Geology of Isola d'Elba
| GENERAL NOTES | |
| The structure of Isola Elba is quite complex though the island is not so large (around 210 kmq.) and it can be divided in 5 parts from the geological point of view that in the last millions of years moved for the tectonics so that to be overlapped or tightly connected one with the other. | |
| The first zone
includes the area of Punta Calamita and all the coast up to
Porto Azzurro and Terra Nera. This part is greatly composed by metamorphic
shist, quartz and carbonatic rocks (such as dolomia). The second zone includes only the area of Terra Nera and Rio Marina and the area inbetween the gulf of Portoferraio and Stella gulf (east of Lacona area). This area is mainly formed by various kinds of schists, marbles, quartzites, and carbonatic rocks. 
The third zone is located north-east of Isola Elba and includes the area of Cavo up to Terra Nera. This area is mainly formed by carbonatic rocks, dolomia, schists. The fourth zone is the largest and includes all the east and central part of Isola Elba, including the south area of Cala Vita, the area inbetween Portoferraio and Capo Stella and the whole area around Monte Capanne. This area is mainly composed by serpentine, gabbros, basalt (typical pillows), carbonatic rocks and schists. The fifth zone is formed by the west and central part of the island, including the area inbetween Portoferraio, Marina di Campo and Monte Capanne. These five zones are perfectly and tightly linked one with the others due to the tectonic movements. During these last few millions of years several fractures permitted to fused lavic rocks to reach the surface, creating the oldest granitic rocks of this area, and later to the inner part of Monte Capanne. During the following millions of years erosions and tectonic movements eliminated the coverage of this granitic parts of the island. This event permitted to unhide the granitic soul of Monte Capanne, exactly what we can see nowadays.
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| THE GRANITES | |
| The granitic part of
Monte Capanne is the specific issue of interest of the present CD, mainly
of the minerals that these granitic rocks contain. These granitic rocks born
more or less in the same period when the Appennini mountains were formed,
all along a time interval that is between miocene and pliocene up to
quaternario, when the vulcanic activity was real intense. Due to the
underground vulcanic activity the granitic soul of Monte Capanne was
created. This mountain is located in the central part of Isola Elba and is
1000 meters high. The granitic rock that forms Monte Capanne is called
granodiorite, a sort of intermediate product between granite and diorite,
due this to a different balance of the main components orthoclase and
plagioclase. Other components of this granodiorite are quartz, mica and all
the minerals that are usually of interest for the collectors (tourmaline,
beryl). Most of the fractures of this granodiorite has been filled by other
lavic materials, forming vugs of white color, rich of tourmalines and most
of the pegmatite minerals.
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| METAMORPHIC MINERALS | |
| All the area that is
around Monte Capanne is composed by various kinds of clays, serpentine, carbonates. When Monte Capanne born the heat of
the magmatic material that was reaching the surface induced a lot of
metamorphic activity creating new minerals such as diopside, garnets, vesuviana, dipiro, humite (found
in the area of
Cavoli), meionite, wollastonite.
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geologY OF PEGMATITES AREA |
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Monte Capanne is a mountain mostly composed by granodiorite. This rock is
now evident and visible thank to the activity of erosion. Pegmatite bodies
of vein-like form are located only in the area of Monte Capanne and are
mainly composed by quartz, plagioclase, feldspar, and mica biotite, with
also the presence of additional minerals such as apatite, zircon, titanite,
tourmaline, hilmenite. Other minerals are clorite, calcite and several
metamorphic minerals (Poli 1989, Bussy 1993). The most credited theory says
that these pegmatite bodies and veins were formed around 6-7 milions years
ago when a great amount of magma coming from the inner part of our planet
reached the surface passing inside the fractures of the earth croast (Keller
& Pialli 1990, Ferrara & Tonarini 1985). The interesting fact is that though
these pegmatite veins are very common all around Monte Capanne, only those
located in a very defined and restricted area formed cavities and geodes.
These last veins are from 1-3 cm to 1-2 m thick and up to 15-20 m long
(Orlandi e Pezzotta, 1996). The area is the one close to the small villages
of San Piero and Sant’Ilario .
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| MineralS OF APLITES AND PEGMATITES | |
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Specific changes took place when there was the cooling of the magma,
especially in the external parts of it where idrothermal events took place.
This happened mainly in the area of San Piero and
Sant'Ilario where vugs and pockets were formed due to this, surrounded by
the granodiorite. These veins show an external part, formed by a more rough
material (aplite) that became better and composed by thinner material in the
inner part (pegmatite), especially close to the geodes or pockets. It is in
these areas of the pegmatites that most of the elements of the magma gave
origin to crystals such as beryl, lepidolite, petalite, polluce, rosterite,
tourmaline, apatite, topaz.
The granodiorite is relatively corrupted while most of the
aplites/pegmatites veins are frequently corrupted. These modifications are due
to the idrothermal changes. Most of the tiny zeolites that can be found
are related to this phenomenon (chabasite, stilbite). Also the color of
tourmaline can depend from this: the black tourmalines formed in a first
moment, then the colored tourmalined formed in this second idrothermal
phase. |
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| Minerals of the aplitic-pegmatitic phase - These minerals are relatively common in the areas of Fonte del Prete, Speranza, Grotta d'Oggi, associated with quartz, albite and feldspar (orthoclase). The most common are beryl, tourmaline, lepidolite, spessartite garnet, pollucite (or polluce), petalite (previously named castore or castorite), apatite, zircon, topaz, cassiterite, microlite, ambligonite. The most abundant minerals that are the matrix for all the others are quartz and feldspar. Toumaline has been described as appearing with at least 40 different kind of crystal shapes and colors. A special mention for beryl: the blue one is known as aquamarine, the pink one is called morganite and the rare tabular one (colorless or light blue) is called rosterite. | |
| Minerals of the idrothermal phase - These minerals formed long after the magma cooled, so they crystallized long after the aplitic-pegmatitic phase, or during the last part of the cooling of the magma. The hot fluids that came from the underground acted on the pegmatite rocks and on their minerals. Quartz and tourmalines were not affected or where just little bit hetched. Lepidolite changed its color from pink/red to velvet and then white. The feldspar (orthoclase) was transformed in clay-like material or in zeolites. In fact this is the phase during which all zeolites formed, such as chabasite, heulandite, natrolite, stilbite, epistilbite, dachiardite, laumontite. | |
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References:
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