Decorative, but a little deadly – Torbernite

 What does it look like?

The crystal structure of Torbernite. Here the atom colours are; blue – uranium, orange – copper, purple – phosphorus, red – oxygen. Image generated by Mercury.

What is it?

Torbernite crystals exhibit exceptionally beautiful shades of green, from emerald to grass-green to apple-green, and thus may entice you to collect these crystals as ornaments for your tables – but beware; these crystals are capable of slowly leaking lethal radon gas which can cause lung cancer.

Image of a collection of torbernite crystals. Taken from: http://www.gemstonesadvisor.com/torbernite/

Torbernite crystals, Cu(UO₂)₂(PO₄)₂)·12H₂O, are formed through a complex reaction of phosphorus, copper, water and uranium and form as secondary uranium deposits in granitic rocks. These materials belong to the autunite group and are found in the alteration zone of hydrothermal veins and pegamites that contain uraninite. Torbernite materials possess a significant environmental interest in that they exert an impact on the mobility of uranium in phosphate bearing systems such as uranium deposits and so can act as a reactive barrier that uses phosphate to limit the transport of uranium in groundwater. As such, the presence of torbernite has been used by prospectors as an indicator of uranium deposits.

Where did the structure come from?

Torbernite occurs in tabular blocks that may be very thin to moderately thick. The crystals have a perfect cleavage parallel to the basal plane and thus can resemble mica. This particular structure of torbernite that we have featured was presented in Locock, A.J. and Burns, B.C. The Canadian Mineralogist, 2003, vol. 41, pp. 489 – 502.

It’s making a list – Santite

What does it look like?

Purple atoms (potassium) interwoven with borate (green boron and red oxygen) chains. The empty hydrate, ice VXI. Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

*sings*

‘and checking it twice, gonna find out who’s nautghy or nice….’

SANTITE is on Crystallography 365!

What is it?

It’s a little bit of a stretch, we know (go on, you go and look for more Christmas-inspired minerals!) but here is Santite! This mineral was first identified from synthetic means, it’s crystal structure found in 1937. Then in 1970 a group of Italian scientists identified this material in the hills of Tuscany (hard field work, eh?). They named it after a former director of the Museum of Natural History of Italy, George Santi.

It’s a very rare borate mineral, where the boron hooks up to oxygen and (rather pleasingly) forms paper chain like features running thought the structure – interwoven with potassium ions.

Where did the structure come from?

Santite, or its synthetic name potassium acid dihydronium pentaborate #9011411 in the Crystallographic Open Database, and was found by Zachariasen in 1937.

A rock with a cleavage – Augite

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Augite is a very dark green to black colourer mineral.  It is made up of silicate chains with principally magnesium, iron, calcium and aluminium atoms sitting between them.  There’s quite a variation in which of these elements are found in the structure, but the chains mark the structures as belonging to the pyroxene family of minerals – like Jadeite.    One of the striking features of this mineral is its, ahem, cleavage.  Many minerals will split open along particular directions, this is usually from a weakness in their crystal structures.  Augite has two prominent cleavages, which meet at about 90 degrees.

Where did the structure come from?

The crystal structure of wurtzite is #1200006 in the open crystallography database.

The final birthstone of the year: Tourmaline

While many of the other birthstones we have investigated this year have had a definite colour to them – sapphire rather than ruby for example, this month’s stone has something for everyone.

What does it look like?

Here is tourmaline. This picture was produced using the Diamond Crystal Structure Visualisation Package. Mg is red, Na is maroon, Al is green, Si is blue, O is grey and B orange.

What is it?

Tourmaline is a trigonal cyclo-silicate which typically forms large prismatic crystals. It has the formula (Ca,K,Na,)(Al,Fe,Li,Mg,Mn)₃(Al,Cr,Fe,V)₆(BO₃)₃(Si,Al,B)₆O₁₈(OH,F)₄, which is a right mouthful! The structure has 6-membered rings of corner sharing Si and Al tetrahedra as well as BO₃ tetrahedra. In the gaps between these large cations of Ca, K or Na can be found.

The brackets around some of the elements in the formula tell us that these elements are interchangeable on those sites. So clearly, there is a lot of scope for different compositions in tourmalines. This variation is what leads to the large variety of colours that tourmalines can come in. Tourmalines are some of the most chemically diverse minerals in the world.

Tourmalines are usually found in a range of igneous and metamorphic rocks, most commonly in granites and granite pegmatites, (just like last month’s birthstone). Mg –rich tourmalines tend to be restricted to schists and marbles.

There are three main types of tourmaline by composition:

1. Schorl

Schorl is the most common type of tourmaline. They were named for the town in Germany where they were first found in large quantities. They are typically black to dark blue to brown in colour. Figure 2 shows a couple of schorl tourmalines in a large book of mica. Schorls are sodium – iron tourmalines.

