INTRODUCTION
Geografically, the district of Bages belongs to the Central Catalonian depression which is the eastern sector of the basin of the river Ebre. This low lands region borders on the mountain range of Pyrenees by the north, on the Catalonian Coastal range by the east and on the Iberian range by the south-west. During part of the tertiary period, this basin or depression was filled up by sediments that came from the relieves around it. Therefore, sedimentary stones from that period like conglomerates, sandstones, mudstones, limestones and eventually evaporites are found in this area. Most of the actual stones in the district of Bages were laid in the superior Eocene period or in the beginning of Oligocene. Nevertheless, modern detrital sediments are found as well mainly near the rivers. These quaternary sediments constitute gravel deposits.
Rocks are natural, inorganic and solid compounds made up of one or more minerals. In some cases, they also can contain pieces of former rocks or remains of living organisms. The rocks in Bages area are conglomerates, sandstones, mudstones, limestones, evaporites and the different stones in gravel deposits. Unless evaporites, all these rocks contain calcium carbonate; thus the resulting soils also contain calcium carbonate.
CONGLOMERATES
Conglomerates are detrital rocks that are made up basically of round, bigger than 2 cm diameter pieces which are called pebbles. The pebbles are included in a matrix made of sand, silt and clay. All these components are stuck by the cement, which here is always the calcium carbonate. The biggest mass of conglomerates is located in the south and in the south-east of the district of Bages. Conglomerates are also frequent in the rest of the territory, mainly on top of the tabular relieves or within layers of thinner particles rocks.
GRAVELS
Gravels are detrital, non-cemented sediments made up of over 2 cm diameter stones. Although with exceptions, in the area of Bages these fragments are usually round as a result of its transport by rivers. They are usually included in a sandy and clayey matrix. They were laid during quaternary period, therefore they’re the newest geological materials of the region. The gravels are common in the riverbeds and in the fluvial terraces. The fluvial terraces witnesses the level of the rivers some hundreds of thousands of years ago. Some deposits of gravels have been exploited.
SANDSTONES
Sandstones are detrital rocks that result from the cementation of sands which, in their turn, are generally included in a matrix of smaller size particles. The sands are fragments of minerals or rocks with diameters between 2 and 1/16 mm. The cement of the sandstones in the district of Bages is calcium carbonate.
Sandstones of both continental and marine sources are common all around the region. Often, iron oxides dye in red, rusty color the sandstones of continental origin. Sandstones are found whether in thin layers into other rocks (mainly mudstones) or in big, homogeneous packs that usually have been or are currently being exploited. Sandstones were used in most of the ancient buildings of the area because their availability and ease to shape.
MUDSTONES
Mudstones are detrital rocks made up of less than 1/16 mm diameter particles. If these fragments are coherent, then the rock is named siltstone in case of the particles are above 1/256 mm size or claystone in case of below 1/256 mm. If these fragments do not have enough coherence, then the rock is called silt and clay. The mudstones that contain 50% of calcium carbonate on average (35%-65% according to Vatan) are called loams. They’re usually bluish color. The color of mudstones in the area of Bages changes from the bluish gray or yellowish of the sediments of marine origin, to the characteristic reddish of those sediment of continental origin which contain iron oxides.
The extraordinary hollow out of the morphology of Bages is a consequence of the scarce resistance in front of the erosion of the mudstones. Mudstone materials predominate in the farmed plains. Mudstones are used in the manufacture of ceramics and bricks for building purposes.
LIMESTONES
Limestones are calcareous rocks made of, basically, calcium carbonate. The most frequent limestones in Bages are arranged in really resistant to the erosion layers. They come from whether marine or lake origin. Nowadays limestones are mined in quarries as a material for construction while, formerly, they were used to obtain lime. Marine limestones of Bages contain a lot of microfossils because their were originated in past coral reefs.
Travertines, which are also found in the area, are light and porous calcareous rocks made up of calcium carbonate that laid on plants, next to water springs.
