Speleothems are mineral deposits of various shapes and chemical and physicochemical origin found in caves.
1. Chemical composition
The amount of minerals found in speleothems is very large, among other salts, we have carbonates, chlorides, nitrates, phosphates, silicates and sulfates, some oxides and hydroxides. The vast majority of speleotems are calcium carbonate (CaCO3), which in the forms of calcite and aragonite make up about 95% of the known speleothem mineral deposits.
Sizes range from microscopic crystals with a microgram mass to stalagmitic sets of tons and hundreds of cubic meters.
In addition to shapes, colors are what draw the most attention to speleothems. Since the vast majority of speleothems are made of CaCo3 in the form of calcite or aragonite, and these, when pure, are white, this is the color of most speleothems. However, the colors vary from white due to impurities present in the deposition of calcium carbonate or the presence of other different compounds.
It is commonly said that yellow is due to the presence of iron or sulfur; red, orange, cream or brown is due to the presence of iron; blue and green due to copper; black and gray due to manganese.
4. Transparency, translucency, brightness and sparkle
Transparency: It is noted in small mass speleothems such as small dog teeth, needles and helictites. It is a characteristic of mono-crystalline portions of the crystallized masses, which allows the unreflected part of a light beam (incident on the surface of a speleothem) to refract within its mass to be poorly absorbed and emerge from the opposite side.
Translucency: Speleothems formed exclusively by crystallization are made up of millions of transparent micro crystals (even those of large mass), where an incident light beam is refracted from crystal to crystal being almost completely absorbed by the crystalline mass, with a small part being able to. cross it.
Brightness: Since speleothems are usually crystalline, their outer surfaces are made up of millions of flat micro faces, and have a high reflection index, making them bright when illuminated (the effect is intensified if the speleothems are wet).
Scintillance: It is caused by punctual reflection on the surface of the speleothems. Each mini-face shines when the beam of light that strikes it reflects, striking the eye of the beholder, the effect is intensified when moving the light source. This is what happens on the ‘star floor’.
5. Forming chemical reactions
The chemical reactions involved in speleothem formation are varied, sometimes complex, but always occur as a function of chemicals deposited or crystallized during speleothem growth.
Firstly there is the formation of carbonic acid, which is extremely weak, this occurs by the hydration of CO2 molecules in the earth’s atmosphere by rainwater, watercourses, soil interstices, infiltration water, etc. This acid ionizes and attacks the limestone of the rock with the formation of calcium bicarbonate, which is highly soluble in water.
In this way, the acidified water erodes the limestone, which is being transported in the form of bicarbonate, through fractures and cracks in the rock mass. When the aqueous solution points in the emptiness of a cave, the chemical process is reversed, the bicarbonate releases carbon dioxide in consequent formation of water and carbon dioxide, which is insoluble precipitates, originating and growing the speleothems.
A set of physicochemical variables (concentration, temperature, environment, solution flow, carbon dioxide and water vapor pressure, air circulation, luminosity, etc.) govern limestone corrosion and carbonate deposition in the formation and speleothem growth.
More particular and little known conditions govern the form of calcium carbonate deposition, in the most varied physical arrangements, some acceptable, but others are totally surprising and sometimes unexplainable.
1) Stalactite: It is the best known among the speleothems. It is the generic name for all the speleothems attached to the ceiling of the caves. Every stalactite has a central conduit, or at least a trace of it, according to which stalactite evolves. Regardless of their external appearance, they all have the same origin, as long as a continuous droplet is formed on the ceiling, precipitation of a thin calcium carbonate disc will occur, shaping the stalactite dropwise.
2) Curtain: These are speleothems that appear on sloping ceilings and may extend over the walls. Curtain growth always occurs by the radial and linear crystallization of calcium carbonate in the form of fillets on fillets.
3) Stalagmites: Together with the stalactites, they form the most representative set of speleothems. They are born and grow normally by dripping radially crystallized solution from the floor. They form large compact masses of various shapes.
