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Onice di Montaione / Characteristics / Genesis and characteristics

Characteristics

Genesis and characteristics

In some areas the carbonate of travertine has crystallised, resulting in a beautiful limestone alabaster with banding primarily of dark colours (blacks, browns and reds), but also yellows and other highly attractive shades.

The particular colours of the rock are due to the presence of iron, manganese and other minerals.

The result is a rock which is ideally suited to cutting, polishing and buffing, and which has been extracted for many centuries, creating a flourishing market which today has unfortunately all but disappeared. In Il Bagno, every phase of the formation of travertine can be seen, from incrustations on all kinds of base, to the most gorgeous onyx.

The characteristics of Montaione onyx
The characteristics of Montaione onyx

Moderate to large plates of travertine are common in Tuscany (Staggia, Rapolano, Asciano, Massa Marittima, Sarteano, Pitigliano, Saturnia and elsewhere). Their origin usually lies in the flow of water rich in bicarbonates, especially of calcium and magnesium, which, arriving at the surface, undergo a series of chemical and physical changes. Specifically, the bicarbonates lose their hydrogen atoms and turn into insoluble carbonates which then precipitate in deposits.

These travertine rocks are nearly always of fairly young formation, even in recent history or current, as in the case of “Il Bagno”, where they continue to form. The processes which lead to the formation of this type of rock are, broadly speaking, as follows.

During its time in the subsoil, oxygen-rich rainwater – both vadose and thermal – can interact in various ways with a wide range of rock types, dissolving, dragging, depositing and taking up many minerals, both soluble and insoluble.

When this water reaches the surface, some of the chemical equilibrium that existed underground is lost, and the water generally can no longer carry the part of its mineral load which cannot be held in solution or suspension. This applies especially to carbonates, which elsewhere give rise to calcite, aragonite and magnesite, basic components which are extremely common in a large number of rocks and formations.

These carbonates form deposits and encrustations, even concretions and increasingly thick layers, around the edges of the spring. Over time the deposits cover and petrify everything in their path, such as organisms and plant matter, which often find optimum conditions in these particular habitats for vigorous growth. In the course of time the organic matter disappears and its imprint remains perfectly petrified.

Travertine rock is in fact a fascinating document, and its layers, which vary in composition according to the water level and content, are open pages on different geological ages. Once these limestone plates (travertine) have formed, they expand and thicken, obeying new physical laws. Meanwhile the input from the underlying strata may cease or increase, shift or change its composition.

Selection of the raw product
Selection of the raw product

Thus the formation of travertine may cease, intensify or move to another area, or give rise to layers of different colours or compositions; the already formed rock may even destroy itself by dissolving again or crumbling into debris. The layers of travertine that thicken and weigh heavily on the underlying strata of various types may also disturb the equilibrium of static layers and break up the rock and itself, sometimes shifting some distance, almost “floating” on the substratum.

The water flowing out of the ground may use the resulting gaps to reach the surface in new sources, to then perhaps re-cement the area, now with different substances, altering the composition of the older travertine.

Rainwater too – and to an even greater extent, as it is extremely active chemically – can penetrate cracks and gaps in the rock, dissolving the already formed travertine and causing the carbonates to re-crystallise in onion-style, in other words in concentric layers of variable composition and shade (limestone onyx), following processes known as secondary chemical formation.

The plates of limestone onyx are arranged according to the feeder springs along the area’s fault lines, on the border between the ancient land and the adjacent sand and clay rocks of the Pliocene era, which extend westwards to Villamagna and beyond, in the Era valley. In this area, the unique geological conditions have resulted in water sources that lack nothing in terms of minerals: besides manganese and iron, they also contain strontium, sodium, potassium, lithium, boron, copper, zinc etc, all of which have contributed to the creation of this very special product and given it its colourful appearance. All these geological events, which generally do not affect the majority of travertine, are fundamental for the genesis of Montaione Onyx.

 

It is also true that, like normal travertine, this type of material is found frequently, but none is remotely comparable to the Montaione variety, particularly in terms of its unique colour effects.

Tutti questi eventi, che normalmente non interessano la maggioranza dei travertini, sono alla base della genesi dell’Onice di Montaione. È pur vero che, oltre ai normali travertini, materiali di questo tipo si incontrano frequentemente, ma nessuno di essi, soprattutto per certe tonalità di colore, è lontanamente paragonabile ad esso che resta unico per la sua preziosità ed effetti cromatici.

This rock has been quarried and worked since the Medici period, and found fame from the nineteen sixties to the eighties, thanks to its properties of being easy to cut, clean and polish and its use for a wide range of objects such as statues, table tops, interior and exterior cladding, paving etc.

The Montaione area

The area of Montaione has been inhabited since Etruscan and Roman times, as we know from extensive archaeological finds such as the remains of necropolises and furnaces at Poggio all'Aglione, Belafonte and other places.

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