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Minerals of Millau viaduct and its neighbourhoods
Millau, town with 22500 inhabitants, sub prefecture of Aveyron department, is
located in the A75 axis. It's the new highway connecting Clermont-Ferrand to the
Mediterranean. The crossing of the Tarn valley required the realization of a titanic
work: the viaduct of Millau. It was initially planned that this viaduct would
be built by the government. The idea of setting in concession appeared only later.
Indeed, it is only in November 1998, that the direction for roads launched the
public investigation for a setting in concession. Of course, it is only after
it, the works could start. This portion was about the A75 route, from the "col
de l'Engayresque (20km in the north of Millau) to Cavalerie (20 km in the south),
zone including the viaduct.
According to a purely mineralogical point of view,
the southern portion that is located on the plate of Larzac didn't bring interesting
specimens. We can only talk about some calcites geodes located in limestones and
in dolomites of Bathonien.
On
the other hand, the part located at the north of Millau, on what we commonly call
the "Red Causse" has a certain interest. As a matter of fact, we can
find dolomitic limestones that are intersected with marly and limestones levels.
They spread out from Trias to superior Lias going to Hettangien and Sinemurien.
Moreover, it is in one part of this zone going from the northern abutment of the
viaduct to Saint Germain (it is in approximately 5 km ) where we found the most
interesting specimens.
The work carried out in this relatively restricted
zone, was very significant. In addition to the earthworks that were necessary
to the realization of the highway (the width of the roadway reaches until 27m50),
they integrate the enormous work about the viaduct tollgate, the northern abutment
for this same work, the rest area of Brocuejouls and the exchanger of "Saint
Germain". If we add to this strictly motorway work, but in the same perimeter,
the secondary service roads and the punt forms installation on the all close zone
of activity of Millau Lévézou, we have hundreds of thousands of
cubic meters that were pickled. Although it is about a causse, i.e. of a karstic
plate, the uneven ones are pronounced. The extracted volumes were totally used
to fill certain zones, particularly in this one of the toll platform.
These
works that were spread out over several years are completed today, at the least,
for the motorway part. Indeed the respect of the timing was essential in order
to carry out the complete installation of the section. This one had to coincide
with the putting into circulation of the viaduct, whose inauguration is planned
for December 20th, 2004. thicknessful companies took part in these works. We have
to say that in spite of national companies as Muller or Roger Martin took part
in the works, the part of works attributed to the local companies (Connes and
Sévigné SA)was really significant.
Because of Baryte/copper
mineralisation was already strongly known there, the mineral amateurs were interested
on what would be on this portion of Red Causse. Brocuejouls and Scales indicators
have been already known at the Roman period. Then, in the XIX century, they still
have been exploited and it lasted until the middle of the 20 century.
The
minerals found comparatively numerous and by place were well expressed. In a quasi
systematic way the géodes are covered with carbonates of the ankerite type/siderite.
It is on this gangue that were established sometimes in a completely interesting
way a certain number of minerals.
Mineralisations were established in the
vacuums of old modelled karstic dating from Sinémurien higher like than
the favour of a posterior fracturing. On the ground, they thus meet in a completely
random way, in the shape of insulated pockets, the form of seams, or then with
the lucky meeting of a large karstic network. One can, in this last case, to have
a number impressing of pockets and seams mineralized, on a surface not larger
than a hundred square meters.
Baryte:
very present, almost over all the length of the route, northern abutment of Saint
Germain. It is in this last part and particularly, on a hundred meters that it
is expressed the best. The geodes, often partially filled with stacking of rocks,
went from a modest size about a few centimetres, to pluri-metric cavities. Translucent
to opaque, from white to yellow. It is sometimes tinted with oxides color giving
it reddish or black colors. It is usually found in crested forms. It 's more rarely
in tabular forms. It should be said that the two facies can be associated in the
same part. The parts known as "floating" were found in "clays".
It is frequently entirely or partially covered with calcite. But it can also be
used as support for a certain number of sulphides. It is incontestably taking
into account the diversity of the found parts, and its relative abundance the
mineral headlight of this site.
In mineralogy, small size specimens are usually
more aesthetic. Yet, we have to specify that specimens with a weight equal or
superior to 50 kg have been found.
Chalcopyrite:
the crystals usually found in the ankerite are usually oxidized. They are covered
by malachite and azurite. These two minerals are sometimes crystallized, including
malachite. They offer colors that can be very sharp.
Being able to reach a
sizeable size (2cm), their brightness and contrast with their support contribute
to present them as original and mineralogically interesting specimens. However,
the specimens of quality are rare, just like the crystal specimens found on Baryte.
Tétraedrite:
it is rarer; this other copper sulphide is also present under similar conditions,
i.e. out of isolated crystals found on a ankerite gangue.
Other
sulphides: among expressed minerals in a macroscopic and perfectly identifiable
way, let us quote:
- Pyrite: strongly oxidized and with a black color
- Bournonite: It is present in certain zones of contact with massive barite (crystals
reaching 2 cm). It is usually covered, even pseudomorphosed in Bendhemite.
- galena: directly deposited in the calcareous gangue or in the ankérite;
it remains rare and the crystals don't exceed more than 5 mm.
Aragonite:
If it is rare, it can also be locally abundant. It can be either in the form of
masses of white color, or in the form of translucent acicular crystals.
Other
minerals: the list above is not exhaustive and other minerals for some of them
are in the process of identification. Among them, we can undoubtedly quote the
manganese, iron Quartz and some oxides.
This building site (it is true
that it is exceptional by its importance) showed once again the interest that
road works could hold on a mineralogical board. This is also true that many mineralogists
who had the opportunity to make research in this sector, did it in a good spirit
which resulted in very courteous relationship with the workmen operating on these
various building sites.
This intelligent and well perceived cohabitation seem
sufficiently rare to deserve to be announced. As long as the mineralogist is satisfied
with the search for minerals, without degrading anything, the communication and
the good management make the remainder.
Bernard
Dincuff
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