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Calcite Aragonite Eccentric Speleotherms

Last Updated: 13th Jan 2015

By Rui Nunes

Calcite Aragonite Eccentric Speleotherms

In the Sesimbra region there are some caves of reasonable dimension and great beauty. After seeing some photos with beautiful cave calcite and/or aragonite formations, at least curious, I made some research to answer to my ignorance on the subject.

So, here I leave a brief note about Eccentric Speleotherms.

Almost always, the caves are formed in carbonated rocks (limestones and dolomites), due to a dissolution process known as karsting. In the exterior too, this transformation leads to a significant midelling lappies or lapis, dolines, potholes, shafts... The mecanism involves the union between the basic minerals comprising these rocks, calcite or dolomite, with certain natural acids, to form a soluble product, calcium bicarbonate, which is transported by water, leading to the formation of gaps. The most common acid involved in this reaction comes from the joining of atmospheric carbon dioxide and water, in other words carbonic acid. In other cases, of organic compounds brought down by water on its way towards the interior or of products generated by the alteration of metal mineralization in the environment which will contribute a fair amount of components that are also going to corrode the carbonated rocks in the subsoil.

The formations embellishing these caverns are generally termed as speleotherms. But straight away we can distinguish between two types of rock formations: the classical or gravitational and the eccentric or helictite. The former are, in all cases, to do with water falling down by gravity. As it infiltrates from the surface, water penetrates the mountain, filling up with soluble products: calcium bicarbonate, iron oxides, metal salts, etc. When the water reaches the roof of a gallery, it falls but, at the same time, undergoes decompression, precipitating part of its load, thus forming stalactites. The other minerals transported reach the floor, building up a deposit which, in almost all cases, covers a larger surface area since the water splashed a fair part of the surrounding area, thus creating stalagmites. Over time, the rocks deposit in the roof and floor tend to join together, and here we have pillars. In the case of shields, tubes, water seeps through a crack, which is not necessarily rectilinear, creating more or less attractive forms. But this does not only precipitate calcium carbonate in the form of calcite or aragonite, but also other chemical products transported by the water. The result is, at time, a whimsical petrifaction tinged by the most unexpected tones.

The other large group of speleotherms that can form inside caverns are the eccentric type. In this case, the most noticeable morphological characteristic is one of anarchy. The threads of calcite or aragonite are interwoven, making whimsical knots beyond any restriction imposed by Earth's gravity. Both the roof, the walls and the floor brim over rocks deposit of this kind, in all cases being associated with large amounts of clay, taking up airlight pockets in the underground system. There must have been an atmosphere of absolute stillness in some caves, where infinite numbers of microdrops of water loaded with bicarbonate ions CO3H- and calcium, Ca2+ would float.

Clays are formed by a series of complex silicates in the phyllosilicate group, accompanied by ions from diverse metals, particularly from iron, which can generate a changing electromagnetic field in the cavern able to mobilize the bicarbonate-calcium ions, causing them to join together and, subsequently, the anarchic precipitation of calcite or aragonite. Another theory postulates a different genesis, relating the eccentric rocks with the arrival in the airlight cavity with tiny amounts of water seeping through a minimum thickness of overflow. As the load precipitates, CaCO3 seals off the small hole through which the water travelled, so that the rock deposits move out laterally to grow, giving rise to an endless number of unlikely forms.

Photos by Francisco Rasteiro, NECA - Núcleo de Espeleologia da Costa Azul

Article has been viewed at least 10741 times.


Rui, thank you for this interesting article and pictures.

Some points I observed in caves and mines are:
- that most (all?) of the stalactites are showing a channel inside, in which the water gets transported to the end of the stalactite by gravitation. Even larger stalactitic "flags" similar to your second picture are showing this channel. In many of the cases I've seen, these channels end with a drop of water and small scalenohedric crystals of calcite growing from the outside towards the center of the channel. Excentrics do not show this channel. Am I correct?
- If you break an excentric consisting of calcite you will find only one perfect cleavage face all through the diameter, means these forms are single crystals. If you break a stalatite you will find a radial and spherical growth showing lots of crystal faces.
- Most of the stalactites and excentrics (except of flos ferri) consist of calcite. In some cases I've found stalactites covered by aragonite crystals in sub-mm size.

Finally: Please do not break off those in natural caves!


Stefan Koch
8th Mar 2012 1:50pm
Stefan, thanks. Not being an expert will try to explain:
Stalactites are formed starting from ceiling cavities growing upright and can be of two different types: stalactites and tubular stalactites.
The tubular stalactites formed from a water exit point on the ceiling (a fracture, crack or fail) through which the leaking water will precipitate the calcite. These tubular always have an internal channel, ie, they are hollow. However, at any time the channels may be blocked and thus ceases to be internally supplied continuing to grow only by mineral precipitation of the water that flows out.
Moreover, the stalactites result from the fact, under certain conditions, water seepage from the ceiling go to a specific point due to any irregularities in the shape of the ceiling and then forming a precipitate calcite stalactite without internal channel. In this case, the stalactites are formed into external "films" resulting that, almost always, they have a larger diameter than the tubular, such as the name implies, have the appearance of thin tubes, more or less long. Note that when a tubular is blocked stalactite may begin to grow as a stalactite, ie through the external water seepage along the same.
The eccentric are very particular forms. They are very thin, grow in all directions of space and its origin is controversial.
They consist mostly of very fine tubes of calcite or aragonite growing from the ceiling, walls or floor, defying the laws of gravity. Its feed channel is so thin that the water flows by capillary action. These forces are greater than the force of gravity and as a result these speleotherms develop in either direction.

Rui Nunes
7th May 2012 11:15am

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