Monday Geology Picture: Serpentinized Harzburgite in Thin Section

For this week’s Monday Geology Picture, I thought I would share a picture of peridotite in thin section viewed under a microscope. This particular peridotite originates from the Samail Ophiolite, Sultanate of Oman, and is a weakly-deformed harzburgite that is ~40% serpentinized. You can see the fine network of serpentine veins throughout the sample. The brightly-colored patches are olivine and pyroxene minerals. I really enjoy looking at rocks in thin section– rocks can be so beautiful in thin section, particularly when viewed under cross-polarized light.

In other news, the defendable draft of my PhD thesis is due on Friday, so blogging (aside from posting cat pictures on Geokittehs) will continue to be light.

OM10_14P_MG_ppl_wscalebar — Serpentinized harzburgite in thin section, plane polarized light.1 mm scalebar

OM10_14P_MG_xpl_wscalebar — Serpentinized harzburgite in thin section, cross polarized light. 1 mm scalebar.

Monday Geology Picture: Listwanite Hills in the Sultanate of Oman

OLYMPUS DIGITAL CAMERA — Listwanite hills in the Sultanate of Oman, January 2012. The reddish parts of the moutnain are listwanites while the grayish parts are less-altered peridotites.

Today I am going to share some pictures of listwanite (also sometimes spelled listvenite, listvanite, or listwaenite), an unusual rock type that I bet even some of the well-educated geologists who read this blog have never seen or even read about. I don’t even think there’s a wikipedia entry about listwanite. Perhaps I’ll write one after my thesis defense next month.

Listwanite forms when ultramafic rocks (most commonly mantle peridotites) are completely carbonated. The pyroxene and olivine minerals found in peridotite commonly alter to form carbonate and serpentine minerals. However, peridotites are usually not completely carbonated. Rather, they typically contain carbonate veins (primarily magnesite; also calcite, dolomite, and other carbonates). Complete carbonation of peridotite means that every single atom of magnesium and calcium as well as some of the iron atoms has combined with CO2 to form secondary carbonate minerals such a magnesite and calcite. The silica atoms in listwanite are found in quartz. Thus, liswanites consist of quartz (a rusty red color) and carbonate and also sometimes talc and Cr-muscovite (a mineral known as mariposite/fuchsite). Geologists are still studying how listwanites form, but they likely form through the interaction of CO2-rich fluids with peridotites at higher than ambient temperatures up to ~200 degrees Celsius. Structural controls (faults and fractures) permit the percolation of the CO2-rich fluids through peridotite, so the formation of listwanites is generally structurally controlled.

Listwanites are important rocks to study for a number of reasons. First of all, listwanites contain large amounts of CO2 which originated from fluids and which is now stored in solid mineral form. Recently, geologists and other scientists have been investigating the potential of storing CO2 in solid minerals (which are more stable than CO2 stored as a liquid or gas) through carbonation of mafic and ultramafic rocks (see, for example, this Nature Geoscience Progress Article by Matter and Kelemen, 2009). Mafic and ultramafic rocks uptake significant CO2 through their natural alteration processes (that’s what I study for my PhD, so expect more on this in the next year or so as I submit my papers for publication). However, the natural carbonation rates of these rocks are too slow to significantly offset anthropogenic CO2 emissions. Therefore, scientists are currently investigating if it is possible to geoengineer CO2 uptake in mafic and ultramafic rocks so that this CO2 uptake happens more quickly. This could be done, perhaps, by fracturing and heating and injection of CO2-rich fluids. This is already being tested in mafic basalts through the CarbFix Project in Iceland.

However, scientists and engineers still have plenty of work to do in order to figure out the right conditions and protocols for CO2 sequestration in mafic and ultramafic rocks. In order to learn about what conditions lead to complete carbonation of ultramafic rocks, scientists such as Peter Kelemen and Gregory Dipple (and their many grad students and collaborators) are working to learn more about listwanites to see if mother nature can provide some clues.

In addition to the recent interest in listwanites for carbon sequestration efforts, listwanites are also important because they are often associated with economic mineral deposits, particularly gold deposits.

So, now that I’ve explained what listwanites are and why they are important, here are some pictures of listwanites which I observed during my trip to Oman back in January. Listwanites are pretty neat rocks, aren’t they?

Mantle Peridotite in the Samail Ophiolite, Oman

Over the past couple of weeks, I posted pictures of pillow basalts and sheeted dikes in the Samail Ophiolite, Oman. To round out the crustal ophiolite sequence, I thought I would post a couple of pictures of mantle peridotite in the Samail Ophiolite. As you can see in the below pictures, mantle peridotite in the Samail Ophiolite is generally highly-weathered and a dullish brown color. Harzburgite tends to be a darker red-brown color while dunite is a lighter tan (or “dun”, hence the name) color.

