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Geology is the Way

A matter of chemistry: the wollastonite crystals of Norsi beach (Elba, Italy)

Rocks are not all equal. Before you call me a rock racist, what I mean is that different rocks have very different chemistry, which determines what chemical reactions can happen, for example when they are exposed to hot hydrothermal fluids. The result? Beautiful crystals which grow only where chemistry allows them to:

Norsi coast

The Norsi coast exposes a complex of limestones and shales of Cretaceous age, known as the Palombini shales, oceanic sediments deformed during subduction in the early Apennine belt.

The Norsi coast showcases cliffs of shale and limestone with a turbulent past. These rocks where strongly deformed during the early stages of formation of the Italian Apennines, which led to the fragmentation of lenses of limestone within the shales (I am actually spending a lot of time on these rocks lately).

limestone and shale outcrop

Broken layers of white/yellow limestone interlayered with black shales. Palombini shales. Norsi, Elba, Italy.

Did these rocks deserve some rest after such an intense deformation? Probably yes, but unfortunately for them, Elba had other plans. In the late Miocene (about 6 million years ago), large volumes of hot magma were emplaced underneath the island. The main magmatic intrusive body in the area, called the Porto Azzurro pluton, is currently buried a hundred meters under sea level below the entire SE Elba (Calamita peninsula), and crops out mostly as small monzogranite/leucogranite dikes and bodies, as the one to the east of the Norsi area, visible in the map below. The rocks close to the pluton were cooked by contact metamorphism at temperatures up to 600-700 °C (1100-1300 °F). The rocks of Norsi were luckier, as they largely escaped contact metamorphism. Nevertheless, they were not be spared by infiltration by huge amounts of super-hot fluids released by the crystallization of magma.

These hot fluids (we are talking of temperatures above 400 °C or 750 °F; Zucchi, 2020) fractured the black shales and deposited green calcsilicates (mostly epidote, chlorite, and amphibole, but also diopside and hydrogrossular) in small veins, with a thickness of a few centimeters at best. The reaction between the fluids and the shales produced thin, white alteration halos enriched in quartz, which are, overall, really unimpressive, generally smaller than a coin:

calcsilicate veins in shale

The Palombini shales in Norsi are rich with these green calcsilicate veins, which were deposited by hot fluids as they fractured the rocks.

calcsilicate veins in shale

White halos, like the one above, are produced by reactions between the fluids and the rock. Basically the hot fluid ‘steals’ iron, aluminium, and magnesium from the shale, leaving behind a margin enriched in silica. Everything happens at a thickness smaller than a coin!

calcsilicate veins in shale

Most of these green calcsilicates are fibrous amphiboles (hornblende or actinolite-tremolite) but it is not uncommon to find also pistachio green epidote or pink garnets!

The situation is completely different for the limestone blocks. Limestones consist mostly of calcium carbonate [CaCO3], calcite. This mineral can be easily attacked by acid fluids and decomposes very easily to the calcium [Ca2+] and carbonate [CO32-] ions at high temperature (decarbonation reaction). Limestone is also chemically very different from the hydrothermal fluids released by plutons, which are typically rich in dissolved silica [SiO2]. Consequently, the limestone blocks were easily attacked by these circulating, super-hot hydrothermal fluids, becoming fertile ground for chemical reactions to take place. Carbonate minerals, indeed, reacted with silica and other elements, and were replaced by calcsilicate (i.e. calcium silicate) mineral, generally starting from the contact with the surrounding shales, along which the fluids were circulating:

In many situations, these reactions were able to run to completion, causing the complete recrystallization of limestone to a marble with beautiful, radiating crystals of wollastonite, a wollastonite skarn.

