Anhydrite

• Anhydritspat

Anhydrite, as Anhydritspat, Gekrösstein and Karstenit or under its chemical name calcium sulfate known, is a common mineral from the mineral class of " sulfates ( and Related ) ". It crystallizes in the orthorhombic crystal system with the chemical composition Ca [ SO4 ] and usually develops coarse-grained, massive aggregates, but also cubical and prismatic crystals up to 20 cm size.

Pure anhydrite is transparent and colorless or white crystalline by lattice defects or much training. It can also be of a bluish, reddish, purple or brown color by foreign admixtures. The stroke color is, however, always white. Visible crystal faces show a vitreous luster, lamellar or granular aggregates, however, more mother- to greasy.

With a Mohs hardness of 3 to 3.5 anhydrite still belongs to the soft minerals that can be scratched with a copper coin. Its density of about 3 g/cm3 corresponds to that of cement.

Special Features

Anhydrite crystals have a good cleavage and therefore often have three mutually at right angles to the gap surfaces. You can set yourself apart from the otherwise externally very similar gypsum crystals.

If anhydrite under constant exposure to moisture, it absorbs water, causing its volume can increase by more than 50 %. Anhydrite is transformed by this water retention in plaster to swell and thereby to.

In mining anhydrite swelling can narrow the tunnel and blow up the adjacent rock layers, as in this true for tunneling, such eg the Engelberg tunnel, Weinberger tunnel or Adlertunnel (CH). In individual cases, this can also lead to elevation of the ground, as is the case for example in Staufen im Breisgau.

Anhydrite in pure form is therefore not suitable as a building material.

Etymology and history

Was first described in 1804 by Abraham Gottlob Werner anhydrite ( 1749-1817 ), the ἄνυδρος the minerals by the Greek word anhydros " waterless " for named.

As Type localities the salt mine near Hall in Tirol in Austria and the potash plant at Leopold Hall in Germany apply.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the anhydrite to mineral class of " sulfates, selenates, tellurates, chromates, molybdates, tungstates ," and there belonged to the department of " water clear Sulfate without foreign anions ", where he with Aphthitalit, glauberite, Kalistrontit and Palmierit formed a distinct group.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the anhydrite in the class of " sulfates ( selenates, tellurates, chromates, molybdates and tungstates ) " and then in the Department of " Sulfate ( selenates, etc.) without additional anions, without H2O ". This division, however, is further divided according to the size of the cations involved, so that the mineral is "With only large cations " to find according to its composition in the subdivision, where it forms the unnamed group 7.AD.30 the only member.

The classification of minerals according to Dana assigns the anhydrite in the class of " sulfates, chromates and molybdates " and there in the department of " sulfates ". Here he is the only member of the unnamed group 28.03.02 within the sub-division of finding " water outdoors acids and sulfates (A2 ) XO4 ".

Modifications and varieties

• Angelite is the trade name for a translucent, gray-blue- purple anhydrite crystal.
• As Vulpinit grained anhydrite unit is called.

Education and Locations

Anhydrite is a sedimentary mineral and often forms as a product of evaporation of sea water, the temperature must be above 35 ° C. At lower temperatures, gypsum forms. It may be because it is insoluble in water, deposited directly from superheated seawater or arise desselbigen at evaporation together with gypsum and halite. Other accompanying minerals include calcite, celestite, dolomite, magnesite, polyhalite, sylvite and sulfur.

Anhydrite takes on short-term effects of moisture on no water. But he is under constant exposure to moisture, so he slowly transforms into gypsum. This leads to an increase in volume, which sometimes can act Busting. Thus, anhydrite layers in the Mining by mine water " grow ", the tunnel narrowed ( dwarf holes Quellungshöhlen ) cause or heave. This is to be observed, for example in road and tunnel construction ( cf. the resulting difficulties eg Weinberger tunnel ).

