows of grapevines with sandstone mountains in the distance.

More grapevines and mountains. Note the thin landslide scar. Click to enlarge the view.

Geology Word of the Week: L is for Lepidolite

A sheet of rose pink lepidolite. Image taken from Wikipedia Commons. Attribution: Rob Lavinsky, iRocks.com.

def. Lepidolite:
A pink, lilac, or gray-white, lithium-rich, Mica Group mineral with the formula K(Li, Al)2-3(AlSi3O10)(O,OH,F)2.

Geologists (and laypeople) often talk about the “mineral” mica, but mica is not really a mineral. Rather, mica is the name of a group of minerals. The micas are phyllosilicate minerals, which means that they are comprised of flat sheets. In Micas, these flat sheets are piled together in stacks. The word “phyllo” comes from Greek and means “leaf.” To remember the word phyllosilicate, I always think of phyllo dough, which is a dough made up of thin, flat sheets piled up and used to make pastries such as baklava or spinach pie. Phyllo dough is generally made up of flour, water, and a little sugar. Phyllosilicates, on the other hand, are made up of thin sheets of silicon and oxygen in a 2:5 ratio. Micas also have some aluminum and potassium thrown into the structure. Micas are basically stacked sheets of aluminum, silicon, and oxygen that are held together by charged potassium (K+). If you want to read more about the structure of phyllosilicates and Micas in particular, I recommend these excellent notes I found on a Smith College Geoscience website.

Sheets of phyllo dough in baklava. Image taken from Wikipedia Commons here: http://en.wikipedia.org/wiki/File:Baklava.jpg.

Sheets of muscovite mica. Image taken from Wikipedia Commons. Attribution: Rob Lavinsky, iRocks.com.

Lepidolite is similar in composition and structure to the silver plates of muscovite mica and the brown-black plates of biotite mica that are common rock-forming minerals in rocks such as granites. However, lepidolite contains a significant amount of the element lithium (in the same place where aluminum sits in muscovite and other micas). In fact, lepidolite is sometimes mined for lithium although generally only because it is associated with other lithium-rich minerals such as spodumene (formula: LiAl(SiO3)2).

Lepidolite is a fairly rare mineral, generally found in something called a pegmatite, which is a very coarse-grained igneous deposit in which large crystals (sometimes amazingly large crystals!) were able to grow because of special conditions. In order to grow large crystals and form pegmatites, igneous bodies must cool very slowly and also have high rates of diffusion (high rates of transport of elements, basically), generally aided through the presence of water or vapor or both. Pegmatites often have high concentrations of lithium because lithium (as well as boron and other large elements) lowers the solidification temperatures (basically by being big edit: when thinking about relative sizes of elements, remember to consider ionic raidus as elements are generally in charged forms in crystal structures) of magmas, giving the crystals more time to grow. Lepidolites often form intermixed with muscovite and other mica minerals as well as with other lithium-bearing minerals such as spodumene, amblygonite, beryl, and tourmaline. Lepidolite most often occurs in pegmatites associated with granite bodies.

Lepidolite is a gorgeous mineral, especially when it is bright pink and lavender. My fellow AGU blogger Jessica Ball recently observed and collected some lepidolite during her visit to the Harding Pegmatite Mine in New Mexico. Jessica was kind enough to send me some pictures of gorgeous purple lepidolite from the Harding Pegmatite:

Lepidolite (purple color) in an outcrop at the Harding Pegmatite Mine. Photo courtesy of Jessica Ball.

Lepidolite collected from Harding Pegmatite Mine. Photo courtesy of Jessica Ball.

A closer look at some beautiful purple lepidolite from Harding Pegmatite Mine. Photo courtesy of Jessica Ball.

References:

Klein, Cornelius. 2002. The 22nd Edition of the Manual of Mineral Science. John Wiley & Sons.

Deer, W.A., Howie, R.A., and Zussman, J. 1992. An Introduction to the Rock-Forming Minerals, 2nd Edition. New York: Pearson Education Limited.

Geology Word of the Week: K is for Krakatau

Anak-Krakatoa 1. Photo courtesy of James Reynolds. — Anak-Krakatau 1. Photo courtesy of James Reynolds.

def. Krakatau (aka Krakatoa):
A volcanic island between the islands of Sumatra and Java in Indonesia. The volcanic island is known for a major eruption in 1883 that largely destroyed the original island. Since the 1920s, the volcanic island has been rebuilding and is today known as Anak Krakatau or “Son of Krakatau.”

I’m flying to Johannesburg, South Africa tomorrow and have had a long couple of weeks preparing for the move, including a 10 hour drive today. So, for this week’s geology word of the week I’ll just share a few links and some photographs. Enjoy!

After I recover from my travels, I’ll share the story of when I was at sea for 50 days (no sight of land!) and then the very first land I saw at the end of the research cruise was Anak-Krakatau. Seeing Anak-Krakatau was wonderful, but seeing the volcano from a ship at dawn after not seeing land for 50 days was amazing, magical almost.

Some Links:
Popular book by Simon Winchester: Krakatoa: The Day the World Exploded: August 27th, 1883

Book: The eruption of Krakatoa, and subsequent phenomena (1888). Thanks to David Bressan for the link

Many links on the Eruptions blog: Krakatoa Tag on Eruptions

Some Pictures, Courtesy of James Reynolds:

Anak-Krakatau 2. Photo courtesy of James Reynolds.

Anak-Krakatau 3. Photo courtesy of James Reynolds.

Anak-Krakatau 1. — Anak-Krakatau 3. Photo courtesy of James Reynolds.

A Video, Also Courtesy of James Reynolds:

Blast from the Past: Rutherford Gold Foil Experiment

I’m still super busy preparing for my move to South Africa on Monday, so here’s another quick “Blast from the Past” post to keep my blog readers entertained. Enjoy!

Normal children draw pictures of rainbows, flowers, ponies, ninjas, and so on. When I was about 10 years old, I drew these pictures of the Rutherford Gold Foil Experiment. Well, technically of the “Rutherford Expriment.” My spelling never was very good.

"Rutherford Expriment" drawing, circa 1994.Click to view larger.

Drawing of Alpha particles interacting with a gold atom, circa 1994.Click to view larger.

Paleontologist Barbie

Paleontologist Barbie arrived today in the mail. After reading this blog post I just couldn’t resist ordering my very own Paleontologist Barbie from ebay. Best $30 I’ve spent in a long time. I think I’ll take her on some adventures with me over the next year.

Wow! She comes with dinosaur friends, a rock hammer, fossils, a map (of Gondwana, I think), a canteen, a field bag, a hat, and a dinosaur frisbee (I think). Look at her awesome shirt with dinosaurs!

Looking for dinosaurs with... Paleontologist Barbie!

Barbie is a silly name, though. I think I’ll re-name her. How about Galena? Any other suggestions?