Figure 2. Black Schorl tourmaline crystal.

 

2. Dravites

Dravites are brown tourmalines. They are magnesium and sodium rich and typically vary from dark yellow – blue in colour but can also form deep green chromium and vanadium tourmaline varieties.

Figure 3. from Wikipedia: By Vassil (Own work) [Public domain], via Wikimedia Commons

 

3. Elbaite

Elbaites are lithium containing and show the most variation in colour. Elbaite was one of the minerals from which lithium was first discovered. They are named for the Italian island, Elba, where they abound. The most common forms are: Red – Rubellite, Light blue – green – Indiocolite, Green – Verdelite and colourless – Achroite. It is common to find variations in composition in zones within individual crystals. This leads to one of the most well-known tourmaline phenomena – watermelon tourmalines – see figure 4.

Figure 4. Watermelon tourmaline. Image from: http://en.wikipedia.org/wiki/File:Watermelon_Tourmaline.JPG

With so much variation in colour, there really is something for everyone if you are born in December!

Where did the structure come from?

This structure came from :

Hamburger G E and Buerger M J (1948) The structure of tourmaline. American Mineralogist 33 532-540. It is available on the American Mineralogist Crystal Structure Database.

Mineral in pink – Spherocobaltite

What does it look like?

Two views of the Spherocobaltite structure, blue atoms are cobalt, red oxygen and brown are carbon.

What is it?

When you think of the word ‘mineral’ and then imagine the types of colours that associate with that – you’ve probably got ‘grey’ or ‘rock-coloured’ in your head first?  I hope that on this blog so far we’ve managed to show that minerals can really come in all colours – through the ‘magic’ of chemistry.

“Spherocobaltite-260478” by Rob Lavinsky / iRocks.com – http://www.mindat.org/photo-260478.html. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Today’s is pink!  Spherocobaltite, is colbate carbonate (or CoCO3), and it’s the cobalt that gives this mineral it’s lovely pink hue.  It’s a hydrothermal mineral, forming originally from a hot soup of elements.  It’s usually found in veins, where the hot fluid has flowed through crack in the rock.

It you think you seen this all before, then you’ve obviously been pretty keen on our blog! The structure of spherocobaltite is the same as calcite (CaCO3), only with cobalt atoms instead of calcium ones.

Where did the structure come from?

The structure of Spherocobaltite that we’ve featured was determined by Graf in 1961 and was published in the journal American Mineralogist. It’s #9000101 in the Crystallography Open Database.

A greenstone – Jadeite

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Jadeite is from a family of silicate minerals, called pyroxenes, which are distinctive for their single chains of silicate tetrahedra (the blue shapes with red oxygens in the corners).  This particular mineral has sodium (gold) and aluminium atoms (light blue) between the layers.  Pyroxene minerals are an important part of the Earth’s crust and mantle, and are found in many igneous and metamorphic rocks.

Gem quality jadeite is only one of the two materials that are known as Jade, the other being nephrite. 

Where did the structure come from?

Jadeite is structure #9000143 in the open crystallography database.

November’s birthstone – Orange Topaz

Helen Brand gives us the low-down on the penultimate of the year’s birthstones.

What does it look like?

The topaz structure. Image created using diamond crystal structure visualisation package. Al is grey, Si is green, O white and F pink.

What is it?

Topaz is another silicate mineral, this time containing aluminium and fluorine. The formula is Al₂SiO₄(F,OH)₂. It is usually colourless and can become tinted by impurities. It is the orange topaz which is traditionally known as the birthstone of November. It has an orthorhombic structure made up of corner-sharing aluminium octahedra and silicate tetrahedra.

Figure 2. A gem quality Orange Topaz. Image from http://www.minerals.net/

Talking about topaz gives me an opportunity to talk a little bit about one of my favourite rocks: Pegmatites. Pegmatites are intrusive igneous rocks which are composed of crystals which are typically > 2.5 cm in size and this is usually where topaz is found. To be classed as a pegmatite, a rock must be all crystalline with almost all crystals >1 cm in size. There is no typical composition for a pegmatite. The large crystal size is the most striking feature of a pegmatite, with individual crystals reaching > 10 cm in size. Some of the largest single crystals in the world (not counting those mega-cryst caves), are found in pegmatites. Most pegmatites are composed of quartz, feldspar and mica, plus a few other minerals and have a similar composition to granite.

An excellent place to find pegmatitic rocks is in Cornwall in South West England. Cornwall has a strikingly different geology to the rest of the UK. Cornwall is underlain by a large batholith – an intrusive body of granite. In various places, this granite is exposed at the surface. Figure 3 was taken at Rinsey Cove in Cornwall. It shows a pegmatitic dyke surrounded by granite.
While I was unable to find any literature to say that there have been topaz crystals found at Rinsey cove, topaz was found about a mile along the cliffs at Megiliggar rocks, where a slightly different part of the complex is exposed.

