GMPV
Geochemistry, Mineralogy, Petrology & Volcanology

Geochemistry, Mineralogy, Petrology & Volcanology

#mineralmonday: tiptopite

#mineralmonday: tiptopite

#mineralmonday: your weekly* dose of obscure mineralogy, every Monday** [*not guaranteed; **or possibly Tuesday-Sunday]

tiptopite. Source: wikimedia.org.

What is it? Tiptopite: K2Na1.5Ca0.5Li3Be6(PO4)6(OH)2•(H2O)

What’s it made of?  Take a deep breath and recite after me: potassium, sodium, calcium, lithium, beryllium, phosphorus, oxygen and water (H2O).

Is it pretty? Yes, it’s a beautiful fibrous mineral. You wouldn’t put it on jewelry though, or even in your pocket, or you’ll end up with needles and dust.

Wikipedia defines ‘tip top’ as ‘a slang phrase which means of the highest order or excellent’. Is this true for this mineral? Not really, it doesn’t have any real use (we are more interested in the minerals that it is associated with, like beryl), is only found in one place (the ‘Tip Top’ mine in South Dakota), and only in some isolated areas in that mine. This mine was opened during world war 2 for strategic reasons, but then shut down again in the 1950s.

Beryllium metal. Not exactly a silver lining, but a shiny grey lining… source: wikimedia.org

Strategic mining you say? Presumably this was for mining dragon glass? Or maybe I’m mixing up my wars. Yes you are thinking of an earlier war. Dragons and white walkers were long extinct by the 1950s. This particular mine was for beryllium (the Be in the formula).

That sounds berylliant. What’s it for? Nothing fun, unfortunately. Beryllium has a useful property of being a neutron emitter and an alpha particle absorber. This makes it really useful as part of the triggering device of thermonuclear weapons. It also makes beryllium quite difficult to buy…

Is there a silver lining? Nope! Beryllium even has a couple of conditions named after it: berylliosis and acute beryllium poisoning. Luckily it’s hardly ever used these days, and the risks are well known.

It seems calling it ‘tip top’ was a little optimistic…

Do you have a favourite obscure mineral? Want to write about it? Contact us and give it a go!

How do crystal aggregates form in magma chambers?

How do crystal aggregates form in magma chambers?

By Penny Wieser (PhD student at the University of Cambridge)

Clues into the inner workings of volcanoes can be gleamed from material which is erupted at the surface, or that which solidified at depth in the crust. Just before eruption, three main phases are present: a gas phase (containing water, carbon dioxide, sulphur, chlorine etc), a liquid melt phase (the magma), and a solid phase (consisting of lots of different kinds of crystals).

Olivine aggregate consisting of at least 17 separate crystals (taking with a normal camera). B-C) Backscatter electron images taken using an SEM showing attached olivine crystals (dark grey) surrounded by volcanic glass (solidified melt; lighter grey).

Petrologists spend their lives studying these crystals, and use them to determine how deep the magmas are stored in the crust, how long the magmas are stored, and how fast the magmas rise to the surface. However, many of the characteristics of erupted crystals are not fully understood, but may provide crucial insights into processes happening at depth within volcanoes. One example is individual crystals attaching together to form aggregates (also known as clusters, clots, or glomerocrysts). Despite these being really common in volcanoes, it’s still not exactly clear how they form. Previously it has been suggested that aggregates form during growth processes such as heterogeneous nucleation (where a new crystal nucleates on the surface of an existing crystal), dendritic growth (which occurs when crystals grow extremely fast, forming finger-like dendrites (this comes from the greek for ‘tree’)), or twinning (where two parts of a crystal grow together, with a specific orientation relative to one another). Or, crystals may “swim”  together as they settle through a liquid magma column (through a process known as synneusis), or land on top of one another after they settle to the solid base of the magma chamber. We wanted to have a look at these different possibilities in some more detail, we did this using a technique called electron backscatter diffraction (EBSD).

We took olivine crystals erupted at Kīlauea Volcano (Hawai’i), and chromite crystals from the Bushveld Complex of South Africa, and prepared flat, very well polished slices. These samples were then placed in the Scanning Electron Microscope (similar to a regular microscope, but with electrons instead of light). An electron beam was focused on the sample; electrons which enter the sample are diffracted by planes of atoms in the olivine and chromite crystals, and leave the sample as “backscattered electrons” (these are collected by an EBSD detector). Following a series of data processing steps, these measurements can be used to determine the orientation of the crystal lattice. This provides a quick, quantitative way to determine the relative orientations of neighbouring crystals within aggregates. These observations can then be compared to the orientations expected from the various different hypotheses.

EBSD-map, color coded according to the orientation of the separate crystals. The fact that all 6 attached crystals show similar colors indicates that their orientations are similar.

Crystals are lazy; they will grow in the way that uses the least energy. During crystal growth, any mismatches in the direction of the new part of the crystal lattice with respect to the existing lattice is energetically exhausting. So if a new crystal wishes to grow on the surface of an existing crystal, it should mimic the orientation of the existing crystal to conserve energy. Similarly, twinning only produces a finite number of possible geometries, called “twin laws” (1 in chromites, 3 in olivines). In contrast, physical aggregation processes can result in a much broader range of attachment geometries. The settling of crystals onto the floor of a magma chamber where they land on other crystals produces totally random aggregate geometries (analogous to a very bad tetris player). The swimming together of crystals in the liquid part of the magma chamber results in certain crystal faces sticking together (which can be predicted from observations of the way that crystals orientate themselves as they fall).

