The transition from the quiet creaking as my thumbs press slowly against the cork easing it out of the bottle neck, to the cork flying across the room to hit the ceiling is almost instantaneous and explosive. After this ritual build up of excitement I can raise a glass to you all and sip the bottled jollity as the bursting bubbles of CO₂ tickle my nose. This seems like a good place to start a blog for Christmas.

The Whin Sill was once a liquid too. Not chilled but hot, very hot. At over a 1000^oC and with over 215km³ of liquid there would have been enough heat to cook all of the UK’s turkey, duck and goose  dinner’s and nut roasts too, and still have a great deal left to spare. All of this liquid cooled down to make the dense crystaline rock, whose obdurate resistance to the ice of repeated glaciations, made the iconic crags of the central section of Hadrian’s Wall. Pagioclase feldspar, pyroxene, iron-titanium oxide and interstitial alkali-feldspar and quartz interlock to make the rock we see now. There is however just a little bit more to the Whin magma (as well as many other igneous liquids) than the elements that make these strongly interlaced crystals.

Out on the rubbly top surface of the Whin Sill at Harkess Rocks, with a beautful view across the bay to Bamburgh Castle, a rare feature can be found.  It even gets a mention in the Geologcial Conservation Review which documents the most special geological features to be found in the UK. Set in shallow holes within the Whin Sill are small pockets up to 0.5m across of what look like ropy lava (or pahoehoe lava as the Hawaiians and geologists would have it).  These pockets are formed from volatiles coming out of solution in the magma. The ropy lava forms inside the gas pocket (or vesicle) where the magma is chilled sufficiently to form a viscous toffee-like skin which is wrinkled by the still molten magma just beneath, flowing and distorting the skin above. This extraordinary phenomena is not only rare, but gives an insight into the direction of flow of the magma and how it was intruded. Gas only comes out of solution when the confining pressure drops. This equivalent of the cork being pulled only happens near to the surface otherwise the confining pressure from the weight of rocks above keeps the volatiles in solution. This volatile release is also rapid and can be explosive. Some of the most dangerous volcanoes in the world have their extreme explosivity caused by the high volatile content of their magmas.    

In the case of the Whin Sill degassing is gentler. There is no evidence for the magma reaching the surface and erupting, suggesting that the pressure relase mechanism is underground fracturing as the sill pushes into and interleaves the Carboniferous sedimentary rocks it is intruding.

Whilst this is all very fascinating, this was not the main purpose of this blog, I was heading for something rather more visually appealing, with the pizazz needed for the winter solstice. The vesicles do however provide the starting point for this. Nature abhors a vacuum, and when there is heat and water it can do something stylish with holes in igneous rocks. In the Whin Sill we can see the beginnings of this, with some of the vesicles, partially filled with crystline calcite and quartz in cream and white colours. Where the residual heat of cooling magma meets ground water, the water is convected through the rock. Hot water is surprisingly good at dissolving things so that this convection cell offers a great way to dissolve and then precipitate minerals. The vesicles provide handy locations in which these precipitating minerals can make a home. This mechanism may seem prosaic, but the products are works of art.

Silica in various forms is the principle medium for this art work. Silica in its simplest form, silicon dioxide, is not only the most prevelant but is also brilliant in it’s achievements in the form of agates. These semi-precious gemstones are my favourite for the fabulous range of patterns and colours they own, and their durability.

Their formation is enigmatic. The components of agate are pure silica (quartz) and chalcedony which is a mixture of quartz and moganite its polymorph, which has the same chemistry but a different crystal structure. Agates are composed of alternating bands of chalcedony and pure quartz, with colours added by trace elements (iron, manganese, copper and others). The crystals form as fine radial fibres, but exactly why they are able to form such fine bands with such intense changes in colour is not clear. 

It may be the Liesegang depletion-diffusion phenomena described in the blog “Balls and Bands” may be at play. The depletion-diffusion controls the precipitation of repeated quartz then chalcedony into fine bands, and in turn controls the way that trace elements are incorporated. Regardless the results are beautiful.

Sometimes the agates will finish with an additional flourish, as the centre of the agate is filled with crystaline quartz. Occasionally these are hollow so that the interior of the agate is lined with quartz crystals, bringing some extra sparkle to the ensemble.

Quartz and chalcedony are not the only artists in the silica movement. There is another group of silicates called zeolites which are micro-porous, and commonly used as catalysts and absorbants. Zeolites are another mineral that likes to make its home in vesicles. When I was working on the Isle of Mull, some of the vesicles in the Palaeogene lava flow were filled with different types of zeolite. These were often in the form of very fine needles of Natrolite one of the dozens of different types of zeolite. The pictures for this section of this blog have been generously sent to me by Bernardo Cesare, a legend in his own field for beautiful art images made using a polarising microscope. These images show basalts which have vesicles in them, and the vesicles in turn contain zeolites. Most of these basalts are from the Val Lagarina at the northern tip of Lake Garda and are of Middle Eocene age. The last of the four images is from the Predazzo area of the Dolomites and is Triassic in age. When viewed in thin section with a microscope using cross-polarised light the difference in light paths through the mineral structure (anisotropy) generates these fabulous colours. This allows for mineral types to be identified, as well as showing up the amazing textures the minerals form and highlighting the exotic beauty of these rocks. In these pictures you can see the circular zeolite filled vesicles surrounded by the basalt. Most of them show radiating needle like textures as the zeolites have grown into the vesicle, with the 4^th image showing agate like banding. In the 3^rd image and in the core of the 4^th image the crystals form a patchwork suggesting that crystalisation has been initiated across the whole vesicle and not just at its edge. In the basalt of the 4^th image, the dark lined round crystals of olivine and rectilnear laths of plagioclase set in the groundmass of finer crystals add to this fabulous display.

There is so much more to discover here and I have provided some links below to do just that. However, that glass of bubbly is waiting. Seasons greeting to you all, and I look forward to seeing you out along the Wall in the spring time.

Cheers! @Northumbrianman

Resources and attributions

Bernardo Cesare

Some of the images in this blog come from this exploration of the microscopic world of the Dolomites. The Invisible Dolomites:

Professor Cesare has two other websites one on his microscope work and the other where you can buy his art work:

…you can also follow him on Twitter @micROCKScopica 

Agate Resources

Books. There are many out there, but this one published by the Natural History Museum is a beautifully presented introduction. The second book looks like an update and I have a copy in the post. Both books available on Abe Books at a good price. 

Agates. MacPherson, H. G. Published by Natural History Museum Publications (1992)

Agates: Treasures of the Earth. John Cromartie, Peter Tandy, Brian Jackson, Roger Pabian. Published by Natural History Museum Publications (2006)

Online. Again there is much out there.

An interesting article in Geology In about the Types of Agates with good images:

A blog post on the National Museum of Scotland website, “What are Agates” – they also have a fantastic collection.

These are commercial organisations selling agates:

Agate Lady.  Agates Anonymous

Attributions

All images by the author other than:

Figure 4:

1. Agate Lady
2. Photo by Avegaon on Flickr and in Types of Agates article in Geology In
3. Recepimal108 IG in Amazing Geology Facebook group

Figure 5:

1. Crazy lace agate: By zygzee
2. Agate Lady

Figure 7: Natrolite: Nasik District, Maharashtra, India Size : (11x9x7cm) By Didier Descouens – Own work, CC BY-SA 4.0, 

The post The Silica Painter appeared first on WallCAP.