#10 UHP Metamorphism, The Eiffel Tower, Queen Elizabeth’s Face, and Quartz Grenades

We’re all used to the concept of pressure, but in many different ways. Some think of peer pressure, the pressure of working life, or maybe of your ears popping on an aeroplane. For geologists however, geological pressure along with it’s siblings geological time and temperature describe conditions in the Earth which are unimaginable to most people.

An illustration of the components which determine pressure.

Pressure is a measure of force per unit area, and it’s quite easy to calculate given just a few variables (i.e. area, force, maybe acceleration too). Beyond the base of the Earth’s continental crust, these geological pressures reach 2,500,000,000 pascals or 2.5 GPa, and continue to rise the deeper you go. From this point on, a rock experiences Ultra High Pressure (UHP) and may be metamorphosed (rather than melted). It is, however, difficult to preserve rocks which have been down so incredibly deep as there are very few processes which take rocks down below the base of the crust and bring them back up to the surface quickly enough so that they preserve information on their burial history. Therefore although UHP conditions prevail throughout much of the Earth, not many rocks on the surface ‘remember’ if they were ever there. More than likely they weren’t.

The majority of tectonic activity on Earth results in rocks being sent deeper in the Earth and new rocks being created in igneous processes at or near the surface. Only special circumstances result in rocks re-surfacing after being very deep in the crust or mantle.

But just how much pressure is felt by rocks when they reach UHP conditions? Well yes we know it’s 2.5 GPa or above, but how can we relate to that? Well, to generate a UHP of 2.5 GPa concentrated on the golden face of the queen on a standard first class stamp, you would need to load that stamp with over 76 million kilograms. That’s equivalent to balancing 10 full-sized Eiffel towers on Lizzie’s Chops! Even that kind of pressure is hard to imagine, but suffice it to say you’d not want to trap your finger under that!

To generate 2.5 GPa on a postage stamp you’d need to load it with over 10 Eiffel Towers!

The photomicrograph shows a garnet crystal with inclusions of various minerals. The yellow dots show the inclusions which were once coesite, but are now quartz. The expansion f the coesite/quartz has created radial fractures in the surrounding garnet.

In my eclogites the mineral garnet commonly contains inclusions of quartz. When the garnet bearing rock experiences UHP conditions for long enough, the quartz changes its structure to a more dense form we call coesite. For the UHP rocks in my field area, these rocks inevitably start making their way to the surface. As the pressure starts to drop, the coesite inside the garnet starts to expand. Like the casing of an imminently exploding grenade, however, garnet resists the expansion. Eventually, however, the coesite reverts back to the quartz structure which takes up a larger volume. This shatters the garnet in the surrounding space just like the explosives in a grenade must shatter the casing to continue expanding.

So when I look at my rocks in thin section, I really like to think that I’m looking at tiny quartz grenades exploding on the face of the queen under the weight of 10 Eiffel towers!

#9 Thin Section Photomicrographs Using a Mobile Phone: Tips and Tricks

Like many of my colleagues, I have a lot of thin sections of many different rocks to look at. But sharing what you observe down the microscope is not as easy and convenient a process as it could, or indeed should be. When looking at sections, I make notes etc directly on my computer, and therefore I wanted the convenience of sitting at my computer rather than at a station in the microscope laboratory which is in a different part of the building.

Recently, the quality of the lenses and sensors on mobile phones has shot through the roof, and most modern smartphones now come with a pretty decent camera. Not to mention that on the same device you have the accessibility to many apps, including e-mail, via which you can share and send your images to other devices. Much easier than removing a memory card and inserting it into your laptop!

Hot To

Cover the other eyepiece to prevent entry of light from your surroundings. Then use both hands to offer the camera up to the eyepiece whilst sealing out as much light as you can.

1. The logistics of taking the photo with your camera.

Step 1
Locate the field of view you wish to photograph. Make sure that the image is in focus. You can of course take the image through either eyepiece, but I prefer using the one without the crosshairs. Make sure this individual eyepiece is in focus.

Step 2
Cover the other eyepiece to stop reflections. Light doesn’t only leave an eyepiece, it can also enter it. Light entering the open eyepiece will bounce around the optics and inevitably exit through the one that you want to take an image through causing nasty artefacts.

Step 3
If you are right handed, hold the phone in your right hand in portrait mode. It’s easier to handle this way, and the “take photo” is probably better accessed by your thumb/finger this way too. Using your left hand, shield the top and sides of the phone. This also allows you to stabilise and manoeuvre the phone into the desired position.

Step 4
Repositioning the camera takes a bit of practice but you’ll get the hang of it in no time. Firstly you want the lens vaguely in front of, and perpendicular to the image coming out of the eyepiece. You should see some sort of image appear on your screen. It will more than likely be overexposed and/or blurred. Once this image is central, using your left hand as a steady guide, adjust the spacing between your phone and the eyepiece. You are not focusing the image here, you are ensuring that the entire field of view is projected onto the camera lens equally and entirely. Too far away or too close and it will look like a small spotlight is illuminating part of the field of view, the rest of it is in darkness. As you move closer to the ideal distance, that spotlight will become large until “BAM!” suddenly the whole field of view is illuminated and you can take your picture once the camera has focused. This ideal distance is quite a narrow zone, so make sure you steady your hands and make gentle adjustments until the image is right.