Could Worldwide anhydrite so far (as of 2011) are detected at around 1100 localities. Noteworthy in light of extraordinary crystal finds, among other things Naica, Chihuahua (Mexico), were found in the geodes with up to 20 cm long Anhydritkristallen and Wieliczka in Poland, where up to 2 cm large crystals were revealed.

In Germany anhydrite occurred among others in the Black Forest, near Heilbronn, garbage home and the Swabian Alb in Baden- Württemberg; in Franconia and Upper Bavaria; many places in Hesse and Lower Saxony; at Aachen, Rheinberg and in the Sauerland region of North Rhine -Westphalia; in the Rhineland-Palatinate Eifel; at Saarbrücken and Saarlouis in Saarland; in the Harz Mountains of Lower Saxony to Thuringia; in the Ore Mountains and near Zwickau in Saxony; at Bad Segeberg in Schleswig -Holstein and in Gera, in Kyffhaeuser and in the Thuringian Forest on.

In Austria, the mineral is found among others in Pöttsching in Burgenland, in the Gail Valley Alps and the Carnic Alps in Carinthia, at the Semmering in Lower Austria, in several places of Salzburg and Styria, in North Tyrol and Upper Austria.

In Switzerland, anhydrite at several locations in the canton of Valais in Felsenaustrasse / Leuggern and Schafisheim in the canton of Aargau, Leissigen in the canton of Bern, in the Grisons valleys Val Cristal Lina and Val Milà, in Airolo and Lavorgo in Ticino, in the salt and sulfur mines near Bex and Sublin in the Canton of Vaud, and at several points during the construction of the Gotthard tunnel.

More Locations are Afghanistan, Algeria, Argentina, Armenia, Australia, Bolivia, Brazil, Bulgaria, Chile, China, Denmark, Ecuador, France, Greece, Guatemala, Indonesia, Iran, Ireland, Iceland, Israel, Italy, Japan, Canada, Kazakhstan, Qatar, Colombia, Democratic Republic of the Congo, Cuba, Lithuania, Madagascar, Malta, Morocco, Mexico, Mongolia, Namibia, New Zealand, Netherlands, Norway, Pakistan, Panama, Papua New Guinea, Peru, Philippines, Romania, Russia, Saudi Arabia, Sweden, Serbia, Zimbabwe, Slovakia, Slovenia, Spain, South Africa, Taiwan, Thailand, Tunisia, Turkey, Ukraine, Uzbekistan, the United Kingdom ( UK), the United States of America (USA).

Even in rock samples of the Mid-Atlantic Ridge, the Central Indian Ridge, the Bismarck Sea, the China Sea and the East Pacific Rise and outside the earth on the moon ( Mare Crisium ) anhydrite could be found.

Anhydrite can also arise through calcination of gypsum. At temperatures around 100 ° C in the gypsum remains something Crystal water, thereby hemihydrate is produced; at higher temperatures, the whole crystal water is removed and there is anhydrite.

Crystal structure

Crystallized anhydrite in the orthorhombic space group Amma ( Raumgruppen-Nr. 63) with the lattice parameters a = 6.99 Å; b = 7.00 Å and c = 6.24 Å and four formula units per unit cell.

Use

Anhydrite is processed in powder form to adhesive for tiles, however, must be a " stimulator ", usually potassium sulfate ( K2SO4 ) or lime ( CaO), are buried. The Stimulator, its proportion is 3-15 %, accelerates water retention, which converts to anhydrite to gypsum. The conversion of anhydrite to gypsum occurs, but only about 65 %, the plaster provides for the fast drying and the anhydride as a scaffold for the high strength. Such anhydrite are air-hardening, non-hydraulic binder of natural or synthetic anhydrite. They are similar in their physical and chemical properties with plaster. Calcium sulfate binder is used for example on housing for the production of calcium sulfate or calcium sulfate screed floor screed.

Powdered anhydrite is a component of cement and is also used in the production of sulfuric acid and aerated.

The purple - blue colored variety Angelite is used as a gemstone.