ExpOOs / Maison des Minéraux

The arch

rgilbert — 2025-07-02T09:58:56Z

Rock: granite
Type: plutonic igneous rock
Age: 300 Ma
Quarry: Le Houx in Languédias (Côtes d'Armor)

A Fine Granite

This granite, with a fine texture, is composed of feldspars (plagioclase, microcline), quartz, muscovite (white mica), and a bit of biotite (black mica). Along with its neighbor, the Hinglé granite, it belongs to a large batholith (from the Greek “bathos” meaning depth and “lithos” meaning rock), the Bobital-Dinan massif, formed during the Variscan (Hercynian) orogeny.

Granite with or without an “e”?

Two spellings, two meanings. “Granit” (without an “e”) is a commercial term used by the extractive industry and refers to a hard, coarse-grained rock. “Granite” (with an “e”) is the term geologists use to name this plutonic igneous rock. This means that a “granit” does not necessarily mean “granite”! For example, the “petit granit” of the Ardennes refers to a limestone, a sedimentary rock.

Forming and quarrying

The elements of this arch were prepared by young stonemasons from the Apprentice Training Center (CFA) of UNICEM, located in Louvigné-du-Désert, about 70 kilometers north of Rennes (Ille-et-Vilaine). This Brittany campus benefits from a training site equipped with state-of-the-art teaching tools. Today, four programs are offered: quarry equipment maintenance, public works and handling machinery mechanics, laboratory technician, and finally, stone-cutting trades.

graphicnode - arche / Unsplash / https://unsplash.com/fr/photos/EA0gEEIGsoU

A Breton Gem

Granite is marketed under two names: “gris perle” (pearl gray) and “beige de Languédias.” These color variations depend on the degree of rock alteration. The beige is slightly altered, while the gray is sound. The “Languédias” quarry has been heavily exploited. It has the highest quarry density in Brittany and has been worked since the late 19th century. Granite was primarily used for paving stones for roadways and is now notably featured on the floor of the Cour Napoléon at the Grand Louvre in Paris.

Nathanael Burton / Wikimedia Commons / CC BY-SA 2.0 / File:Paris 75001 Cour Napoléon Louvre Aile Turgot 02b 20141107 125236.jpg

Geode

amenez — 2022-07-07T20:15:18Z

Stone : quartz
Type : mineral
Age : 304 million years
Quarry : Kergantic, Ploemeur (Morbihan)

Quartz

This geode features beautiful quartz crystals. It comes from a large vein located in the Kergantic quarry, where kaolin, a white clay, is extracted. Quartz is the most abundant mineral in the continental crust and is composed of silicon and oxygen (SiO₂). It is known for its high resistance to weathering.

Kaolin: The Product of Granite Leaching

Kaolin mainly forms from the alteration of feldspar-rich rocks, such as granite. Granite is composed of quartz, micas, and feldspars. Feldspars are particularly vulnerable and transform into clay minerals. This transformation is driven by the circulation of hydrothermal fluids ( 550°C) through fractures in the granite massif near the end of its emplacement. The silica released during this process is what forms large quartz veins. Later, about 38 million years ago, Brittany's tropical climate intensified the kaolinization of the massif.

Blank Page and Ceramics

Kaolin is used in a wide range of industries. It is a key ingredient in paper production, ceramics, rubber, paint, certain plastics, fiberglass, and more. The first porcelains were made in China 1,800 years ago, using white clay from a "high hill" — Kao Ling in Chinese.

Faïence de Quimper signée Henriot (vers 1910)

Patrick.charpiat / Wikimedia Commons / CC BY-3.0 / File:Quimper7.JPG

France's Largest Kaolin Deposit

Kaolin from Ploemeur has been mined since 1904, and it is the largest deposit in France. The operating company, Imerys, extracts 60,000 tonnes of kaolin per year. From 1 tonne of raw material, you get approximately:

250 kg of kaolin (one quarter)
50 kg of white micas
550 kg of quartz-rich sands
Besides Brittany, kaolin deposits have been or are also mined in Drôme and Allier departments of France.

Vue aérienne de la carrière de Kergantic à Ploemeur

fonds IGN / Mégalis de Bretagne

Paths

amenez — 2022-07-07T20:15:15Z

Stone : sand
Type : sediment
Age : between 7 and 3.5 million years
Quarry : "Carrière du Moulin", Radenac (Morbihan)

Stone : red siltite
Type : metamorphic rock
Age : 480 millions years
Quarry : "Carrière de Trékoët", Muel (Ille-et-Vilaine)

Sand: A Highly Sought-After Material

The sands of Radenac have been exploited since the early 20th century. These river sands are highly valued for making concrete. Every year, humanity consumes over 50 billion tonnes of sand (80% of which is used for concrete production). That's equivalent to a one-meter-thick layer of sand spread across all of Brittany. Sand is the second most-used material on Earth, after water.

