Salt properties

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Author: Hans-Jürgen Schwarz, NN

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Abstract

Salts have certain properties. Some of them may help to explain their behaviour in solution and their "damaging" effect on objects.

Density

The density is a location-independent property of every mineral. It is measured in [g/cm³]. The density is only valid for a given phase; if this is modified to another phase, the density changes, e.g.:

• Calcite (trigonal): 2.71 g/cm³
  
• Aragonite (rhombic): 2.04 g/cm³
  

Mechanical strength and hardness

Beside density, strength and hardness determine the physical properties of a salt/mineral. Both refer to the cohesion properties that depend on the type and strength of the bonds in the crystal lattice and are thus subject to anisotropy. Also cohesion properties depend on defects in the crystal structures, i.e., measurements of strength and hardness may differ from the theoretical values. Like most minerals salts are quite brittle. When a crystal breaks, the fracture can be irregular, conchoidal or splintery, or it can have an even cleavage face. The reason for cleavage is found in the crystal structure. It is facilitated along the crystal lattice planes, where the occupation in the lattice is especially dense and there are only a few bonds per unit area. Cleavage along planes with hydrogen bonds or Van der Waals-bonds is almost always perfect. Particularly, if the mechanical stress applied leads to a shift of the crystal lattice in such a way that blocks are facing the same charge, so that a repulsion is achieved.

These lattice planes are also important crystal planes, so that cleavage indicates the symmetry and microstructure of a mineral. It’s especially important to watch the cleavage cracks when studying minerals with a microscope.

The hardness of minerals:

Intrusion on the surface of a mineral can occur by scratching, impressing a punch, grinding or planing. Therefore, different scales of hardness can be defined depending on the method used for measuring it. Nonetheless, the values can be compared with each other. In mineralogy, the standard method is by scratching, the so-called scratch hardness according to Mohs. He defined a hardness scale from 1 to 10 on the basis of ten minerals:

Table 1: Hardness scale according to Mohs

Mohs hardness	Mineral	Chemical formula	Absolute hardness	Image
1	Talc	Mg3Si4O10(OH)2	1
2	Gypsum	CaSO4·2H2O	3
3	Calcite	CaCO3	9
4	Fluorite	CaF2	21
5	Apatite	Ca5(PO4)3(OH–,Cl–,F–)	48
6	Orthoclase Feldspar	KAlSi3O8	72
7	Quartz	SiO2	100
8	Topaz	Al2SiO4(OH–,F–)2	200
9	Corundum	Al2O3	400
10	Diamond	C	1600

The anisotropy that the minerals may have is mostly irrelevan when using the scratching hardness according to the rather rough gradation defined by MOHS (e.g. Al₂SiO₅ - Disten).

Melting point

The most "damaging" salts don’t melt. They either decompose or change into another mineral phase before reaching the melting point. Therefore, the melt obtained will have a different chemical composition than the original salt.

Thermal conductivity

The thermal conductivity of a crystal may be dependant on its direction (anisotropy) behaving similarly to light transmission following optical orientation. The thermal conductivity anysotropy of crystals results from differences in atom packing of a crystal structure. In directions where atoms are densely packed a higher conductivity will be observed as compared to those where the atoms are more spaced out. Amorphous solids, having only a short-range order of components and lacking the long-range order of a crystal structure, show an isotropic thermal conductivity, i.e., heat conduction is the same in all directions.

Thermal expansion

Like most materials, minerals expand upon heating. The thermal dilataion of crystals is far less than for the case of liquids. The thermal expansion will be different in different directions for anisotropic crystals, i.e., crystals expand differently depending on the heat flow direction. Some minerals may even show a contraction, i.e., a negative expansion, in one direction, while expanding in another. The irregular deformation of crystals by changes of temperature is the main reason for the so-called temperature weathering.

For example, the calcite crystsl expands along the axis and contracts along the Also, it is possible to produce a ceramic glass (PLEASE DEFINE) that will not show thermal expansion by carefully controlling the cooling down process and crystal growth within the glass bulk.

Electrical conductivity

The conductivity of ionic crystals can be considered as being the result of ions moving through a crystal lattice via the defects present in it. In other words, ideal crystals do not conduct electricity. As crystal defects are a thermodynamic property (concentration equilibrium) influenced by pressure, temperature and composition, there are hardly any perfect crystals. A thermal induced oscillation of the crystal components around the rest position of an ideal lattice site induce some of the components to leave their site and leave a vacancy. Ion conduction only takes place at high temperatures, because only then the concentration of crystal defects and the thermal energy are sufficiently large to induce electric conductivity (semiconductor property).
Only when dissolved in water are hydrated ions are good electric conductors, so called ion conductors.

Color

Many soluble salts are highly colored. These idiochromatic minerals contain chromophores, coloring elements such as Cr, Mn, Fe, Cu, etc. Allochromatic minerals gain their color by integrating idiochromatic particles, chromophoric trace elements, or certain defects in their lattice. These colors are not typical of the mineral and can be very different for one and the same mineral, as for example, quartz, rock crystal when pure and well crystallized, can be purple (amethyst), yellow (topaz) or even greenish (aventurine. In anisotropic minerals, of course, the colour of the mineral depends on the direction. This effect the pleochroism is in some minerals pronounced, such as alexandrite, where the color will change from dark red to greenish orange depending on the direction the crystal is looked at.

The soluble salts that are relevant for the deterioration of buildings and monuments are mostly colorless, i.e., they may be transparent or appear white depending on the crystal size.

Luster

The luster of a mineral is an optical effect induced by the reflection of light. High reflections cause a high luster that can be described as a metallic, diamond-like or glassy luster. The specific surface shape also has an influence on the luster (greasy, silky, pearly).

Luminescence

Luminescence (lat. lumen = cold light) is the property of minerals to convert input energy into visible light. Radio-luminescence: Stimulation by radioactive radiation. Thermoluminescence: stimulation by application of heat. UV fluorescence: excitation by UV light. Phosphorescence is a light phenomenon in minerals that continues after the energy input has stopped. It could be said to be a "delayed" luminescence. The process is temperature-dependent, where the light energy is stored on an intermediate level and the emission takes place by rising the temperature. Some electrons stay longer on the intermediate level, so that the glow continues for some time after switching off the stimulating rays and fades away slowly.
Other optical propertis, such as birefringence anddifferent forms of refraction are used to identify the salt minerals with a polarizing microscope. Detailed description of these procedures are found in the corresponding chapters.

the property of a material being directionally dependent, as opposed to isotropy, which implies identical properties in all directions

The relationship between a mineral's crystallographic axes and optical indicatrix.

the property of exhibiting different colors as a function of the vibration direction

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