South West Minerals

Part 3 - Gemstones in the British Isles

So, having explored the formation and nature of diamonds (and other precious gems) and the mechanism for their transport and exposure, we now get down to the crux of this series; are there gems (and more specifically Diamonds) in the British Isles?

The British Geological Survey have done a fair amount of research in this area, so I apologise if section of this are (hopefully understandably) taken from their publications.

Scotland

 

The diagram above (from the BGS publication) shows that across Scotland there have indeed been a number of gemstone occurrences. These range from those that are definitely 'semi-precious' such as Cairngorm and Amethyst (both Quartz) through Tourmaline and Aquamarine and finally to the big hitters of Ruby, Sapphire (both forms of Corundum) and 2 reported Diamond discoveries. Just to add to this list, I personally have seen a specimen of what was believed to be Emerald collected from a Scottish Quarry, but which was definitely not gem quality

Diamonds

The only 2 reported diamond finds in the UK have both been in Scotland.

The Ben Hope Diamonds

The BGS report cites the eminent Scottish mineralogist, Prof M Heddle discovered a rock which he described as containing either '...colourless garnets or Diamonds..'

His son-in-law Alexander Thoms, stated in the posthumous published account that when Heddle gave the specimen to him, he indicated that he thought diamond the most likely identification. Given Heddle's qualities as a scientist and Chemist, this was definitely an intriguing observation.

The specimen itself disappeared not long after being first described in Heddle's book 'The Mineralogy of Scotland' in 1901, and so it was not possible to verify Heddle's claim. However, in 2003, purely by chance the specimen was re-discovered at the Hunterian Museum in Glasgow as part of the little known Alexander Thomas collection (it was in a batch destined to be thrown out). Of course its re-emergence allowed the museum's mineralogist Dr Faithfull to examine the specimen using modern scientific techniques, and the results were clear - there were no diamonds in the sample. As Heddle had said 100 years before, '...either colourless garnets or diamonds' these indeed were colourless garnets.

Although 3 further crystal have been reported near Ben Hope more recently (1958) these have not been verified, and it is probable that they are the same as Heddle's 'diamond'.

The New Cumnock Diamonds

In the 1890s there was a reported find of a crystalline material, hard enough to scratch quartz having been found near New Cumnock in southern Scotland. Mindat list the site as Cragmain, to the South West of the town. These crystals are accepted as diamonds, occurring as they do as minute crystals within a graphite seam, where coal has undergone thermal metamorphism.

This occurrence is clearly not a 'text book' occurrence, as there are no pipes or other volcanic mechanisms involved.

Ireland

Cambridge Mineral Resources have via surveys and analysis discovered Kimberlite indicator minerals in Ireland (Co. Fermanagh) near a stream where an alluvial diamond was discovered in the 18th century. Alluvial Sapphires were also discovered in Co. Donegal, as well as tiny rubies.

There have indeed been a number of areas where indicator minerals have been discovered in stream sediments, which will undoubtedly come under further scrutiny.

England & Wales

 

In the South West of England there have been a number of semi-precious stones discovered; Beryl, Cairngorm, Topaz, Amethyst and Tourmaline. As the map shows, none of the more precious stones, the gem quality forms of corundum (Sapphire and Ruby) and Diamond have been reported in the region.

The Beryl and minor Topaz occurrences within the Meldon Aplite, at the margin of the Dartmoor Granite.

Also associated with the granite emplacement the Cairngorm, Amethyst and gem quality Tourmaline have been found within the Dartmoor Granite, with other occurrences within the Bodmin Granite to the South.

Beyond the South West, there are very few precious or even semi-precious stones discovered within England and Wales, with the exceptions being the Blue John Flourite in Derbyshire and Jet near Whitby.

Summary

So, in summary, the known occurrences of gems and semi-precious stones are limited to Scotland and parts of the Northern half of Ireland, with nothing spectacular beyond that.