The Red Siltstone of Muel

The Muel quarry is operated by the company Pompéi, a public works firm based in Brittany for over 100 years. Opened in 1948, the quarry mainly supplies aggregates for roadworks. It is still active today. Geologically, these siltstones belong to the Pont-Réan Schist Formation (see installation 12).

In an Abandoned Ria

The "red sands" extracted at Radenac are ancient sedimentary deposits laid down in the bed of a deep valley near an estuary (a ria or aber). Their presence at over 80 meters in elevation reveals fluctuations in sea level, and therefore, climatic changes. Similar deposits from the same period crop out in other parts of the Armorican Massif. At that time, the island of Brittany (yes, it was once an island) enjoyed a humid subtropical climate, much like Florida today.

Sand, Solar Panels: Silicon, My Love

This quarry is no longer active and will not be reopened — the resource is depleted. A landscape, ecological, and agricultural redevelopment project is under consideration, which also includes plans for a solar power plant. Truly, silicon is everywhere: it forms the quartz grains in sand and the cells of solar panels.

Le silicium, présent dans le quartz, est un métal utilisé dans la fabrication des panneaux photovoltaïques

Ceinturion / Wikimedia Commons / CC BY-SA-3.0 / File:SolarPowerPlantSerpa.jpg

Tumulus

amenez — 2022-07-07T20:15:14Z

Stone : pink granite
Type : plutonic magmatic rock
Age : 300 millions years
Quarry : la Clarté, Perros-Guirrec (Côtes d'Armor)

A Young One

This pink granite forms the outer part of the Ploumanac'h granite complex. It is part of the lineament of “red granites” of the Armorican Massif. These granites, about 300 million years old, are the youngest in Brittany. They stretch from the island of Ouessant in Finistère to Barfleur in the Manche department.

Les granites rouges sont les plus jeunes granites de Bretagne

An Icon of Breton Geological Heritage

This granite is the origin of one of Brittany's main tourist attractions: the famous “Pink Granite Coast” in the Côtes d'Armor department. These coastal landscapes, made up of massive pink granite boulders stacked in a wildly chaotic manner with improbable shapes, captivate visitors. Over 800,000 people visit the Ploumanac'h site each year.

Couronne du Roi Gradlon à Trégastel

Archives départementales des Côtes d'Armor / Wikimedia Commons / Licence Ouverte 1.0 / File:Trégastel - Rocher du roi Gradlon - AD22 - 16FI6270.jpg

Why Is It Pink?

Clarté granite is a coarse-grained granite, especially rich in large feldspar crystals (potassium feldspars) that range in color from pink to bright red. This coloration is due to the presence of small amounts of iron oxide impurities within these minerals. It also contains quartz and black mica.

The “Rose de la Clarté”

Its commercial name is Rose de la Clarté. This granite is commonly used for memorials (e.g., the General de Gaulle memorial in Colombey-les-Deux-Églises) and funerary furnishings. It is also used for flooring (like on rue de Siam in Brest), building facades (such as the European Court of Justice in Luxembourg), outdoor and indoor fixtures, furniture, and decorative elements.

Labyrinth

amenez — 2022-07-07T20:15:12Z

Stone : mylonitic granite
Type : metamorphic rock derived from a magmatic rock
Age : 310 million years
Quarry : Poulmarh, Grand Champ (Morbihan)

A Deeply Crushed Rock

The term “mylonite” refers to any rock that has been crushed to a finer or coarser degree (from the Ancient Greek “mulôn”, meaning “mill”). The minerals within the rock retain signs of this deformation. The mylonite exposed at the Poulmarh quarry is a former granite — specifically, the Sainte-Anne d'Auray granite. This rock was crushed more than 15 kilometers deep and marks the path of a major fault in Brittany: the South Armorican Shear Zone (see page 10). Other mylonites are found along this major tectonic feature.

Perfect for Aggregate Production

This naturally crushed rock is ideally suited for producing aggregates. Aggregate refers to rock fragments smaller than 125 mm (including gravel, crushed stone, and sand). The Grand-Champ quarry, opened in the 1950s, now operates an ultra-modern processing plant — one of the largest in Europe — capable of producing over 6,000 tonnes of material per day, or about 2 million tonnes annually. It primarily supplies the local construction and public works markets.

Caution: Nesting Area

The large sand piles stored in the quarry attract migratory swallows. They come to settle there during the summer breeding season, digging their nests into the artificial sand cliffs. Quarry operators, mindful of this beneficial coexistence, create special nesting mounds — about 4 to 5 meters high — so the birds can breed there peacefully.

