When I was teaching with the course of Geology I in Pisa, I used to tell students: “the best way to learn how to recognize minerals is to start from big samples”. I think we are exaggerating here.
Some time ago, stressed by lockdowns, restrictions, and stuff, we went on a hike on the Moriglion di Penna, a mountain near Lucca that watches a metamorphosed carbonatic massif. While we were climbing to the top, something on the ground suddenly captured my attention…
What is this glassy, reflective rhombohedral stuff? Wait, these are calcite crystals larger than my fists! A huge vein filled with crystals that climbs over the mountain for several meters on the trail. I could not tell where the vein ended because where the rock was grey dull and looked like the border of the vein, suddenly other big calcite crystals appeared upon closer inspection. I gave up trying to understand when the vein ended and just enjoyed some mineralogical beauty.
Many calcite crystals appeared well formed, sometimes surrounded by microcrystalline calcite, sometimes hanging in voids filled by the rusty soil of the area. Many of them showed a color from white to brownish or pale orange. They reacted with acid, so I cannot tell if they contain some inclusions of dolomite or some oxides that may give these orange hues when altered.
I have seen calcite crystals many times in class, but this was the first time I have seen some this big. It was possible to look around and see their crystal shape unfolded in three dimensions. Most of them had incredibly well developed rhombohedral cleavage planes that were ‘kidnapping’ rhombohedral fragments from the big crystals. I remember there were rhombohedral fragments of calcite scattered all over the place!
I have highlighted some cleavage planes in the crystal above, which looked like the most beautiful example to me. In reality, the choice was tough, as you can see from the pics below.
Finally, this last calcite crystal captured my attention. I was looking at its well-developed cleavage when I noticed it had ‘stripes’ with different color.
What are they? I suspect they are lamellar calcite twins, as they follow the same crystallographic orientation of cleavage planes.
Do you have any other suggestion? Please tell me in the comments! In the meantime, I send you a big ‘ciao’ from the top of the world!
Hiking on the Monti Pisani
We hike a lot on the Monti Pisani, a wild area right in our backyard we discovered during the lockdown.
If you want to go there, I recommend the map we use, which shows all the trails, villages, ruins, and itineraries and is available in five languages.
For the geological stuff, there’s also a free map available here.