MINERAL DEPOSITION IN CAVES

           In corrosion, erosion, dissolution, transportation, rock crumble, produce the development of the cave. But, the continuous process occurs to our eyes. Several materials modify specially in the soil of the cave, coming either from the internee as external midst; internal, firs deriving from the calcareous dissolution and crumbled blocks and external, mud, firs, sand, water, leaves, roots, vegetables rests, animals and detrits, in general, are making part of the soil of the cave, specially when the cave is submited to variations of inundation of river, giving a dinamic aspect to the aparently static interior of the cave.

           The speleothems (speleo = cave; thems = deposit) are deposits of minerals in caves formed basically by chemical process of dissolution and precipitation, which gives a more permanently character or even structural. In a general and simplified form, the speleothems have their origin in the external and superior midst of the cave. The water, combined with carbonic gas of the atmosphera and soil, form the carbonic acid which invade in the soil and reaches the calcareous rock, penetrating in it by its natural orifices, it provocates a chemical reaction of dissolution of the calcium carbonate, forming a calcium bicarbonate, that after cross all the roof of the cave, emerges in its roof.

           The drop of this aqueous solution hangs on the roof until volume and weight be enough to overcome the surface tension and falls . In this period, in the space of the cave, that solution is exposed to environmental conditions which are very different from the previous, when it passed underpressure to the narrow fissures of the rock. This change of conditions – greater ventilation, changes of temperature and pH, reduced CO2 pressure, humidity of air – creates chemical instability of the solution by the liberation of carbonic gas in the environment of the cave with a precipitation of part of the dissolved carbonate. Thus it is formed on the surface of the drop, greater instability area, the first calcite crystals (calcium carbonate); these, organizing themselves during the period of contact of the water with the roof, form an initial crystalline ring which will serve as a base for a future stalactite. Drop by drop, a hollow tubular stalactite grows in a downward direction. The drop, when it at last falls, carries with it a solution of carbonate which slowly forms a succession of layers on the floor immediatly below, and which become a new structure. The structure formed on the roof is mostly tubular and hollow, called stalactite, and the form in the floor is a stalagmite, but depending on the physical and chemical – climate conditions (humidity of air, environment temperature, pH, saturation, ventilation, CO2 pressure, volume of water flowing out, water temperature, wall inclination, others minerals on the precipitation) of the cave, these speleothems can assume other forms and others properties that will be described later.

1. Acidulation of water (formation of carbonic acid)

H2O     +     CO2       =        H2CO3

                      Water              Carbon dioxide         Carbonic acid                     

2. Dissolution of rock by carbonic acid:

H2CO3                +               CaCo3            =            Ca(H CO3)2                     

    Carbonic acid                                Calcium carbonate                         Calcium bicarbonate                    

3. Inversion of the equation and precipitation of calcite

  Ca(H CO3)2       =       CaCo3     +      H2O    +    CO2 

Calcium carbonate                       Calcite                      Water              Carbon dioxide

Little changes can occur in this chemical reaction, if the attacked rock is not carbonatic.

In calcareous caves, prevail the mineral of calcium, as the calcite, aragonite and gypsite.

Calcite (calcium carbonate) – is a white or transparent mineral in its pure form of rhombohedral crystallization, the most frequent minerals and that more types of speleothems are formed inside the cave.

Aragonite (orthorhombic calcium carbonate) – is a polymorph of calcite but has a different type of crystallization (orthorhombic). It is much more soluble than calcite, and the process of precipitation is more difficult, thus it is relatively rare. Speleothems formed from aragonite, in certain conditions, can change in calcite. The forms created by aragonite are usually more delicate and rare.

Gypsite (calcium Sulphate) – it is not as common as calcite and aragonite. These usually occurs like flowers, crusts of elongated and twisted crystals or irregular heaps of extremely fine transparent crystals. It is probably formed by dissolution and transportation of the gypsum in limestone (S2Fe).

Other minerals are found in Brazilian caves, as the silica, opal, chalcedony, quartz and other elements as iron, copper and manganese.

In spite of the white coloration be formed by calcite, aragonite, gypsite, silica, quartz, other colors can appear in the precipitations of speleothems, by stainent depositated between the crystals or wrapping the surface of the piece, or the substitution of the calcium ion for another, as the copper, in the own crystalline structure of carbon. As an exemple, the oxides of iron – which produces a coloration that goes from orange to red; oxides of manganese – that goes from blue to black; copper – with a bluish color ; malaquita – green blue; nickel – yellow or green. In Brazil, it has been identified about 20 minerals in caves.

As has been seem in the exemple, in the formation of stalactite-stalagmite-column, the whole process of calcite precipitation is based on dripping. But, as it was already seem, from the begining of the eclosion of the drop on the roof of the cave, several factors can alterate the precipitation and deposition process of the mineral, making beyond gravitational dripping, can also occur other forms as: flow – linear or laminar; spray, exudation, precipitation in a liquid environment, and flocculation, among others – which will be translated in the formation of speleothems of different forms and aspects.

As an exemple, it can be considered some variation:

•         If the roof is inclinated, the drop, other than drop, it can flow out on the inclinated surface, forming a drappering;

•         If the water flow is too low and the evaporation level is high, the precipitation can occur against gravity, forming helictites and heligmites;

•         If the precipitation occurs inside the liquid volume, it can form rimstones, pearls and volcanoes.