A hydrothermal solution is an accumulation of water in rock layers , which due to the prevailing pressure conditions can still be liquid at well above 100 ° C, but only up to the critical point of the water at 374.15 ° C. Hydrothermal solutions often contain large amounts of dissolved volcanic gases and minerals , including those that would be completely insoluble at room temperature and normal pressure. These are generally present in the solution as ions and / or complexes , but at lower temperatures also as colloids or sols. Metals are usually dissolved as sulfides and polysulfides .
The cooling of the hydrothermal solutions in fissures and crevices and when they emerge from the rock leads to the deposition of these minerals on the fracture surfaces and on the adjacent soil layers, depending on the pressure conditions, pH value , redox potential, etc., in characteristic mineral associations . These are mostly compact mineral aggregates or fine crystalline layers. In open fissures and in other cavities ( drusen ), coarsely crystalline crystal lawns with free crystal ends can also form.
Hydrothermal solutions are divided into four categories according to their formation temperature : In "highly thermal solutions" ( katathermal ), the minerals precipitate at a temperature of around 350 to 300 ° C, in the " medium thermal range" ( mesothermal ) between 300 and 200 ° C, in “Low thermal solutions” ( epithermal ) between 200 and 100 ° C and finally in the “telethermal range” ( anothermal ) below 100 ° C.
Hydrothermal solutions are essential for the mobilization, transport and concentration of mineral raw materials in buildable deposits . Hans Schneiderhöhn differentiates between three types of hydrothermal deposits:
- Hydrothermal ore and mineral veins in tectonic crevices and rustling zones , which simultaneously serve as routes of ascent and as places where the solutions precipitate.
- Hydrothermal displacement reservoirs . Here the hydrothermal solutions have penetrated the host rock (mostly limestone) and have completely chemically converted it in situ .
- Hydrothermal impregnation deposits in porous host rock.
Gemstones in crevices and druses
Agate , malachite and others are fine-grained, layered excretions from hydrothermal solutions, which can show multicolored bands when the ingredients change. Minor impurities lead to different colors of the quartz crystals , as with amethyst , citrine and other varieties of the base mineral (not with smoky quartz , in which the coloration is caused by lattice defects).
Pleasantly banded pieces, those with interesting optical phenomena and large crystals are processed by the jewelry industry and the arts and crafts . Crystals can be given a cut , gems are cut from banded layers , or the stone is smoothed into a rounded shape. In the trade, suitable material, even if it did not come from a hydrothermal solution, was sometimes referred to as " semi-precious stone ". Today the term gemstone has established itself .
- Martin Okrusch, Siegfried Matthes: Mineralogy: An introduction to special mineralogy, petrology and deposit science . 7th edition. Springer Verlag, Berlin, Heidelberg, New York 2005, ISBN 3-540-23812-3 .
- Werner Lieber: The mineral collector . 6th edition. Ott Verlag, Thun and Munich 1973, ISBN 3-7225-6229-5 , pp. 29 .