EVAPORITES
Common salt or halite, potassium salts (silvita and carnalite) and gypsum are evaporites, these are soluble rocks that come from the precipitation of the salt from ancient seas or lakes. In the district of Bages, there are several outcrops of gypsum and the single outcrop of salt in Catalonia, the Mountain of Salt of Cardona where salt has been exploited since the Roman times. The Mountain of Salt of Cardona is made of mainly common salt, though it contains potassium salts as well. The evaporites of the Mountain of Salt of Cardona raised to the surface in one geological structure called diapir that keeps raising because their plastic behaviour combined with the differential pressure from the materials above them.
Looking at the mountain of salt closely, the thin layers of salt and clay display a tangled picture while its surface looks like a rake of sharp edges and minute channels as a consequence of the solution of salt by rain water. The colourful combination of thin and twisted layers of different materials astonishes the visitor of the mountain of salt of Cardona. Halite, the most abundant mineral, is colorless or white, orange or pink according to its content of different impurities, while mudstone is gray.
THE MINERALS
Minerals are homogeneous, inorganic solids of natural origin with a specific chemical composition (however often this composition is not totally fixed) and they are arranged in atomic structure (crystalline structure). Most of the minerals are produced by inorganic processes, but minerals like aragonite from the shells of mollusks are made by living organisms. Crystal means any solid with crystalline structure, but often the name is applied just to the minerals with a polyhedral shape dispaying the internal atomic order. Minerals with a perfect polyhedral shape are very rare in nature because they require specific conditions of origin. Minerals, usually in small or microscopic crystal shape, are the main components of the rocks. Anyway, different processes of mineralization may concentrate some minerals until growing relevant size crystals.
All rocks in the area of Bages were originated by sedimentation. Therefore, the minerals of the area are not particularly diverse. On the other hand, minerals that are made up of sulfates and carbonates are quite common and minerals of chlorides from the Cardona geological formation are really outstanding.
CALCITE (CaCO3, calcium carbonate)
Calcite is a mineral that crystallizes in several shapes, producing crystals generally white or colorless, although sometimes they can be different colors. Calcite defines the third degree of hardness of Moh’s scale. It cannot be scraped by the nail, but it is easily scraped by a knife. Calcite crystals are often found coating the cracks of CaCO3 containing stones and in caves. Calcite is the main component of limestones that are used in the manufacture of cement and lime.
MALACHITE (Cu2(OH)2CO3, copper basic carbonate)
Malachite eventually appears as a prismatic acicular crystals of the monolithic system or, often, as patinas, powdery coatings or reniform or globular masses. It has a beautiful, characteristic green color, green streak and 3,5 – 4 hardness degree. Malachite is a source of copper and, after its beauty, it is used in jewels.
In the district of Bages, malachite is located mainly in Artés, in the so-called red-bed copper mineralization. The mineralization process consists first in the washing (lixiviation) of copper ions with water and then their precipitation as sulfides when suddenly a reductor level was reached. Finally, the carbonatation of the sulfides turns the copper mineral to malachite and azurite. The process of malachite formation is the real interesting thing in Artés, rather than the size of the crystals or the amount of the deposit. Malachite can be found in Cabrianes (Sallent), Santpedor and Ferrerons (Moià) as well.
AZURITE (Cu3(OH)2(CO3)2, copper basic carbonate)
Azurite produces different kinds of crystals of the monolithic system, but it is commonly found as patinas or reniform or powdery masses. Usually, azurite and malachite are found together, but azurite can turn into malachite by chemical alteration. Azurite has a beautiful Indian blue color, blue streak and 3,5-4 hardness degree. Azurite is a secondary source of copper and it is used as an ornamental stone too.
Although less abundant than malachite, azurite is found in Sallent and Artés, in the so-called red-bed copper mineralization (see malachite) that are located at the bottom of paleochannels (ancient riverbeds) in Artés geological formation.
PYRITE (FeS2, iron disulfide)
Pyrite crystallizes in the regular system, in cubes and pentagonal dodecahedrons of yellow brass color. The hardness of pyrite is 6 -it cannot be grated with a knife but with a quartz crystal-. In Bages, pyrite is found in gray mudstones stratums of marine origin. The crystal size usually is less than 4 millimeters. Often, pyrite builds aggregates that refill ancient galleries that were opened by marines invertebrates -the so-called burrows-. One of the world most important deposit of pyrite is located in Río Tinto (Huelva). Pyrite is exploited mainly to manufacture sulfuric acid and also as a source of iron.