Geology Word of the Week: S is for Speleothem

Posing with a pseudostalagmite, Oman, January 2009.

def. Speleothem:
An encompassing term used to describe all types of chemical precipitates that form in caves.

If you’ve ever been in a cave, you’ve probably seen speleothems. Speleothems generally precipitate from groundwater which has percolated through the bedrock surrounding the cave and leached various elements and compounds. When the enriched water reaches the cave, changing conditions (a large open space has very different pressure and temperature than pore spaces in bedrock) allow gases, such as carbon dioxide, to escape from the water. Evaporation can also occur. The changing composition of the water encourages the (usually very, very slow) precipitation of speleothem minerals from cave waters.

Chemically, the speleothems which form in a particular cave are similar in composition. Most caves are formed in limestone, and so the speleothems will generally be formed of calcite, the dominant mineral in limestone. However, depending on how and where the speleothem is precipitated, it can take on a variety of shapes. Scientists and other cave explorers have given different names to these various morphologies. Examples of speleothems are stalactites, stalagmites, flowstones, cave coral, cave drapery, cave curtains, and cave crystals. There are dozens of names for various cave mineral formations, so speleothem is a nice catch-all phrase for geologists to use.

Here is a great picture (from Wikipedia) of some of the most common types of speleothems:

Photo by Dave Bunnell of some common speleothems. Taken from Wikipedia here. Click photo to enlarge.

Most speleothems form over thousands upon thousands of years. Thus, you shouldn’t touch or remove speleothems unless you’re doing so for legitimate science. Even when collecting speleothems for science, one should be conservative. Geologists should take small samples and obtain the necessary permissions. Fortunately, for my own research in Oman I am often able to collect speleothems which have fallen on the ground and are no longer growing.

At the top of this post is a picture of me with a stalagmite in Oman. I didn’t sample this one, but I did sample some of its neighbors. This particular stalagmite isn’t forming in a true cave but rather in an open hallow underneath a layer of rock. Water percolated through the layer of rock and formed speleothems in the hallow underneath. The speleothems I study in Oman are thus really pseudospeleothems– they are not in true caves but rather in little overhangs and hallows.

Now, for those of you who still confuse stalactite and stalagmite, here’s a reminder of something you probably learned in grammar school but may have forgotten by now:
StalaCtites hang tight (or tite) to the Ceiling while stalaGmites grow up from the Ground.

Finally, below are few more pictures of some Omani pseudospeleothems. These pseduospeleothems are forming in overhangs in travertines (carbonate precipitates) which are forming at the surface of the peridotite layer of the ophiolite. Be sure to click on the two panoramas to enlarge them. Note the location of the pseudospeleothem column in the two panoramas. Many pseudospeleothems are located in the overhang around this column. The last picture has my colleague Lisa standing next to this column for scale; this shows the enormous size of the travertine deposit.

Travertine pseudospeleothems, Oman, January 2009.

Water dripping from a pseduospeleothem straw, Oman, January 2009.

Panorama of Wadi Sudari Travertine I, Oman, January 2010.

Panorama of Wadi Sudari Travertine II, Oman, January 2010.

Lisa standing next to an enormous column of travertine, Oman, January 2009.

Geology Word of the Week: P is for Peridot

Peridot gemstone. Image taken from here.

def. Peridot:
Peridot is a gem-quality olivine [(Mg,Fe)₂SiO₄], a beautiful green mineral found in mafic to ultramafic rocks.

My engagement stone is a peridot– my fiance was pleasantly surprised that my favorite gemstone is among the cheaper gemstones. Though far less durable than diamond, peridot has a beautiful green color which I love.

Most gemstones have alter ego mineral names. Below are some examples:

Peridot- Olivine: (Mg,Fe)₂SiO₄
Ruby- Corundum (Red): Al₂O₃
Sapphire- Corundum (All other colors except red): Al₂O₃
Moonstone- usually Potassium Feldspar: KAl₂Si₃O₈
Tanzanite- Zoisite (Blue): Ca₂Al₃(SiO₄)(Si₂O₇)O(OH)
Amethyst- Quartz (Violet): SiO₂
Aquamarine- Beryl (Blue/Turquoise): Be₃Al₂(SiO₃)₆
Emerald- Beryl (Green): Be₃Al₂(SiO₃)₆

These are just a few of the many examples of gems with both gem names and mineral names. Note how some minerals have multiple gem names depending on their color. Makes learning geo lingo a little more difficult, doesn’t it?

To be fair, some of the gem names undoubtedly originated before the mineral types were discovered/invented. Also, while color is usually a poor way to identify a mineral, color is very important for gemstones. Thus, it makes sense that some minerals such as corundum and beryl (which come in many colors) have multiple gem names. Interestingly, diamonds are always diamonds– no matter the color.