Oiseaux en cours de nidification / Carrière de Grand-Champ / CMGO / Carrières et Matériaux Grand-Ouest

6.4 Tonnes of Aggregate per Person per Year

According to a report by the National Union of Aggregate Producers (UNPG), the average consumption of aggregates in France was 6.4 tonnes per inhabitant per year between 1978 and 2015. Each year, more than 400 million tonnes of aggregates supply the construction and public works sectors. Building a road or railway requires between 10,000 and 15,000 tonnes of aggregates per kilometer. A house requires between 100 and 300 tonnes, and a high school between 2,000 and 4,000 tonnes.

Centrale à granulats / Carrière de Grand-Champ / CMGO / Carrières et Matériaux Grand-Ouest

Small bench

amenez — 2022-07-07T20:15:10Z

Stone : red sandstone
Type : sedimentary rock
Age : 480 million years
Quarry : Lourtuais, Erquy (Côtes d'Armor)

Multicolored Sand

Sandstone is a sedimentary rock formed from ancient hardened sand grains. The polished surface of the rock layer reveals horizons of different colors. These indicate variations in the composition of the sands. The darker layers are very rich in heavy minerals, such as iron or manganese oxides. Some of the lighter layers show cross-bedding; these features, well known to geologists, indicate a more turbulent depositional environment. The sands, transported by rivers with variable flow rates, were deposited on a piedmont plain at the base of hills.

Millions of 10x10x10

The exploitation of the sandstones at Erquy began in the late 18th century and ended in the 1960s. The quarries mainly produced paving stones (10x10x10 cm) but also rubble stones, curb stones, and door and window lintels and frames. Quarry activity peaked between 1900 and 1930. Quarry waste was used in the construction of the Rance tidal power station dam in the early 1960s. All materials were exported by sea. Today, an interpretive trail allows visitors to discover the history of the quarry workers. The extraction site at Lourtuais is located 400 meters north of these former quarries.

Archives départementales des Côtes d'Armor / Wikimedia Commons / Licence Ouverte 1.0 / File:Les carrières industrielles de l'Ouest (Erquy) - La taille des pierres et la voie ferrée Decauville utilisée avec des wagonnets pour le transport.jpg

Opening, Expanding, Closing, Redeveloping

Opening or expanding a quarry is not something done on a whim. The company must prepare an authorization request and submit it to the prefecture services. This study details how the quarry will be operated and proposes solutions to manage its environmental impacts (water, noise, dust, biodiversity, landscape, etc.). It also outlines the site's rehabilitation plan after operations end. The company is required to restore the site. The maximum duration of operation is set at 30 years but is renewable.

Ancienne carrière des lacs bleus

Large bench

amenez — 2022-07-07T20:15:09Z

Stone : siltite (almost transformed into a slate)
Type : metamorphic rock derived from a sedimentary rock
Age : between 590 and 540 million years
Carrière : Ripennelais, Saint-Aubin des Landes (Ille-et-Vilaine)

Rustic Slates

This rock has been exploited for over two centuries, initially used to make lauzes, those rustic, thick slates used on roofs in certain regions of France. However, due to its poor slate quality, this rock—over 500 million years old—was no longer in demand, and slate production had to be abandoned by the end of the 1990s.

A Friend Against the Rain

Since it was easier to crush the material into small pieces than to extract slates, by the 1960s, the company diversified its production and specialized in producing flakes to utilize quarry waste. These flakes were then used in the manufacture of bituminous shingles (shingle in English). They protect tar from solar and lunar radiation and remain effective allies for rain protection.

FASTILY / Wikimedia Commons / CC BY-SA-4.0 / File:Shingle roof 2 2017-06-14.jpg

The Age of Mulching

In the 2000s, the family company ventured into "mineral mulching" for the landscaping sector. This mulch, complementary to plant-based mulching, has numerous advantages: it retains soil heat, provides shelter for wildlife, remains permeable to water and air, and helps limit weed growth.

Made with… Brilliance

This siltstone belongs to a geological formation dated to the Briovérien period, a geological time frame between 670 and 540 million years ago. The term "Briovérien" comes from the Latin briovera, meaning "bridge over the Vire," which is the former name of the town of Saint-Lô in Normandy, where this geological formation also outcrops.

Tectonics

amenez — 2022-07-07T20:15:07Z

Stone : granite
Type : plutonic magmatic rock
Age : 327 million years
Quarry : Lande du moulin, Bignan (Morbihan)

Aurora Yellow or Grey—You Choose

This is a granite composed of quartz, feldspars, and two types of mica (biotite and muscovite). The minerals range from medium to coarse in size. Its color varies: grey when fresh, and yellow when weathered. Both colors coexist in the same quarry—yellow near the surface and grey at greater depths. Conclusion: surface granite is more weathered than the same granite found deeper underground!

