Cosa sono i 'sassi bianchi' incastonati nelle rocce di Sant'Andrea (Isola d'Elba)?

I was a student the first time I visited the outcrops of Sant’Andrea in the far 2011. We must have looked really weird: a class of noisy people with hammers and yellow helmets! Nevertheless, this did not discouraged the people walking on the cliffs to aask us a very simple question about the curious-looking rocks that were all around us:

“What are the white ‘stones’ in the rocks?”

Sant’Andrea is an outcrop of the Monte Capanne monzogranite, a type of intrusive rock deriving from the solidification of magma at depth. Differently from lava erupted at surface by volcanoes, which cools down in some hours or days producing glassy and fine-grained volcanic rocks, the cooling of magma at depth is very very slow, occurring in thousands if not millions of years.

This happens because rocks are extremely good thermal insulators. The monzogranitic rocks here, part of the larger Monte Capanne monzogranite, emplaced at about 6 km and cooled down very slowly, buried under a pile of crustal rocks. Consequently, the melt was able to crystallize as coarse-grained crystals. In the rocks of the Monte Capanne monzogranite, we can, indeed, see crystals of grey-transparent quarzo, white feldspars (plagioclasio e alkali feldspar), and black biotite that give these rocks a granular appearance.

Most of the crystals in the monzogranite exposed here have a grain size of 5 and 15 mm – and this is already considered ‘coarse-grained’ in intrusive rocks – but the white ‘stones’ are even larger, reaching the impressive size of 15 cm! These are actually megacrystals of alkali feldspar that crystallized from the molten magma.

But why are some crystals big and others small? The crystallization of granitic magmas is a slow process that occurs over a temperature range between about 700 and 600 °C. Some crystals may start to grow earlier in the crystallization sequence and may become larger and bigger because they have had more time to grow in a molten magma before the final crystallization. When a granitic melt approaches its solidification temperatures, the contemporaneous growth of many crystals forces them to compete for space, generally limiting their size to a few millimeters.

The megacrystals of alkali feldspar of Sant’Andrea are interpreted as the earliest products of crystallization in the Monte Capanne pluton. This structure, defined by large crystals in a finer groundmass, is called porphyritic texture in geology and is actually common in a wide range of igneous rocks.

Beyond the geological meaning of the textures of these rocks, the alkali feldspar crystals at Sant’Andrea are also very beautiful and we can observe many features typical of feldspars within them.

The megacrystals are often well-formed (euhedral) with a tabular crystal habit, showing typically rectangular and more or less stocky sections.

Thanks to erosion, which highlights the crystal features of the alkali feldspars with respect to the more erodible monzogranite, we can use these outcrops as an open-air mineralogy lesson!

Many alkali feldspars show small ‘ears’ or triangle-shaped edges that appear out of the feldspar: these are actually two crystals that are interpenetrated within one-another following a crystal law called Carlsbad twinning.

There are several, wonderful examples of Carsbad twinning around!

 

On broken surfaces, we can actually see that these are two individual crystals linked together by a twinning law, because, in section, they are separated by a sort of median line, which puts two different crystals in contact. If you move in a position where light reflects on the section, you can see that light reflects at different angles in each twin, which means that are in different crystal orientations.

Another mineralogical feature that can be observed on the alkali feldspar megacrystals is the presence of cleavage planes. Feldspars tend to break more easily on two crystallographic planes of weakness which are nearly perpendicular to one another. If you look at the feldspars here, you can see the cleavage planes as intersecting, orthogonal fractures in the crystals.

When we look at crystals in sections, we can learn a lot about their crystallization history. Normally this is done with a microscope, which allows to observe the internal features of crystals, but these are so big that we can see something interesting within them even at this scale. The crystal below contains a concentric zonal arrangement of biotite inclusions (black) that closely matches the external shape of the crystal. I don’t know what caused the crystallization of biotite at some point during the growth of this crystal, but it is there now, preserved within the feldspar, registered as an event in the crystallization history of the Monte Capanne.

The coast of Sant’Andrea may be famous for its scenic beauty, but it is also a wonderful open-air mineralogical and geological museum. I have honestly learned more myself on feldspars here than in class. And this is just the first part of the story, because there is much more to see here! I am leaving you a little teaser of the objects I will talk about in the next part: what are the black balloons in the granite?

This post was possible only thanks to Sandra McLaren (Thank you!). If you like my geological posts and you wish to support me, you can do it by offering me a coffee at ko-fi!

Bibliografia
Barboni, M., Annen, C., & Schoene, B. (2015). Earth and Planetary Science Letters, 432, 436-448.
Gagnevin, D., Daly, J. S., & Poli, G. (2008). Mineralogical Magazine, 72(4), 925-940.