Wollastonite is a calcium silicate with formula CaSiO3 that forms after the reaction between decomposing calcite (releasing Ca) and the SiO2 supplied by the fluid. In Norsi, you can find it as beautiful, radiating aggregates with a diameter up to several centimeters! Other greenish calcsilicates (mostly amphibole and epidote) occur closer to the contact with the surrounding shales, as they also contain other elements, like iron and aluminium, which are released by the phyllosilicates of the shales.

wollastonite skarn

Radiating aggregates of brown wollastonite surrounded by white, recrystallized carbonates.

wollastonite crystals

This one was 4-5 wide (sorry for the lack of scale, it was on a vertical wall!).

Do you think these rocks are beautiful at outcrop scale? Well, be prepared, because I am about to show you what they look like under the microscope! [slide 2 times from the right and enjoy the view]

CPL + AP
CPL + AP
CPL + AP
CPL
CPL
PPL
PPL

Introducing the MICROMEGA series
Micromega markerThis is the first of many posts that will show rocks from the mega- to the micro-scale. Geology deals with many scales of observations and to me they cannot be divided! I personally could not imagine looking at rocks in the field without checking samples under the microscope or looking at thin sections without seeing the rocks in the field: that’s the philosophy that will be behind the MICROMEGA series. Look for the green MICROMEGA marker on the map!

CPL + AP
CPL + AP
CPL + AP
CPL
CPL
PPL
PPL

Under the microscope, wollastonite (the fibrous crystals with brownish/yellow colors) are so large in this sample that I simply have no way of fitting them in a single shot (field of view only 3 mm wide). As you can see, there is little of the original limestone left in these rocks. Calcite (the mineral that is transparent at PPL with very high interference colors at CPL) has recrystallized to coarse-grained crystals due to the heath, while wollastonite is basically ‘eating up’ calcite and taking its calcium to grow (calcium is good for bones and wollastonite knows that!).

fibrous wollastonite

Radiating aggregates of fibrous wollastonite, associated with calcite and other (tiny) calc-silicates in the Norsi skarn. CPL. Width: 3 mm.

Rarely you might see other minerals, other than wollastonite, at least here in the core of recrystallized limestone (on the rims there is a world of minerals I will show you another time). However, there are also some interesting accessories that can tell more about the evolution of these rocks. For example, I have found this diopside rim overgrowing a wollastonite core (video below, 3 mm wide, seen at CPL). Diopside consists of calcium, silica, and magnesium [CaMgSi2O6], so I wander if its presence indicates that some magnesium entered the rock at some point or that maybe there maybe was also some dolomite [CaMg(CO3)2] in the rock to break down? Anyway, this stunning microstructure suggests that diopside formed after wollastonite. This is something we can learn only from the analysis of rocks at the microscope, which was absolutely not visible in the field!

Why are skarns, like this one, important?
Skarn is an old Swedish mining term that means ‘wasterock’, because these rocks are normally found around sulphide and oxide ores, where important elements like iron, copper, tungsten, silver, and gold can be found. Elba is famous for her pyrite and hematite mines and for its skarns that surround the ore bodies. The term ‘wasterock’ is, in my opinion, misleading, though: skarns offer a plethora of information about how an ore body formed, as they preserve microstructures suggesting how chemical reactions, movement of elements, and interaction between hot fluids and rocks took place. Unfortunately there are not many studies on the Elba skarns but – who knows – perhaps this blog post will motivate some future Elban researcher 😊. In the meantime, stay tuned for the next posts!

I would like to thank Sandra McLaren, Silvio Ferrero, Marta Codeço, Ellie, Morgan, and Jerry Nelson for supporting my blog! If you like my geological posts and you wish to support me, you can do it by offering me a coffee at ko-fi!

References
Keller & Pialli (1990). Bollettino della Società Geologica Italiana109(2), 413-425.
Zucchi (2020). Geothermics85, 101765.
Papeschi, Vannucchi, Hirose & Okazaki (2022). Tectonics, 41(7), e2021TC007164.

The cropped map shown above is after Barberi et al. (1967). Carta geologica dell’Isola d’Elba alla scala 1:25000. Absolutely the most beautiful map ever realized for the island!

A full gallery with microphotos of this rock is available in the wollastonite and skarn pages.

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