The image shows what you expect to see at various distances from the eyepiece. You want to aim for full illumination which is in the middle of the range.

2. Some Common Issues
Dark crescents around the edge of the image – You are either too far away or too close to the eyepiece
Dark spots or areas inside otherwise well illuminated field of view – You phone is more than likely not perpendicular to the eyepiece, so adjust the attitude of the phone relative to the eyepiece.
Colour is off (too warm) – It’s likely the phone is reducing the exposure too much, so try reducing the diaphragm (or power of light) into the microscope. The former will increase the relief of the mineral boundaries making them more defined.
Camera won’t focus – Limitation of your camera phone and/or it’s camera software.

Reducing the diaphragm on the microscope gives better colour reproduction for some camera phones. It also increases the apparent relief of the minerals.

3. Things to do to increase quality
– Particularly when shooting in plain polarised or simple plain light, reducing the diaphragm aperture on the microscope will increase the image quality and colour reproduction.
– Make sure that you get a good light seal on both eyepieces.
– Make sure pin-sharp focus is achieved.
– Better microscopes = better images, better camera phones = better images.

4. Post-processing
If you want to really get the best quality image you can from your phone, I recommend that you take the following post-processing steps. I personally use photoshop, but most image editing packages will achieve the same results.

Step 1 – Adjust the levels. Using the histogram, move the left slider to either: A) to be under the peak of the blacks; or B) to exclude the black peak. Use whichever gives the best result for that image. Then adjust the righthand slider for whites to the left to achieve the desired brightness. You can also adjust the middle slider (contrast) to suit.

The levels for the image before they were adjusted.

Levels for the image after adjustment.

Comparison of the image before and after the levels were adjusted as described.

Step 2 – Apply Sharpening

This may or may not be necessary depending on your image. It’s always worth trying it to see. In photoshop, go to Filters > Sharpen > Unsharp Mask. I applied a strength of 46 % and a radius of 2.1 pixels. This will vary for each image. As a general rule I like to keep the strength slider higher than the radius slider to avoid black halos.

The unsharp mask in Photoshop is a very powerful tool.

It may not seem like your image needs sharpening, but after a unsharp mask is applied the results are stunning.

Step 3 (Optional)

If the colour balance is not completely right you can edit the colour balance manually. Go to Image > Adjustments > Colour Balance (or Cmd + B). This image, however, does not require it.

Final comparison of images. The only thing that has been done is a levels adjustment and sharpening applied.

5. Interconnectivity

Once great way of taking photomicrographs and keeping them together with any observations you may have is to use Evernote. Evernote is a great note taking application for your PC, Mac, Unix Machine, Mobile phone. iPad or tablet. Everything syncs wirelessly over the internet. I can write some notes on my computer about the slide I am looking at, and when I spot something interesting I can open up the Evernote app on my phone and take a photo which gets inserted into my notes. A few seconds later the photo automatically appears in the notes of my computer screen without the need for me to tell it to do anything! Truly a great tool for the geologist!

Evernote is a very useful tool for anyone who takes notes with any kind of media and/or the need to access the notes on a number of different devices.

#8 Britain’s Only Carbonatite

(A brief introfuction to the Loch Borralan Carbonatite)

Between the NE shore of Loch Urigill and Loch Borralan lies Britain’s only carbonatite pluton. The presence of a carbonatite in the Assynt area was first reported to the geological community by Young, Parsons and Threadgould in 1994 . The pluton was actually discovered by an undergraduate from the University of Aberdeen in the late 1980s who was mapping in the area. He puzzled over the rocks and eventually was bold enough to confront his supervisors about it. Sure enough, his speculation was soon corroborated and a fuller investigation was launched.

So what is known about the carbonatite? Well there has been relatively little work conducted on it since it was reported in the Journal of the Geological Society of London in 1994. It was reported that there are 4 varieties that occur:

1. porphyritic white sövite
2. phlogopite sövite
3. sövite breccia
4. foliated silicocarbonatite

The latter of three of the four types above were only observed in situ after part of the pluton was excavated. Since then mineralogical and whole rock analyses were conducted on the four different lithologies. Those chemical analyses revealed that the Loch Borralan carbonatite is chemically and isotopically (carbon) distinct from the surrounding Durness Dolomites in which the pluton is enveloped.

There are many unanswered questions about the pluton. Amazingly, there has been little interest in the body since the mid 1990s. I visited the carbonatite earlier this summer, and collected a sample of the porphyritic sövite. A thin section of this has revealed some interesting minerals which I am in the process of having analysed. Hopefully a report into further findings in the mineralogy of the body will follow some time next year.

A sample of Sövite from the Loch Borralan Carbonatite, showing well equilibrated calcites amongst some other as yet unidentified minerals.