Vermiculite

Vermiculite is a rarely occurring mineral from the mineral class of silicates and the order of the layer silicates. It crystallizes in the monoclinic crystal system with the general chemical composition ( Mg0, 5, Ca0, 5, Na, K) 0.7 (Mg, Fe, Al) 3 [( OH) 2 | (Al, Si) 2Si2O10 ] · 4H2O or somewhat simplified Mg0.7 (Mg, Fe, Al ) 6 ( SiAl ) 8O20 (OH) 4 · 8H2O. The elements in brackets can be represented each other, but are always in the same proportions to the other components of the mineral (substitution).

Vermiculite developed exclusively foliated, scaly or massive aggregates that are either colorless or may be colored gray-white, yellow-brown, gray-green or green by foreign admixtures.

Vermiculite is one of the clay minerals, which contribute by their ion exchange influences the ground fertility. They are similar both structurally and from the appearance of the mica minerals and it forms flaky crystals.

Etymology and history

The name of the mineral is derived from the Latin word vermiculus ( " worms" ) from and alludes to the property trioctahedral vermiculite on to balloon in case of sudden heating at 200 to 300 ° C in the direction of the crystallographic c- axis worm- like structures.

Classification

Already in the now outdated but still in use 8th edition of the mineral classification by Strunz was one of the vermiculite to mineral class of " silicates and Germanates " and then to the Department of " layer silicates ( phyllosilicates ) ," where he indeed belongs to the large family of the " clay minerals ", but by itself the independent group VIII/H.21 formed.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the vermiculite also included in the category of " silicates and Germanates " and there in the department of " layer silicates ( phyllosilicates ) ". This division, however, is further subdivided according to the internal structure of the layers, so that the mineral according to its construction in the subdivision of " layer silicates ( phyllosilicates ) with mica sheets, composed of tetrahedral and octahedral nets " can be found, where it together with Tibiscumit the unnamed group 9.EC.50 forms.

The mainly common in English-speaking classification of minerals according to Dana assigns the vermiculite in the class of " silicates and Germanates " and there in the department of " layer silicate minerals " one. Here he is with hydrobiotite, illite and Brammallit in the " mica group ( hydromica subgroup ) " with the system no. To find " layers of six-membered rings with 2:1 layers phyllosilicates " 71.02.02d within the subdivision.

Education and Locations

The most important vermiculite deposits are mainly formed by hydrothermal or weathering of phlogopite / biotite, chlorite, and pyroxene in basic to ultrabasic rocks. Natural dioctahedral vermiculite have been out in the clay fraction of soils, but from which they can not be purely separated, so far not found, however.

Locations include the Kola Peninsula in the Russian Federation, Phalaborwa in South Africa and Milbury in the USA.

Crystal structure

Vermiculite crystallizes in the monoclinic space group C2 / c ( Raumgruppen-Nr. 15) with the lattice parameters a = 5.35 Å; b = 9.26 Å; c = 28.89 Å and β = 97.1 °, and four formula units per unit cell.

Structurally, vermiculite can be described as di - and trioctahedral 2:1 layer silicate:

Two layers of tetrahedrally coordinated cation form such a 2:1 silicate layer with an intermediate layer octahedrally coordinated cations. In trioctahedral vermiculites can be found in the Oktaederlage mainly magnesium ions, which provide an almost complete occupation of the octahedral cation sites. The tetrahedral layer has a Si4 : Al3 ratio of 1:2 to 1:3. This isomorphous replacement in the tetrahedral layers ( Al3 for Si4 ) results in a negative excess charge of the silicate layer, which can be reduced by isomorphous replacement in the Oktaederlagen (eg Fe3 for Mg2 ).

In sum, vermiculite carry a negative excess charge from 1.2 to 1.8 elementary charges per unit cell. This excess negative charge is balanced by hydrated cations in the interlayer. Depending on the nature of the Zwischenschichtions and the chemical composition of the 2:1 - silicate layer and a function of water vapor and the temperature of the surrounding atmosphere, the mineral, are differently large water amounts in the intermediate layer of vermiculite.

The amount of water and the corresponding base plane spacing d001 vary as a function of water vapor pressure and temperature is not continuous but are discrete hydration states with sharp transitions of these states to watch. With increasing temperature or decreasing the pressure of water vapor vermiculite go gradually in hydration states on with less and less interlayer water and thus smaller d001 - base spacings.

Use

Vermiculite is used industrially in the production of cat litter and carnival articles ( fireworks ). Since it has no effect other than asbestos cause cancer, it is also used for sound and heat insulation as well as fire protection. Half of the annual production comes from South Africa.

Vermiculite is also used in the produce industry as a cover material of seedlings after sowing. It is lightweight and has the ability to reflect light and to retain moisture. This prevents excessive heating of the seedlings and ensures balanced substrate moisture.

Also in the reptile breeding and reptiles attitude, in which the constant humidity in the terrarium, the wet- pit and in the incubator are vital for the animals, vermiculite is often used because there is excess moisture from the air picks up, and releasing them as required.

Vermiculite is due to its low thermal conductivity ( λ = 0.06-0.07 W / ( m · K) ) as a thermal insulation board or noncombustible material panel ( A1 or A2 according to DIN 4102) in ship interiors and also in building its use. Worldwide, this product is marketed under the brand name " Fipro ", " Miprotec ", " Vermilite 2000 " and " Thermax " offered.

Vermiculite as a high melting point ( 1315 ° C), is electrically non-conductive and no stratification occurs during freezing, it is used as a core material for infrared heaters.

In conjunction with iron powder, water, cellulose ( or polypropylene) salt and charcoal vermiculite is made ​​into hand warmers. When unpacking the hand warmer (ie when air gets to the package ) is an extremely fast oxidation process (in this case the rusting of iron ) is triggered. The salt acts as a catalyst and the activated carbon helps to distribute the heat evenly in the hand warmer. Vermiculite acts as an insulator and cellulose ( or PP) supports the distribution of air between the ingredients, if you use the hand warmer in a humid environment. The resulting temperature can reach 75 ° C ( 180 ° F). The heat is kept depending on the outside temperature, to twenty hours. However, this kind of hand warmers are not reusable. They are disposed of with normal household waste, because they are not harmful to the environment.

For use in various fields in which it comes to absorption capacity and good thermal insulation, vermiculite is treated by dry heating. At temperatures between 700 and 1000 ° C, the crystal water present is evaporated and presses the crystal platelets apart. Volume and the absorption increase considerably, while the bulk density decreases. This material is referred to in the industry as expanded vermiculite. Since vermiculite - even after expansion - is non-combustible, absorbent but to some extent, it is often used for the packaging of liquid hazardous material. For example, bottled in glass bottles hazardous materials are packed in cartons or tin cans and completely filled the space between the glass and outer packaging with vermiculite. In this way, leaking chemicals are on the one hand may be absorbed, on the other hand, glass breakage is avoided in advance, since the surrounding vermiculite layer protects well from impacts against the package.