INSIDE GeoLaB - Examined closely!

Dr. Fiorenza Deon from the GFZ holds a PhD in Natural Sciences, having specialised in mineralogy and experimental petrology. This means she analyses minerals and rocks at a scientific level. Drawing on her expertise, Fiorenza has travelled the world working on geothermal projects. For GeoLaB, she is examining sections of the drill cores from the Tromm exploration phase – using powder and thin sections. 

Fiorenza, have you hit a rock?

Something like this: In our GFZ workshop, sections are sawn off the 500-metre-long Tromm drill core. I break these up with a hammer, grind them, and then use a mortar to reduce them to a fine powder. I then analyse this in the laboratory using an X-ray diffractometer. This instrument allows us to determine the mineralogical composition of rocks – for example, how much quartz, feldspar or mica is contained in the drill core. So far, I have worked with a total of 36 samples, which mainly contain granite. In the deeper layers, from 410 metres downwards, metamorphic rock such as gneiss is also found.

But the chemistry has to be right, too?

I also want to determine the chemical composition of the granite precisely. To do this, I prepare a thin section from a piece of the drill core. It is wafer-thin, 20 to 30 micrometres thick – about the same as a human hair. I examine this sample using an electron probe microanalyser. It measures the elemental content with pinpoint accuracy, thereby enabling us to determine the mineral chemistry. This allows us to determine the composition of the minerals and to understand chemical changes and, for example, weathering processes.

Why are these studies so important for GeoLaB?

Let’s take the quartz content in Tromm granite, for example. Generally speaking, the quartz content averages between 25 and 35 per cent – which makes the granite particularly hard and durable. In Tromm granite and in the drill cores, I have so far found a quartz content of around 20 to 30 per cent, depending on the depth.  This is crucial information for the planned construction of the underground laboratory. This is because the drilling engineers need the most accurate information in advance. For example, they need to know which drill bit to use or how often it needs to be changed. The exact composition of the rock is crucial for this. This is because the equipment must be used as efficiently as possible to save time and money. 
 

What else are you examining closely?

The mica content is also particularly important for GeoLaB. Mica consists of thin, leaf-like platelets, and this is relevant to our planned geomechanical investigations. The mica content influences the rock’s shear strength and cohesion. Shear strength refers to the rock’s resistance to displacement or failure. Cohesion is a kind of internal adhesive – it holds the rock together even without external pressure. We have found metamorphic rock, such as gneiss, on the Tromm – it starts at a depth of 410 metres. It is important to determine exactly at what depth it occurs. Put simply, granite is more stable than gneiss – and this information is crucial for the construction of an underground laboratory.

What is weathering all about?

White veins can be seen in the fissures of the Tromm granite. These are made of calcite, i.e. calcium carbonate. In everyday life, we know it as lime or chalk. The calcite is thought to have been formed by water. Water plays an important role in the chemical weathering of rocks. It transports dissolved ions such as calcium, sodium or potassium, which have been leached out of the minerals. Elsewhere, these are deposited as new minerals. However, without further investigation, it is not possible to determine when the calcite deposits in the Tromm granite formed – we can only establish that such processes have taken place. Other minerals, such as feldspars, have also shown signs of weathering: we are currently investigating these – they provide fascinating insights into the rock’s formation history. 

And to top it all off, it turned out to be quite a precious find? 

In the drill cores, we discovered something particularly interesting in a thin, dark layer: tourmaline. This mineral is used worldwide as a gemstone. Depending on its chemical composition, it comes in a wide variety of colours, ranging from black and red to yellow and green. Beautiful! Scientifically speaking, however, it is not a single mineral but a group of minerals belonging to the silicate class. It is commonly found in granites – specifically those that have been exposed to hot water.