In the final part of this series, we will explore the new theories of gem emplacement, and how that may bring forth new areas for exloration; and most importantly how this may lead to the discovery of Diamonds in South West England.

References:

http://www.minersoc.org/pages/Archive-MM/Volume_50/50-357-481.pdf http://www.mindat.org/loc-3026.html

 

 

Part 2 - Diamond Creation and Exposure

This part is mainly about the conditions for Diamond formation, and rock types, but to be fair Pat has summed it up pretty well in this comment to my 1st part, see Here.

Pat's information is a direct quote from a Diamond exploration company, so it relies upon a certain level of existing geological knowledge and understanding, so (in as simple terms as possible) here is my understanding of the processes:

Diamond Creation

The formation of natural diamond requires very specific conditions. Diamond formation requires exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars and at a relatively low temperature range between approximately 700–1300 °C. Existing models suggest that these conditions are met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.

The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the aforementioned requirements of temperature and pressure. These depths are estimated to be in between 140–190 kilometres, though occasionally diamonds have crystallized at depths of 300–400 km as well.  Under oceanic plates the temperature rises more quickly with depth than beneath the continents, which puts it beyond the range required for diamond formation at the depth required.

 
The Diagram above shows the thin Lithosphere, straddling the (red) Mohorovičić discontinuity, which marks the crust / mantle boundary.

Therefore, the correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as cratons exist.

Surfacing

Rocks containing Diamonds are only brought to the surface rarely, and this is because these rocks only reach the surface as a by product of deep seated volcanic eruptions. As we saw earlier, diamonds only form at great depth, and only about a third of all volcanic eruptions start at such a depth. 

Where the rising magma reaches the surface, the volcanic craters extend downward in formations known as volcanic pipes, and these contain material that has been transported to the surface by the eruption, but was not ejected when the volcano stopped erupting.

The magma in volcanic pipes is usually one of two characteristic types, which cool into igneous rock known as either kimberlite or lamproite. The magma itself does not contain any diamonds; instead, it acts as an elevator that carries deep-formed rocks (xenoliths), minerals (xenocrysts), and fluids upward.

It is these particular igneous rocks that are used to identify the potential for the presence of diamonds, as both kimberlites and lamproites are characteristically rich in magnesium-bearing olivine, pyroxene, and amphibole minerals which are often altered to serpentine by heat and fluids during and after eruption. Certain indicator minerals typically occur within diamantiferous kimberlites and are used as mineralogic tracers by prospectors.

A Kimberlite Pipe (black lines added by me!)

These minerals are rich in chromium (Cr) or titanium (Ti), elements which impart bright colors to the minerals. The most common indicator minerals are chromian garnets (usually bright red Cr-pyrope, and occasionally green ugrandite-series garnets), eclogitic garnets, orange Ti-pyrope, red high-Cr spinels, dark chromite, bright green Cr-diopside, glassy green olivine, black picroilmenite, and magnetite.

Once diamonds have been transported to the surface by magma in a volcanic pipe, they may erode out and be distributed over a large area. A volcanic pipe containing diamonds is known as a primary source of diamonds. Secondary sources of diamonds include all areas where a significant number of diamonds, eroded out of their kimberlite or lamproite matrix, and accumulated because of water or wind action. These include alluvial deposits and deposits along existing and ancient shorelines, where loose diamonds tend to accumulate because of their approximate size and density. Diamonds have also rarely been found in deposits left behind by glaciers (notably in Wisconsin and Indiana in the USA); however, in contrast to alluvial deposits, glacial deposits are not known to be of significant concentration and are therefore not viable commercial sources of diamond.

Well, that's the background.  Next time I'll look at known occurrences of gemstones within the British Isles.

Thanks for reading on.

Kieron.

Part 1 - What are Diamonds, and where are they found?

This is a question I'm going to explore in a series of articles over the next few months. In Part 1, I'm looking at what Diamonds are and where are they found.

Mineralogically, diamond (from the ancient Greek adámas) is a form of carbon where the atoms are arranged in an isometric-hexoctahedral crystal lattice (as per the diagram below).

 
The arrangement of Carbon atoms in Diamond and Graphite

After graphite, diamond is the second most stable form of carbon. Its hardness and high dispersion of light make it useful for industrial applications and jewellery. It is the hardest known naturally occuring mineral, with a Mohs Scale value of 10    See here for details : http://www.amfed.org/t_mohs.htm


Diamond Occurrences

Diamonds have been found widely across the globe:

Asia
 It is certainly true that the earliest known deposits were in Asia, particularly those in Indian, which today are almost completely exhausted. Nearby are the deposits in Borneo, but the supply from this island, in comparison with the rich treasure of India, has always been limited.

America
In South America the famous Brazilian diamond-fields were discovered at the beginning of the eighteenth century, and have compensated for the exhaustion of the Indian mines; the mines in the States of Minas Gerais and Bahia have given the richest yield of stones. Finds have been made in another parts of the South American continent, including Guyana, Venezuela and elsewhere in Brazil. 

In North America, there are occurrences of diamonds, although commercially non-viable; a small number of stones having been found in the eastern States of Georgia, North Carolina, South Carolina, Kentucky, Virginia, Wisconsin, and in the western States of California and Oregon.

In Canada, the discovery of Kimberlite Pipes in the North West Territories in the early 1990 led to the establishment of the Diavik Diamond Mine, after which a number of smaller operations have also started up in the region.

Africa
The continent of Africa has been the major source of diamonds in the world for decades. The main deposits are near the town of Kimberley in South Africa (from where the usual host rock Kimberlite gets its name), and there is also a less important source in the adjoining Orange River region.

Other areas that produce diamonds include Botswana, Namibia, Angola and the Democratic Republic of Congo.

Despite the emergence of new Australian diamond fields, Africa is still the world's largest producer of diamonds (by value), producing as much as 50% of global production. To date, Africa has produced over 75%, in value, of the world's diamonds with more than 1.9 billion carats worth an estimated $US 158 billion mined.

 
An 'uncut' South African Diamond

Australia
Diamonds have been found historically in Australia, especially in New South Wales, but more recently there has been a significant discovery of diamonds in Western Australia. From isolated discoveries in stream sediments in the 1800s, following a systematic search for the source in the early 1980s, the Argyle Diamond Pipe was discovered. 

From that point, in terms of volume, the Argyle Diamond Field now out produces any other source of diamonds in the world.  One important fact that sets this diamond pipe aside from almost all others is that the diamonds are found in a type of volcanic rock called Lamproite, as opposed to the usual Kimberlite, which is something we will explore later.

Europe
In Europe, diamonds have been found in the Urals and more recently Siberia in the east, and in Lapland in the west. In Lapland the stones are 'few and far between', whereas the occurrence in Russia is of major economical importance.

Extra-terrestrial
The final known source of diamonds is in meteorites, and impact craters do contain diamonds, both from the meteorites themselves and also created by the heat and pressure exerted by the impact.

Antarctica
It is speculated that there is a large source of diamonds beneath the ice cap in Antarctica (and recent new models for diamond implacement support this), but of course current restrictions on mining on this continent precludes any exploration, let alone commerical exploitation. This could change in years to come.

So, how are Diamonds formed?


As mentioned above, in the search for new sources of gem stones (particularly diamonds) across the world, much analysis and modelling has been performed on the mechanisms for creation and transportation of diamonds, and that is what I'm going to explore in the next article.

A Beautifully cut diamond

Links: 

Argyle Diamonds of Australia: 
http://www.costellos.com.au/diamonds/industry.html
Russian Diamonds: 
http://www.khulsey.com/jewelry/kh_jewelry_diamond_mines_russia.html
Canadian Diamond Exploration: 
http://gsc.nrcan.gc.ca/diamonds/index_e.php
Overview of African Diamond Mining: 
http://www.mbendi.com/indy/ming/dmnd/af/p0005.htm