Salt Data

Author:Hans-Jürgen Schwarz
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Abstract

All necessary data, graphics and images for the identification and characterization of a salt are presented. The structure of the salt systems is subject to the major anions.

The Salts

Here you will find a general overview of the salts. If you need detail information, please go to the pages of the individual salts. Recent reviews on salt and salt damage can be found at [Leitner.etal:2003]Title: Mauersalze und Architekturoberflächen
and [Steiger.etal:2007]Title: Special issue on salt decay

Carbonates

Carbonates are salts of the "carbonic acid"(H ₂ CO ₃ ). The main structural element is the planar complex anion [CO ₃ ] ^{2 -} . The carbonates have a number of characteristics that distinguish them well from other minerals / salts. The most important characteristic is that they all decompose more or less easily releasing CO ₂ when treated with acids. The alkali and alkali earth carbonate salts are colourless. Most of the other carbonates are usually coloured pale, only the heavy metal carbonates can show strong colours (such as azurite, (Cu₃CO₂((OH)₂. Noteworthy is the extremely high birefringence of the carbonates, which leads to higher order interference colors. Under thermal treatment the carbonates dissociate mostly at temperatures between 500-900 °C. The solubility in water is relatively good, especially in carbonated water.

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Nitrates

Biogenic reactions are a major source for nitrates (having the [NO₃] ^- ion) produced by bacteria which process nitrogen-containing raw materials, such as animal waste, manure and other organic substances. Nearly all nitrates are water-soluble. The nitrates have remarkable structural similarities with carbonates; characteristic is the isomorphy (same crystal structure) between nitronatrite/calcite and aragonite/niter.

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Sulfates

Sulfates are compounds with the divalent anion [SO₄]^2-. The sulfate anion is formed in nature by the oxidation of sulfur or sulfur compounds, particularly sulphides. In many cases, microorganisms (e.g., thio - or sulfur bacteria) are involved in this process. A large proportion of atmospheric sulfate and their precursors (e.g., sulfur dioxide SO₂) is anthropogenic and produced by combustion processes. The relatively large [SO₄] - complex tends to form stable compounds mainly with large divalent cations such as barium (probably one of the most insoluble compounds considering that Ba⁺⁺ is toxic but barium sulfate suspensions are given to patients that need X-rays of their intestinal tract). The smaller the cations are, the more unstable the sulfate compound, and they are stabilized by addition or incorporation of H₂O. Therefore, most sulfates, especially those with mono- or three-valent elements contain water or have additional anions, such as [OH]. For the case of building materials, most sulfates encountered have a number of common characteristics: they are colorless; their optical birefringence is usually lower than for the case of carbonates and nitrates; and, a very large number of its complex salts tend to be very soluble in water.

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Chlorides

The chlorides of alkali and alkaline earth metals are all ionic compounds. Since the cations have a low charge and a relatively large ionic radius, their polarizing effect is small on the chloride ions. The result is that these minerals are usually colorless and transparent. Other common features are the low refractive index and low density. Chlorides, particularly with alkali ions are very soluble in water. In contrast, the fluorides are sparingly soluble. This is due to the relatively small size of the fluoride ion (1.33 Å), and thus due to other types of coordination of the fluoride in the crystal lattice.

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Other inorganic Salts

To Other inorganic salts

Organic anions Salts

Salts with anions from organic acids may play a great role in very specific cases, as for example acetates, which may originate from construction materials used in museum showcases that can contaminate objects exhibited in them thus leading to their deterioration.

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Metal Salts

Salts formed with transition metals are usually colored (as mentioned above for the case of azurite). Thus, the formation of salts with these metals may stain building materials, as is the case with the formation of copper carbonates when bronze detailing is present in the structure, or when iron beams corrode leading to orange or brownish stains. Particularly susceptible to this problem are the bases of metal sculptures, since air pollution will also attack metal elements leading to the formation of sulfates or carbonates.

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Literature

[Leitner.etal:2003]	Leitner, Heinz; Laue, Steffen; Siedel, Heiner (eds.) (2003): Mauersalze und Architekturoberflächen, Hochschule für Bildende Künste, Dresden
[Steiger.etal:2007]	Steiger, Michael; Siegesmund, Siegfried (eds.) (2007): Special issue on salt decay, Springer Verlag, Url, 10.1007/s00254-006-0591-8

an atom or molecule with a negative electrical charge.

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral