# plaster

plaster
Gypsum crystal specimen from Friedrichroda , Thuringia
General and classification
other names
• Plasterboard
• Calcium sulfate dihydrate
chemical formula Ca [SO 4 ] • 2H 2 O
Mineral class
(and possibly department)
Sulphates (selenates, tellurates, chromates, molybdates and tungstates)
System no. to Strunz
and to Dana
7.CD.40 ( 8th edition : VI / C.16)
06/29/03/01
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group A 2 / a (No. 15, position 4)
Lattice parameters a  = 6.52  Å ; b  = 15.18 Å; c  = 6.29 Å
β  = 127.4 °
Formula units Z  = 4
Frequent crystal faces {010}
Twinning very often contact twins according to {100} dovetail, Montmartre, penetration
Physical Properties
Mohs hardness 2
Density (g / cm 3 ) measured: 2.317; calculated: 2.31
Cleavage very perfect after {010}, clearly with fiber formation after {111}
Break ; Tenacity shell-like
colour colorless, white, yellowish, reddish, gray, brown
Line color White
transparency transparent to opaque
shine Glass gloss, pearl gloss, silk gloss
Crystal optics
Refractive indices n α  = 1.519 to 1.521
n β  = 1.522 to 1.523
n γ  = 1.529 to 1.530
Birefringence δ = 0.010
Optical character biaxial positive
Axis angle 2V = measured: 58 °, calculated: 58 ° to 68 °
Other properties
Chemical behavior Slightly soluble in water

Gypsum , geologically also known as gypsum spar , is a very common mineral from the mineral class of " sulfates (and relatives)" with the chemical composition Ca [SO 4 ] · 2H 2 O and thus, chemically speaking, hydrated calcium sulfate or calcium sulfate dihydrate.

Gypsum crystallizes in the monoclinic crystal system and develops mostly tabular or prismatic to needle-like crystals , but also granular to massive aggregates . In general, plaster of paris is colorless or white. However, it can take on a yellowish, reddish, gray or brown color due to the absorption of foreign ions or admixtures of various types ( sand , bitumen ). However, its streak color is white.

Most of the time, the mineral gypsum consists of mono-mineral rocks with only small admixtures of other minerals such as anhydrite , quartz or clay minerals, which are also referred to as gypsum or gypsum stone.

## Etymology and history

Iridescent, transparent gypsum crystal specimen (selenite) from Lubin, Poland

The name plaster is derived from the Greek word γύψος gypsos (baked plaster, chalk), which in turn was adopted from the Semitic language area. The Latin word is gypsum . Other ancient, but not always synonymous, names for gypsum are selenites (moonstone), alabastron and lapis specularis (mirror stone ). In German-language encyclopedias of the 18th, 19th and 20th centuries, the term "Gyps" and corresponding compounds are used.

Plaster of paris was used as a building material as early as the Neolithic Age . As early as 7000 BC In the 2nd century BC, plaster of paris was used to decorate the interior of the city ​​of Çatalhöyük in Asia Minor . In the cuneiform scripts of the Sumerians and Babylonians there are references to the use of plaster, also in Jericho (6000 BC). From 3000 BC In Uruk and later in Egypt, plaster of paris was also used as a mortar, to which lime or stones were added as impurities or for stretching. For example, on the Sphinx (2700–2600 BC), a calcareous plaster mortar was used for certain work. Translucent disks made of alabaster were also known to the Egyptians. The Minoan culture used plaster of paris and alabaster instead of marble as flooring or wall covering and as a building block (Palace of Knossos , 2100–1800 BC, and Palace of Phaistos ), and the Greek naturalist Theophrastus of Eresos described the manufacture of in a treatise Plaster. In Greece, gypsum was also used for building ornaments on houses because it was easy to work with .

The Romans only used plaster of paris for ornamentation indoors, as they were familiar with the much more durable lime for the outdoors.

In Europe, the use of gypsum increased again from the 11th century, gypsum was used for grouting masonry and for lining interior walls and from the 17th century for stucco work . Gypsum is extracted and burned in gypsum works .

## classification

Already in the outdated 8th edition of the mineral classification according to Strunz , gypsum belonged to the mineral class of "sulphates (selenates, tellurates, chromates, molybdates and tungstates)" and there to the section "hydrous sulphates without foreign anions ", where it was named after "gypsum Series "with the system no. VI / C.16 and the other member Ardealit as well as in the appendix Bassanit and Hoch-Bassanit .