Figure 3. A pegmatitic vein from Rinsey cove, Cornwall.

These granites were intruded approximately 300 – 275 million years ago as the northern boundary of a mountain building event called the Variscan-Hercynian orogeny (orogeny just means mountain building event) which occurred when the ancient continents of Euramerica and Gondwana collided to form a super-continent – Pangaea.

The granites were molten when they were emplaced and then subsequently crystallised. The hot magma rose upwards and moved through weaknesses in the country rock. As it did this it changed (– metamorphosed), and consumed, the country rock surrounding it.
The granites in Cornwall have shaped the economy of the area. They have provided resources which have been exploited by the inhabitants for years. Hydrothermal fluids concentrate precious metals such as tin and copper, they carry the ions in solution and deposit them when new minerals crystallise. Cornwall is littered with mines which extracted these precious metals and also famous for wide occurrence of tourmaline minerals, the birthstones for December which I’ll tell you about next month!

Where did the structure come from?

Diego Gatta G, Nestola F, Bromiley G D, Loose A (2006) New insight into crystal chemistry of topaz: a multi-methodological study. American Mineralogist 91 1839-1846.

It is available on the American Mineralogist Crystal Structure Database.

Sign of an impact – Stishovite

What does it look like?

The crystal structure of stishovite, image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/.

What is it?

As we’re coming up to the home straight of the Crystallography 365 project, crystal structures keep popping up that have us questioning ‘how have we not done that one yet!’.  Today’s material, stishovite, is a further polymorph of silica but one that’s extremely dense.  In fact, so dense it was quickly realised that no ‘Earthly’ process could have formed it.

It was first made in the laboratory in 1961, within a high pressure press, by a Russian scientist called Sergey Stishov.  The pressures he used to form this are so great, that the silicon atoms are forced to bond with six oxygen atoms, whereas normally they are content to be associated with four.  It’s actually the same structure as the rutile family of compounds.

A year later a geologist, Edward Chou, discovered the very same material in the bottom of Meteor Crater, Arizona, US, making it a mineral which he named after Stishov.  Finding stishovite in the field has now become a way of identifying sites of impact craters, as well as revealing when a ‘rock’ is, in fact, a meteorite.

Where did the structure come from?

The structure we’ve pictures comes from the determination from a piece of stishovite found in meteor crater, US.  It’s #9007530 in the Crystallography Open Database.

What are comets made out of? One potential ingredient, Melilite

What does it look like?

The crystal structure of melilite, image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/. Blue and red silicate units are interspersed with atoms of magnesium, calcium, potassium, aluminum and sodium,

What is it?

One of the objectives of the Rosetta space mission, which has had us all enthralled over the last week, is to work out what comet 67 P is made out of.  The  idea is that armed with this information, we can then work out where in space the comet actually came from – giving us clues as to the origin of the solar system.  To do this Rosetta carried a number of instruments, all of which were tested with minerals that could be making up the comet before it launched.  

Along with primary minerals, such as Fosterite, and alteration minerals such as Talc, one mineral that was tested on Rosetta’s instruments was Melilite.  Melilite is actually a family of minerals, like feldspar, and can have a range of chemical compositions.  It’s characterised by isolated silicate units, with many other elements (on Earth these are usually calcium, potassium, sodium, aluminium and magnesium).  In fact it is the magnesium in melilite which is very important – as isotopes of this element found in meteorites can be used to date processes back to the earliest point in our solar system  

Where did the structure come from?

The structure of melilite we’ve picture comes from work by Smith in 1953, and the structure parameters can be found in the American Mineral Database.

A mineral imposter – pseudomalachite

What does it look like?

The structure of psuedomalachite, the blue atoms are copper, red oxygen. In contrast to malachite, there’s no carbon in this structure – instead phosphorus (lilac). Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Side by side, it’s hard to tell apart Malachite and it’s rarer ‘imposter’ mineral pseudomalchite – both are green, form a crystal that looks a bit waxy and grow in a rounded (or botryoidal) form.  But diffract with an x-ray beam and the differences really reveal themselves.

Rather than an copper carbonate hydroxide, pseudomalachite is a copper phosphate hydroxide.  And it isn’t that the phosphate atoms take the place of the carbon atoms – it’s a whole different crystal structure.  Rather than forming the sheets of copper, oxygen and carbon atoms that malachite does – pseudomalachile forms more of a framework structure, as a result it is a harder mineral.

The way to tell them apart in the field is to drip a couple of drops of warm hydrochloric acid onto each rock – malachite will react, but psuedomalachite won’t.

Where did the structure come from?

The crystal structure of pseudomalachite was found by Shoemaker et al, and the crystallographic information file for it can be downloaded from the American mineral database.