HEXAHYDRITE (MgSO4·6H2O, hydrous magnesium sulfate)
Hexahydrite is a fibrous mineral that coats with white, delicate efflorescences the surface of magnesium containing humid rocks. Hexahidrite has the typically bitter taste of magnesium and it is so soft (hardness 2-2,5) that can be grated with a nail. Manresa is a well known location of this mineral which is usually rare. Hexahydrite is quite similar to epsomite (MgSO4·7H2O) by the appearance and the chemical composition, they only differ from the water of hydration.
GYPSUM (CaSO4·2H2O, hydrous calcium sulfate)
Gypsum is a colorless, white, gray or red mineral, but always with an obvious white streak. Gypsum is soft; it defines the hardness degree 2 -the minerals that can be scraped with the nail- in the Moh’s scale. Gypsum crystallizes in different shapes: translucent sheets with an excellent exfoliation, colorless crystals with acute arrowhead shape, fibers, compact masses made by tiny, white crystals (variety alabaster). The most frequent gypsum is the so-called secondary gypsum that comes from the hydration of anhydrite (Ca SO 4 ). Once more, anhydrite was originated from a primary gypsum.
Most of the gypsum in Central Catalonia deposited in the period Eocene, during the first stages of evaporation of the sea that covered this region. This process ended up with the deposit of halite and silvine of the geological formation Cardona. There is gypsum in the borders of this formation, for instance Artés, Avinyó, Igualada, Òdena and Castellterçol, and also below the salt deposit. Furthermore, there is gypsum in Súria and Sallent that comes from the evaporation of continental lagoons. Gypsum is currently extracted in Igualada and Òdena, two villages in the district of Anoia close to the south-west of Bages. Gypsum was also mined in Súria and Artés, always for construction. The clayey stratums of the Artés geological formation are often crossed by narrow veins of white, fibrous gypsum.
HALITE (NaCl, sodium chloride)
Halite or common salt has a hardness of 2 -can be grated by the nail- and crystallizes in cubes that can achieve big sizes (even more than a cubic meter!). Pure halite is colorless, but the presence of impurities turns it to different colors. In Bages, halite is often reddish because the presence of iron oxides. Most of the Cardona geological formation -a pack of stratums 300 m thick in the unfolded areas- is made up of halite. This formation spreads in the subsoil in a large area of the Central Catalonia that comprises the north of Bages district. Its unique outcrop is the salt mountain of Cardona, where halite is exploited probably since the prehistory. Halite was also mined in Súria.
SILVITE or SILVINE (KCl, potassium chloride)
Silvite, like halite, crystallizes in cubic system and has a hardness of 2. Crystals of silvite are colorless if pure, however often they’re reddish in Bages because the content of iron oxides. Silvite is easily identified by its spicy taste, more or less bitter according to its amount of carnalite (hydrous magnesium and potassium chloride) traces. Silvite is exploited in mines of potash nowadays in Súria, Balsareny and Sallent and, until 1990, in Cardona as well where the mine achieved -1000 m depth. In plain, unfolded areas, silvite is found in two layers, 100 m below the top of the Cardona formation. Usually, centimetric sheets of silvite and halite are blended. Then the rock is called silvinite. The word potash means any potassium containing sedimentary rock. Silvite is used mainly in fertilizers. In the photography, the appearance of a just mined mineral: a mass of tiny, orange crystals of silvine that is crossed by a layer of colorless halite crystals.
CARNALITE (KMgCl3·6H2O, hydrous magnesium and potassium chloride)
Carnalite crystallizes in rhombic system and has a hardness of 2.5, the same as the nail. It usually displays granular structure. Carnalite can be colorless, pinkish or reddish, always with a vitreous gloss. Carnalite is deliquescent, it solves even in the air humidity. Its taste is salty, spicy after the potassium and bitter after the magnesium content. In the sequence of the Cardona geological pack, carnalite is found in a higher position than silvine. Thus, carnalite is the first potassium mineral that appears, however its presence is irregular.
[Oriol Oms, Florenci Vallès and Josep Biosca, with the collaboration of Jordi Badia and Joaquim Sanz]