In the Lapis mineral directory according to Stefan Weiß, which, out of consideration for private collectors and institutional collections, is still based on this old form of Karl Hugo Strunz's system , the mineral was given the system and mineral number. VI / C.22-20 . In the “Lapis system”, this also corresponds to the section “Hydrous sulfates, without foreign anions”, where gypsum, together with ardealite, bassanite and rapidcreekite, forms an independent but unnamed group (as of 2018).

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and was updated by the International Mineralogical Association (IMA) until 2009, also classifies gypsum in the category of "sulfates (selenates, etc.) without additional anions, with H 2 O". This is, however, further subdivided according to the relative size of the cations involved , so that the mineral can be found according to its composition in the subsection “With only large cations”, where it is the only member of the unnamed group 7.CD.40 .

The systematics of minerals according to Dana also assigns gypsum to the class of "sulfates, chromates and molybdates" and there in the category of "water-containing acids and sulfates". Here he is the only member of the unnamed group 06/29/03 within the subdivision of " Water-based acids and sulphates with the general formula AXO 4  • x (H 2 O) ".

## Crystal structure

Perfect, transparent gypsum crystal, viewing direction on the b-axis

Gypsum crystallizes monoclinically in space group A 2 / a (space group no. 15, position 4) with the lattice parameters a  = 6.52  Å ; b  = 15.18 Å; c  = 6.29 Å and β = 127.4 ° as well as four formula units per unit cell .

## properties

Splitting off of crystal water with CaSO 4 in the DTA

### Physical Properties

Gypsum has a very low Mohs hardness of 2 and, along with halite, is a standard mineral on the Friedrich Mohs hardness scale . Its density is between 2.2 and 2.4 g / cm³ and, in contrast to the often associated mineral halite, it is only sparingly soluble in water. The solubility in water is 2.1 g / l under normal conditions , while that of halite is 358 g / l. Of pure aqueous solution of calcium sulfate is crystallized below 66 ° C always as gypsum, above 66 ° C as anhydrite . In the presence of other ions, for example sodium , the solubility equilibria shift.

### Chemical properties

When heated, the going crystal water lost (TG curve = mass loss = onset of dehydration, peak = Maxima onset of the reaction), and at first it creates a hemihydrate (also hemihydrate, plaster of Paris or Bassanite called) with the chemical formula CaSO 4  • ½ H 2 O, with further loss of water the insoluble anhydrite II (CaSO 4 ) is finally formed via the soluble anhydrite III , the latter two are simply called anhydrite in mineralogical terms .

### Rock formers

Under special natural circumstances, gypsum can be subject to a rock-forming process. Due to the evaporation of calcium sulphate seawater, gypsum and anhydrite precipitate in the early phase of carbonate separation. Primarily gypsum sediments. The rock that forms in larger layers or aggregates is counted in petrography to the group of evaporites and is also known under the cultural term alabaster . The genesis leads to cryptocrystalline or crystalline formations with a grain size down to the centimeter range.

In the vicinity of such deposits, crystalline new formations of the mineral gypsum, called Marienglas , can arise .

## Varieties and modifications

Swallowtail twin from Nordhausen in the Harz Mountains; exhibited in the Mineralogical Museum of the University of Bonn
Bird sculpture made from alabaster silk spar
Sand rose

Plaster of paris comes in solid form, in fine-grained form as colorless, white, yellow, red or gray alabaster , as well as fine- fiber plaster of paris . For the latter, the term silk spar or, more precisely, alabaster silk spar and occasionally the term atlasspat is in use. However, the name atlaspat is inconsistent and is also used for fine-fiber calcite with a silk gloss.

Alabaster eyes are made from calcium sulphate, which collected in individual places within a host rock before it had solidified and then later hardened into alabaster balls. In addition, there are sometimes see-through crystal tablets known as Marienglas or Fraueneis ( selenite ).

The mineral is found in different crystal forms: The crystals are often very large, plastically flexible, completely fissile, thick-tabular, often curved, sometimes twinned; on the other hand is also fused gypsum rosette-like as so-called sand Rose , gypsum Rose or Desert Rose before.

A variety of plaster of paris, which is associated with potassium sulfate and magnesium sulfate , is misleadingly referred to as polyhalite . It occurs in the rock salt deposits of Staßfurt , Berchtesgaden and Bad Ischl .