Shear and Shakes

The Armorican Massif is crisscrossed by faults of varying sizes, not all formed at the same time. These are large fractures along which rocks have slid past one another. These jerky movements generate seismic activity. Geologists use seismometers installed across the region to record these tremors. Brittany is the third most seismically active region in mainland France.

A Magmatic Droplet

This granite belongs to the Guéhenno granite massif, whose distinctive teardrop shape on geological maps reveals much about its formation context. The intrusion stretches southwestward to meet a major crustal fault over 200 kilometers long. The magma was emplaced concurrently with fault movement. This relative motion can be identified in the rock structures.

Le pluton de Guéhenno, et ses deux voisins, se sont mis en place durant le coulissage de la faille

Echo of Tectonics

This site ties into the theory of plate tectonics. The Earth's outer shell is made up of small and large lithospheric plates. These plates are mobile and shift in relation to one another by a few millimeters per year. On a geological time scale, these "rafts of stone" travel thousands of kilometers!

Modèle récent (2022) des plaques tectoniques

Sandwich

amenez — 2022-07-07T20:15:06Z

Stone : biotite-bearing granodiorite
Type : plutonic magmatic rock
Age : between 540 and 520 million years
Quarry : Bécanne, Lanhélin (Ille-et-Vilaine)

What Is a Granodiorite?

Geologists classify plutonic igneous rocks based on their quartz and feldspar content. The Lanhélin granodiorite contains 44% plagioclase feldspars (predominant), 26% quartz, as well as black micas (biotite) and other potassium-rich feldspars. Granodiorite differs from granite mainly by its higher proportion of plagioclase feldspar.

On the Champs-Élysées...

This rock is marketed under the name “Bleu de Lanhélin.” It is mainly used for roadworks, construction, funerary art, and sculpture. In Paris, it's used for paving stones and curb stones on the sidewalks of the Champs-Élysées. It is also exported to many regions in France and internationally.

Benh LIEU SONG / Wikimedia Commons / CC BY-SA-3.0 / File:Champs Elysees Paris Wikimedia Commons.jpg

Precision Metrology

This rock possesses highly desirable physical characteristics—stability, hardness, durability, etc. It is used to manufacture extremely flat surfaces with a precision of 3 to 10 micrometers (about the thickness of a very fine hair). These “precision marbles” are used in the automotive, aerospace, and space industries, as well as in research centers. The Lanhélin quarry can provide 10-meter monoliths for use in large precision measurement benches.

Autre usage original : le bâteau de pierre (Maen vag en breton) réalisé par le sculpteur Jean-Yves Menez

Jean-Yves Menez / Wikimedia Commons / CC BY-SA-3.0 / File:Maen vag.jpg

Always Tiny Holes

The straight, cylindrical semi-grooves visible on the vertical faces of the “sandwich” block reveal the cutting technique used to extract it from the quarry wall without causing cracks. Quarry workers drill holes around the perimeter of the block, then use metal wedges or expansive powder inserted into the drill holes to delicately split the rock.

Carrière de Lanhélin / SOCAL / https://socal-granit.com

Bridge

amenez — 2022-07-07T20:15:04Z

Stone : biotite-bearing granodiorite
Type : magmatic rock
Age : 540 million years
Quarry : Godard, Louvigné-du-Désert (Ille-et-Vilaine)

Flaming and Anti-Slip Treatment

The surface of the bridge has been flamed to make it less slippery. This treatment involves using a blowtorch to intensely heat the rock's surface in order to fracture its minerals. The surface then becomes rougher, providing better grip for our shoes. This process is widely used in roadwork to create anti-slip surfaces.

The “Toad-duct”

Across the surface of the bridge, one can see numerous light or dark inclusions of various shapes and compositions. Quarry workers call these inclusions “crapauds” (literally “toads”). These inclusions have two possible origins: either they are fragments of surrounding rock that were torn off and incorporated into the magma, or they are the result of imperfect mixing between different magmas. Both types of inclusions occur in equal proportion within the granodiorite.

Paving and Multi-Use Material

Granodiorite is marketed both in France and abroad. It is mainly used for paving roads and urban development, but also in construction (masonry or exterior cladding), interior design, monuments, funeral structures, and sculptures. It is also used as aggregate.

When the Continental Crust Melts

The “blue-grey granite of Louvigné” is not actually a granite, but a granodiorite. It has a light grey color and a coarse-grained texture (grains of 1 to 5 mm). It is mainly composed of plagioclase feldspars (33% by volume), quartz (29%), potassium feldspars (21%), and biotite (16%). Its high aluminum content confirms its origin from the continental crust (resulting from its partial melting). This granodiorite is part of the Cadomian batholith, a large intrusive super-pluton emplaced in Brioverian sediments toward the end of the Precambrian, around 540 million years ago.

Le batholite cadomien est composé de plusieurs plutons dont ceux de Lanhélin (cf. installation 19) et Louvigné