Olivine and chromite aggregates show too broad a range of attachment geometries to have formed during crystal growth. Instead, we observe that large numbers of olivine crystals are attached along the crystal faces predicted from settling. So, just before eruption, olivines in in the liquid part of the volcanic plumbing system at Kīlauea Volcano must “swim together” as they settle. Chromite aggregates from the Bushveld complex show completely random orientations. This is best explained by the settling of individual crystals onto a firm substrate at the base of the chamber, where they randomly land on other chromite crystals. The fact that crystals in liquid magma are present as aggregates in some volcanic systems (e.g. Kīlauea), but individual crystals in others (e.g. Bushveld) has implications for calculations of the rates of crystal settling. This, in turn, affects the viscosity of erupted lavas (as slower settling rates means more crystals, and “stickier” magmas), and is important to understand the formation of economic deposits of platinum group elements (which are concentrated within settled piles of crystals within the Bushveld Complex).

Want to know more? Read all about it here!

Penny Wieser is a PhD student in volcanology at the University of Cambridge, UK. She uses a variety of analytical techniques to interrogate erupted crystal cargoes and their melt inclusion records to gain insights into the pre-eruptive storage of magma beneath Kīlauea Volcano, Hawai’i.

#mineralmonday : sengierite

#mineralmonday : sengierite

#mineralmonday: your weekly* dose of obscure mineralogy, every Monday** [*not guaranteed; **or possibly Tuesday-Sunday]

What is it? Sengierite: Cu2(UO2)2V2O8.6(H2O)

What’s it made of? A few useful metals – copper (Cu), vanadium (V), uranium (U), plus oxygen (O) and water (H2O).

So by ‘useful’ you mean ‘radioactive’? Pretty much. The main reason people have been interested in this kind of mineral is the (very radioactive) uranium, which is important for nuclear power and military uses. The copper (not radioactive) is also useful though – this is a secondary mineral that is often found in mines where copper and uranium are extracted together.

Sengierite, from Leon Hupperichs via wikimedia.org

Secondary mineral? Like second class? That seems harsh. Basically, primary minerals are the ones that form when the rock is forming in the first place, so if the rock is crystallising from a liquid or magma, the minerals that form at that point are primary. If this rock then gets weathered, or maybe some water passes through it, new minerals can sometimes form – these are the secondary minerals.

I see, so it’s more like an caterpillar becoming a beautiful butterfly? Something like that. These secondary minerals are often hydrous, meaning they have water explicitly in their structure. That doesn’t mean the mineral is just wet, but rather there is a specific place for the water in the lattice of atoms. You can see this is the case for sengierite because of the 6(H2O) at the end of the formula.

So is it pretty? Absolutely. This one is a beautiful bottle green to apple green colour. It also has what we call a vitreous to adamantine lustre (lustre describes how light interacts with the mineral – vitreous is kind of like glass, adamantine is like a diamond). Probably best to avoid radioactive crystals for use in jewelery though…

Do you have a favourite obscure mineral? Want to write about it? Contact us and give it a go!

Are mantle melts heterogeneous on a centimeter scale?

Are mantle melts heterogeneous on a centimeter scale?

The mantle makes up the majority of the volume of the Earth, but there is still a lot about it that we don’t understand. This is because we can’t observe it directly – forget ‘Journey to the center of the Earth’ – even our deepest drill holes (about 12 km deep) are merely tickling the surface of the planet (about 6400 km to the center).

The journey to the centre of the Earth, by Édouard Riou (it’s not that easy). Source: wikimedia.org

Most of what we know about the mantle comes from secondary sources of information. For example, we can use magmas (liquid rock, which later become lavas when they erupt), which form when the mantle melts slightly. When these erupt onto the Earth’s surface, they bring with them some information about the rock (the mantle) from which they formed. This is just like how our DNA can tell us something (but not everything) about our parents. So by analysing the magma, we can start to piece together what the mantle was like where it formed. The problem here is that magmas are liquid, and can easily mix with other magmas, so the original information gets lost.

One thing we do know about the mantle, though, is it is not the same everywhere, it is heterogeneous. Some of the differences might be left over from four and a half billion years ago when the planet formed, others are due to rocks from the surface being pushed down into the Earth. The question is the scale of this heterogeneity –  is it heterogenous on the scale of hundreds of thousands of kilometers, kilometers, tens of meters, or even smaller?

The drill used to sample the rocks. Open your mouth and say ‘aaahh’. Credit: Sarah Lambart

A new perspective comes from a team from Cardiff University and the Vrije Universiteit of Amsterdam, led by Sarah Lambart (now at the University of Utah), and published in Nature Geoscience. The team took samples drilled from below the bottom of the North Atlantic ocean, and from these they extracted some minerals – clinopyroxene and plagioclase. These were selected as the most primitive minerals – that means the ones that formed first from a magma, and so they should most closely mirror the melt’s composition. From these millimeter sized crystals, they drilled out tiny samples of powder (the drill is a lot like a dentist’s drill, but with less fear and pain), which were then dissolved and analysed, to measure the concentration of some different isotopes (the same element, with different mass) of strontium and neodymium.

What they found was startling – these tiny samples were extremely variable in their isotopic compositions. The range was so big that it represented almost the whole variability of the North Atlantic – a whole ocean-sized range of chemistry in samples totalling about the size of a grain of rice. This implies that the mantle is able to preserve heterogeneous melts over a scale that is far smaller than previously thought. So, why do we not see this in the erupted lavas? Because they get mixed up, and the tiny variations are lost – this mixing has to happen somewhere between the site from where the crystals were sampled, and the sea floor.

So what’s next? Sarah says the answer might lie in experiments. By recreating the conditions of mantle melting, and simulating the movement of melt through solid rocks, we might be able to understand better how such tiny scale variations can be preserved in the mantle and then be completely lost by the time the melts erupt onto the ocean floor.