## Education and Locations

The gypsum deposits in Germany are predominantly evaporites , which means that they were created by crystallization from mineral-oversaturated seawater (see also Zechsteinmeer ). In the meantime, the plaster of paris has often been converted to anhydrite due to sedimentary load and later hydrated again. However, gypsum is also found as a weathering product of sulfidic ores and in volcanic chimneys (so-called white smokers ), where it can be formed by the reaction of escaping sulfuric acid with limestone . The natural deposits are mostly provided with admixtures that favor a parallel development or successive formation of different minerals ( paragenesis ). In paragenesis, for example, gypsum occurs with anhydrite, aragonite , calcite , celestine , dolomite , halite and sulfur .

Gypsum is widespread and so far (as of 2015) over 6600 sites are known. He performed particularly frequently in Algeria , Argentina , Armenia , Australia , Belgium , Bolivia , Brazil , Bulgaria , Chile , China , Germany , France , Greece , Indonesia , Iran , Ireland , Italy , Japan , Canada , Kazakhstan , Madagascar , Morocco , Mexico , Namibia , Norway , Austria , Peru , the Philippines , Poland , Portugal , Romania , Russia , Sweden , Switzerland , Slovakia , Spain , South Africa , the Czech Republic , Turkey , Hungary , the United Kingdom (Great Britain) and the United States (USA).

In Germany, the mineral can be found in the Neckar-Odenwald district (around Mosbach ), near Osterode am Harz , Eisleben in Saxony-Anhalt , Borken near Kassel and in the Segeberger Kalkberg , as well as part of the Grabfeld formation ( Gipskeuper ) in the Steigerwald , the Frankenhöhe and north of the Swabian Alb . Here it was mostly formed by hydration of existing anhydrite during the Pleistocene glacial periods and is therefore preferably located on exposed western sides.

In Austria there are deposits in Preinsfeld near Heiligenkreuz , Puchberg am Schneeberg , Wienern am Grundlsee , Spital am Pyhrn , Moosegg near Golling , Abtenau and Weißenbach am Lech .

The Naica mine in Chihuahua (Mexico), where giant gypsum crystals of up to 15 meters in length were discovered in various caves, is known for its extraordinary gypsum finds . In the Mina Quien Valley Pensara ( Mina Rica ) near Pulpí in the Spanish province of Almería , miners found an oval giant geode with a diameter of 1.8 × 1.7 meters and a length of 8 meters (internal dimensions), which averaged half a Meter-long Marienglas crystals is lined and is called the "Geode of Pulpí" ( Pulpí-Geode for short ).

Furthermore, gypsum could also be detected in mineral samples from the seabed of the Barents Sea (Arctic Ocean), the Mid-Atlantic Ridge , the Central Indian Ridge, as well as in the Bismarck Sea (Pacific Ocean) and the East Pacific Ridge .

Outside the earth, gypsum was detected by probes on Mars , more precisely at Juventae Chasma in the Valles Marineris , in the Terra Margaritifer and Yellowknife Bay in the Aeolis quadrangle and in the Endeavor crater in the Meridiani level.

## Composition of various building materials that are traded as plasters

Source:

material Natural gypsum (Trias, Keuper ) Natural anhydrite (Trias, Keuper) Flue gas gypsum (REA gypsum) Phosphorus gypsum Fluoroanhydrite (neutralized)
Calcium sulfate dihydrate 95 0.5 98 96 0
Calcium sulfate (anhydrite) 1 96 0 0 96
Calcium carbonate 1.5 1.5 1 0 0
Magnesium carbonate 1 1 0 0 0
Sand and clay 1.5 1 1 2 1
otherwise. accompanying substances no no Calcium sulfite 1% phosphates, 0.5% fluoride, 0.5% strontium sulfate , heavy metals 1.5% fluoride, 1.5% potassium and zinc sulfate, traces of calcium hydroxide
PH value 6.7 7th 6.7 2.9 12

## Chemical production of plaster

### Historical

Gypsum distillery , Théodore Géricault , 1822–1823

In the Middle Ages, rock containing gypsum was mined in quarries or mined, sorted and further crushed in crushing mills so that it could be fed into the burning or cooking process. The gypsum distilleries operated kilns or pit ovens that were fired with wood or peat . The plaster of paris was then finely ground in a plaster of paris mill. Another method consisted of making a fire in the tunnel and then knocking out the plaster of paris. → Schleitheim plaster museum

These activities were mostly done by farmers or millers during the period of underemployment. Depending on the purity and fineness, a distinction was made between building plaster, screed plaster and stucco plaster.