4) Columns: Columns originate from the evolution of stalagmites and stalactites, which come to be welded into one piece, when dripping ceases and the growth of the speleothem occurs only by lateral thickening.
5) Waterfalls: It is the generic name of the formations originated by the flow, through inclined or vertical walls, of water emerging from the ceiling or wall. If the flow is voluminous and continuous, there is a micro-travertine coated surface, if the flow is intermittent, small and saturated solution, there is a calcite micro-monocular coated surface.
6) Travertines: These are formations derived from damming of waters, and their sizes vary from one whose volume barely fits a drop of water, to what is a true pool with a few meters deep and others of superficial extension.
7) Ooids: This is the name given to the family of speleothems that grow loose in travertine dams or in wells that are subject to continuous high frequency dripping, the turbulence of the flow causes the pearls in the nest to move round. Growth occurs by crystalline layers in radial direction. They have a smooth ceramic appearance and occur in clusters called litter. The embryo of each pearl is any crystal or loose particle in the pool that functions as a crystallization germ.
8) Volcanoes: Conical shaped speleothem and concave top, spongy consistency. It is formed exclusively submerged and limited in height by the water level of the travertine in which it is located.
9) Rafts: It is one of the floating speleothems and they are loose. They are formed by the crystallization of calcite on surfaces free of stagnant saturated water, fluctuates due to surface tension and any disturbance causes it to sink.
10) Marquises: When the rafts touch the edges of a reservoir wall, they end up welding and increasing their thickness by lower crystallization. If for some reason the water level drops or the dam dries up, the rafts that have been suspended horizontally are now called marquises.
11) Dog Teeth: Each reasonably sized single crystal (a few inches) that leads to the center of a geode, or the inner wall of a travertine dam, is called a dog’s tooth.
12) Star floor: These are crystal-coated floor surfaces or wall flaps that, with their multiple planes or parallel microfaces, clutterly reflect any incident light. Whatever direction of the incident light, the observer will see it reflected in hundreds of positions. The impossibility of fixing luminous points and their appearance of movement is such that it gives the idea of scintillation and even takes the balance of the observer.
13) Needles: These are mono-crystalline formations of white or transparent calcite, with a few millimeters in diameter and a few centimeters in length. They occur by covering the inside walls, with free ends, in recesses of curtains.
14) Calcitic sand: These are deposits of calcite monocrystals in supersaturated solution, subject to drip turbulence. Constant accumulation gives rise to a white sand of crystalline calcite, rarely found.
15) Moon Milk: It is a fixed speleothem of pasty consistency. Depending on the degree of moisture, it may look like dry talcum powder or a damp mass. Its probable origin is related to the attack of microorganisms on calcite.
16) Helictite: They grow on the floor, ceiling, walls, over other speleothems, cover columns, etc. They have varying shapes, but all play with gravity, some are threadlike, worm-like, longer than 1 m and less than 1 cm in diameter, can form intertwined sets of twisted crystals, resembling a jellyfish. Its composition is calcite or aragonite and is usually white.
17) Calcite Flowers: Carbonate precipitates as calcite, crystals are cylindrical, curved and sometimes branched.
18) Aragonite Flowers: In these the carbonate precipitates in the form of aragonite, the crystals are elongated, rectilinear, with fluffy ends and radiate from a common base, like a bundle of loose needles and standing in the air. They are of rare beauty and fragility.
19) Gypsum flowers: The flower is formed by a set of fibrous-looking crystals with juxtaposed, curved bases that are generally subdivided into independent bundles.
20) Gypsum needles: The crystal beam has a diameter of the order of mm2, the growth is rectilinear, reaching up to decimeters in length, composed of several thinner needles.
21) Angel Hair: The crystal beam is multiple, and each unit of the curly beam grows independently of its neighbors, reaching decimeters in length. The beams hang, swaying to the taste of air movement.