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.

A crystal sandwich – Vermiculite

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?

Vermiculite is a layered silicate (also known as a phyllosilicates) mineral.  It is very similar to Kaolinite in that respect by differs in the fact that it has an extra silicate layer (the blue tetrahedrons) that sandwiches the layer of magnesium (in this case) and oxygen (which are you can see as orange tetrahedrons).  These layers themselves then sandwich other elements (often water molecules with some small metals) that can sit in-between.  This layered structure means that the structure of a vermiculite and swell up to take up elements from its surroundings, but can shrink at high temperatures when the water is expelled from the sandwich.

Where did the structure come from?

There’s a lot of variation in the elements found in the vermiculite structure.  This particular example is magnesium vermiculite, and is #9000146 in the open crystallography database.

Surreal Microscopic Environments – Negative Crystals

A few months ago a story did the rounds about a “negative crystal floating in space” [1] which was accompanied by a rather spectacular image (Image 1).

Image 1: Negative crystal in spinel. Image by Danny Sanchez

This picture was described as a negative spinel and was one of the most perfectly formed crystal inclusions found and photographed by Danny Sanchez using photomicrography techniques [2]. This unique photography technique allows small regions within crystals to be photographed with exceptional clarity, creating extraordinarily surreal images of crystal inclusions. Negative crystals aren’t the only type of crystal inclusion however, and they certainly aren’t the only ones to produce stunning images.

Crystal Inclusions

Generally speaking, negative crystals are a specific kind of crystal inclusion. Inclusions are when a foreign material is trapped within a host crystal. The foreign substance can be solid, liquid or gaseous and can become trapped within the crystal either during its growth or once the host crystal has formed. The exact method of inclusion formation depends on the type of inclusion and the conditions under which the crystal was grown [3].

Solid inclusions

The most well known solid inclusions are ancient insects or plant life suspended in amber. However, solid inclusions can also include different types of gemstones embedded inside another crystal, typically quartz (Image 2).

Image 2: Rutile on hematite in quartz. Image by Danny Sanchez

These mineral inclusions either form simultaneously with the host crystal or were pre-existing and the host crystal has grown around it, encasing the foreign material. In the case of quartz which usually forms in hydrothermal conditions (basically a hot water soup of dissolved minerals) the included gem existed in the solution prior to the growth of the quartz and over time the quartz grows around it (Image 3).

Image 3: Purple and blue fluorite in quartz. Image by Danny Sanchez

Alternatively, as many crystal growth conditions are under high temperature and pressure conditions and in aqueous media, liquid inclusions highly saturated with dissolved minerals can form. As the crystal cools these minerals begin to precipitate forming a second crystal inside the host.

Liquid inclusions

As mentioned, many famous crystals (such as the Naica crystal caverns) were formed in hot, mineral-rich water solutions. As a result, sometimes this solution can become trapped inside the crystal during growth. A rather unique liquid inclusion which occurs under pressurised conditions is liquid CO2, which remains trapped in liquid form due to the pressure maintained inside the inclusion.

Quite often liquid inclusions come paired with a bubble of gas, such as in Image 4 in which liquid petroleum and a bubble of methane are trapped in quartz.

Image 4: Petroleum and methane bubble in quartz. Image by Danny Sanchez

Gaseous inclusions

Finally, “negative crystals” form when a pocket of air is trapped within a crystal. These gaseous inclusions can be a specific gas, depending on the conditions the crystal grew in, or just plain old air. These inclusions can form either due to crystal growth which occurs in multiple directions that then intersect or because crystal growth in a particular direction is inhibited temporarily. Since gaseous inclusions are enclosed by crystal faces the shape will reflect the crystal habit (or defined external shape of the crystal) of the host and they are always oriented parallel to the host crystal (Image 5). While they may look like a typical gemstone, it is actually an inversion with air “inside” the crystal boundaries and crystal material on the “outside”. Hence these inclusions are named “negative crystals”.

Image 5: Negative crystals in amethyst. Image by Danny Sanchez

A special aspect of any type of inclusion is that the foreign material is suspended in time. In this way, the inclusion contains a “piece of the past” which provides insight into what the earth and the environment were like when the crystal grew, often millions of years ago. For example, the discovery from air inclusions in amber that the oxygen content in air reached 35% during the Cretaceous period, before suddenly dropping to near the current level of ~21% [4].

There are many other forms of inclusions in addition to those shown. Quite often they can produce stunning visual effects and increase the value of the gemstone. For example, star sapphires which produce a unique six-rayed star effect under certain light sources contain tiny inclusions of aligned needle shaped rutile (Image 6).

Image 6: Star sapphire under a direct light source. Image from http://www.gemselect.com/.

Ordinarily, most inclusions are microscopic in size and can only be properly visualised under a microscope. For the majority of images featured here highly specialised and expensive equipment is required.

More images of crystal inclusions can be found in Gübelin and Koivula’s Photoatlas of inclusions in gemstones [5] or at http://www.dannyjsanchez.com/.

[1] Hooper, R.; New Scientist 2014, 2975, 24.
[2] The Art of Photomicrography: Gemstone Inclusions by Danny Sanchez <http://www.dannyjsanchez.com/&gt; (Accessed 11/14)
[3] Benz, K. W.; Neumann, W. Introduction to Crystal Growth and Characterization; Wiley, 2014
[4] Kump, L.R.; Kasting, J.F.; Robinson, J.M. Global and Planetary Change 1991, 5, 1.
[5] Gübelin, E.J.; Koivula, J.I. Photoatlas of Inclusions in Gemstones; Vol 1-3, Opinio Publishers

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.