### Industrial

Because calcium sulphate is a secondary product in many chemical processes (usually in the form of gypsum) , for example in the production of citric acid , tartaric acid and oxalic acid , targeted industrial production on a large scale is unnecessary. The so-called phosphogypsum formed in the production of phosphoric acid is u. a. contaminated with uranium and a problem waste . The classic process is precipitation from sulfuric acid water with milk of lime or limestone :

${\ displaystyle \ mathrm {H_ {2} SO_ {4} + CaCO_ {3} \ longrightarrow CaSO_ {4} + H_ {2} O + CO_ {2}}}$

Even Goethe , a passionate scientist and chemist, described this process in his novel The Elective Affinities :

“What we call limestone is a more or less pure calcareous earth, intimately connected with a delicate acid that became known to us in the form of air. If a piece of such a stone is placed in dilute sulfuric acid, it seizes the lime and appears with it as plaster of paris; that delicate, airy acidity escapes "

where the poet's chemist meant carbon dioxide .

Gypsum is also produced in all wastewater treatment processes when it comes to the neutralization of sulphate-containing process wastewater or sulfuric acid pickling.

The production of hydrofluoric acid from fluorite ( fluorspar , calcium fluoride) and concentrated sulfuric acid also produces gypsum (so-called "fluoroanhydrite"), which is used as an anhydrite screed in the cement and construction industries .

Gypsum is also produced as the end product of the flue gas desulphurisation ("FGD gypsum") from coal-fired power plant exhaust gases . As a rule - depending on the impurities - such gypsums (drained filter cake) can be used in the building materials industry or for further processing into calcium sulphate modifications (hydrates). This synthetic route made the mining of natural gypsum deposits in Europe partially superfluous at the end of the 1980s, today the production figures are declining due to this process, since low-sulfur Australian hard coal is often used. In 2014, 7 million tons of the 11 million tons of gypsum were extracted in Germany by the FGD, while 4 million tons were extracted from natural gypsum.

### Gypsum-like calcium sulfate modifications

• α-Hemihydrate (CaSO 4 · ½ H 2 O) is produced in a closed vessel ( autoclave ) under a wet steam atmosphere or without pressure in acids and aqueous salt solutions. It is the starting material for harder plasters (type III, IV and V) and requires less water but more time to set .
• β-hemihydrate (CaSO 4 · ½ H 2 O) is formed when burning in an open vessel under normal atmosphere. When mixed with water, hydration to the dihydrate occurs within minutes. It is the raw material for the softer plasters.

In the case of α- and β-hemihydrate, they are different crystalline forms of hemihydrate.

• Anhydrite III (CaSO 4 ) is formed from the hemihydrate at temperatures of up to 300 ° C. In the presence of water, including humidity, hemihydrate is formed very quickly.
• Anhydrite II s (CaSO 4 ) is formed at temperatures between approx. 300 to 500 ° C, the s stands for "poorly soluble". When mixed with water, hydration occurs within hours and days.
• Anhydrite II u (CaSO 4 ) is formed from anhydrite II s at temperatures of 500 to 700 ° C , the u stands for “insoluble”.
• Anhydrite I (CaSO 4 ) is the high temperature modification of gypsum, it forms at 1180 ° C.

## use

Gypsum is also marketed under names such as alabaster white , analine , anhydrite , Bolognese chalk , electrician's plaster , feather spar , light spar or Marienglas , Plaster of Paris .

### As a raw material

Gypsum as a raw material is predominantly extracted as gypsum rock, but is now also often a by-product of various chemical large-scale processes.

Technically, the ability of gypsum is used to absorb the crystal water that has been partially or completely lost by heating (burning) when mixed with water and to bind it in the process. When heated to around 110 ° C, so-called burnt gypsum (the hemihydrate mentioned above) is formed, and at 130 to 160 ° C, stucco , a mixture of a lot of hemihydrate and little anhydrite . Anhydrite is formed at 290 to 900 ° C, whereby the crystal water is completely burnt out. Very-high temperature gypsum is also "dead burned gypsum" or alanine or Annalin named because he no longer abbindet with water.

### As a building material

A brick cast from high-fire plaster , manufactured around 1870

In construction technology, gypsum (as hemihydrate or multi-phase gypsum) is mostly used today in the form of REA gypsum for gypsum wall panels for partition walls and for gypsum plasterboard for dry construction , as a base material for various plasters , fillers and dry screeds , and also as a filler . By mixing with lime is produced for plaster, brick and stucco Gipskalk that is longer process than pure stucco and malleable as plasticine , before it hardens.

Since the hardened gypsum has a certain solubility in water, gypsum building materials are predominantly only used for interior work. Outside, gypsum building materials must be protected from regular driving rain . In the past, plaster of paris was also used for stucco work on facades and impregnated with linseed oil.

Because plaster of paris is hygroscopic (water - attracting) and therefore tends to discolour and fungus if it is too often soaked, poor maintenance or ventilation, it can only be used to a limited extent in the wet and basement areas. During renovation work is construction or plaster of Paris used to close small cracks, holes and cable slots in the walls and einzudübeln wood and other components. In new buildings, gypsum plasters and plasterboard are used to create a surface that is ready for painting and wallpapering on rough and uneven masonry. Partition walls that are not statically loaded are now often made from plasterboard with a metal substructure or from plasterboard .

Also screeds are made of gypsum or anhydrite produced.

In addition, plaster of paris is used to fasten flush-mounted elements for electrical installations in structural walls. The speed of setting in alkaline formulations - for example gypsum plaster - is regulated by adding tartaric or citric acid . Neutral formulations can be delayed with protein compounds, cellulose glue or white lime hydrate . The setting process is accelerated by adding potassium sulfate or finely ground plaster of paris.

In structural fire protection, plaster of paris is preferred because it offers great fire resistance while being relatively light ; Protection is provided by the water of crystallization in the dihydrate, which evaporates in the event of a fire and forms a protective vapor curtain on the side facing the fire.

The building material gave the plasterer (today plasterer ) its name.

### As model and mold plaster

When used as model or mold plaster, for example with Bozzetti , increased requirements are placed on the purity of the plaster raw materials and on the preparation. A more even surface structure is achieved through finer grinding and lower proportions of foreign minerals. By using α-hemihydrate (produced under water vapor pressure and has a higher density), higher strengths of the molded parts can be achieved. In this context hard plaster is also used.

### In art

In the fine arts , plaster of paris is used to create sculptures and, just like in technology, to make forms and models. Marienglas still plays an important role in church and alabaster restorations, while dead-burned plaster is also often used as an additive (extender) for paints, as it leads to cheaper products without significantly impairing the color quality.

Analin is also for primers of canvas , in the panel painting or as a gold background ( Assis used). Also Chalk and Chalk exist in Germany usually mostly made of plaster.

### In the medicine

In medicine, plaster of paris is used for the plaster cast : the affected limbs or joints are wrapped with moist plaster bandages to immobilize and stabilize them, which then harden within minutes and are fully resilient after about twelve hours.

In dental technology , plaster of paris is the most important raw material for dental plaster for the production of models that are created from impressions of the oral and dental situation. According to the EN ISO 6873 standard for dental plasters, a distinction is made between five types:

• Type I: Casting and impression plaster, β-hemihydrate, 0.15% setting expansion and 4 N / mm² compressive strength
• Type II: Alabaster plaster of paris, β-hemihydrate, 0.3% setting expansion and 9 N / mm² compressive strength
• Type III: hard plaster of paris, α-hemihydrate, 0.2% setting expansion and 20 N / mm² compressive strength
• Type IV: super hard stone, α-hemihydrate, 0.15% setting expansion, 35 N / mm² compressive strength
• Type V: super hard stone, α-hemihydrate, 0.3% setting expansion, 35 N / mm² compressive strength

Internationally, the exact specifications are given, in particular the mixing ratio (ml of water per 100 g of plaster) and the compressive strength (in MPa or N / mm² after a certain time and when dry). Depending on the intended use, the percentage setting expansion and the duration of the processing and setting times are also important.

### Further areas of application

Unburned or dead-burned plaster of paris is used instead of chalk to mark the playing field .

To make tofu , the protein from ground soybeans is coagulated with calcium sulfate. Calcium sulfate is also used as a food additive (E 516). It belonged to the original canon of the twelve Schuessler salts used in alternative medicine .

In some areas of Germany, such as in the southern Harz region , a weathered gypsum product is created which, due to its similarity to table flour, is popularly known as "heavenly meal " or "gypsum ash". In times of famine , this gypsum flour was used either as a flour substitute or for stretching real flour to prepare food. However, the heat of baking, for example, creates burnt plaster, which sets in the gastrointestinal system and can lead to deadly intestinal obstruction .

## Figurative meaning

Since gypsum is abundant worldwide, there has never been a military dispute over this raw material in human history. The proverb “Don't tell me anything about the war on the plaster” is based on the power-political insignificance of the plaster of paris in order to make it clear to someone, ironically colored, that they should not tell stories about non-existent events.

## literature

• Martin Okrusch, Siegfried Matthes: Mineralogy. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin [a. a.] 2005, ISBN 3-540-23812-3 , pp. 71-72 .
• Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Villager Nature ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 147 .
• Basics . In: Fritz Scheidegger (ed.): From the history of construction technology . tape 1 . Birkhäuser, Basel 1990, ISBN 3-7643-2385-X .
• Franz Wirsching: Gypsum - natural raw material and residue from technical processes . In: Chemistry in Our Time . tape 19 , no. 4 , 1985, ISSN  0009-2851 , pp. 137-143 .
• Markus Arendt: Circular economy in the construction sector: Controlling future material flows using the example of gypsum . 2001 ( dissertation at the University of Heidelberg ).

Wiktionary: plaster of paris  - explanations of meanings, word origins, synonyms, translations
Commons : Gypsum (Gypsum)  - collection of images, videos and audio files

## Individual evidence

1. Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  393 .
2. Webmineral - Gypsum (English)
3. ^ Gypsum . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 67  kB ; accessed on September 28, 2017]).
4. Mindat - Gypsum (English)
5. State Office for Geology, Raw Materials and Mining in the Freiburg Regional Council: Sulphates ( Memento from April 9, 2014 in the Internet Archive )
6. Encyclopedic entries on "Gyps": Adelung-1793: "Gyps, der", Brockhaus-1809: "Der Gyps", Brockhaus-1837: "Gyps", Brockhaus-1911: "Gyps", Herder-1854: "Gyps" , Meyers-1905: "Gyps [2]" · "Gyps [1]", Pierer-1857: "Gyps"
7. Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
8. Ernest H. Nickel, Monte C. Nichols: IMA / CNMNC List of Minerals 2009. (PDF 1816 kB) In: cnmnc.main.jp. IMA / CNMNC, January 2009, accessed March 10, 2020 .
9. Entry on plaster of paris. In: Römpp Online . Georg Thieme Verlag, accessed on September 28, 2017.
10. Entry on sodium chloride. In: Römpp Online . Georg Thieme Verlag, accessed on September 28, 2017.
11. EPI - Institute for Gemstone Testing. Name search, trade names and what they mean (entry of Atlasspat required)
12. Mindat - Number of sites for plaster of paris (English)
13. a b c List of places where gypsum was found in the Mineralienatlas and Mindat
14. The emergence of the natural space. Zechstein period, Harz uplift and ice age, post-ice age at the Society for the Promotion of the South Harz Biosphere Reserve (GFB) eV ( Memento from February 28, 2009 in the Internet Archive )
15. Stefan Schorn and others: Mina Quien Valley Pensara (Mina Rica) and "Corta San José". In: mineralienatlas.de. Mineral Atlas , accessed October 18, 2019 .
16. Cynthia Reynolds: Messinian Crystals. In: solvitur.de. June 12, 2000, accessed October 16, 2019 .
17. Jet Propulsion Laboratory -News: NASA Mars Rover Finds Mineral Vein Deposited by Water, December 7, 2011
18. ^ Franz Wirsching: Gypsum - natural raw material and residue of technical processes . In: Chemistry in Our Time . tape 19 , no. 4 , August 1985, p. 137-143 , doi : 10.1002 / ciuz.19850190405 .
19. ^ BGR: Raw materials in Germany. BGR, 2014, accessed November 15, 2017 .
20. ^ Siegfried Ernst, Hans H. Caesar: The non-metals . Verlag Neuer Merkur GmbH, 2007, ISBN 978-3-937346-31-1 , p. 58 ( google.com ).
21. Christian Reinboth: Digital plaster exhibition in the Walkenried local history collection - Himmelsmehl. July 16, 2011, accessed September 28, 2017 .
22. Thomas Hofmeier: Attention plasterers. 100 years of Grassi & Co. AG in Basel . 2nd Edition. Books on Demand, Norderstedt 2009, ISBN 978-3-8370-5095-0 , pp. 16 ( limited preview in Google Book search).