Kaolinite

The mineral kaolinite is a very common layer silicate from the kaolinite - serpentine group with the crystal chemical composition Al4 [( OH) 8 | Si4O10 ]. It is a typical representative of the two-layer clay minerals. Kaolinite crystallizes in the triclinic crystal system and usually develops scaly, flaky, earthy or massive aggregates, rarely but also pseudo-hexagonal crystals of predominantly white color. Impurities its color can also play into the reddish, brownish or bluish. His stroke color is white. Kaolinite can occur as rock-forming kaolin.

Kaolinite has a Mohs hardness of 2 to 2.5, a density from 2.61 to 2.68 g / cm ³. In water, the mineral is plastically deformable.

Etymology and history

The name is derived from the kaolinite, kaolin rock, the main component of it. Kaolin in turn is derived from the first locality, the Chinese place高岭Gaoling (from Chinese: Gao Ling = high hill ).

Although the production of rock and Tongut is as old as the settling down of the people, the production of porcelain from kaolinite and feldspar in a primitive form has probably occurred only in the seventh century AD in China.

The mineral was used there are already in Füllstoffmineral 105 as in papermaking. 600 years later, it was then used near the hill above as raw material for Chinese ceramics and porcelain industry. The development of this type of pottery was accompanied by the development of high temperature kilns, which could provide a sufficiently high temperature of 1450 ° C for vitrification ( Vitrification ) of kaolinite and feldspar are available. China was also initially because one of the main trading goods between Europe and China. Since 1707, the first kaolinite deposit to porcelain production in Europe was exploited near Meissen.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz kaolinite belonged to the mineral class of " silicates and Germanates " and then to the Department of " layer silicate minerals ( phyllosilicates ) " where there is eponymous which together with dickite, halloysite - 7 Å and nacrite kaolinite group VIII/H.23 formed.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the kaolinite in the class of " silicates and Germanates " and there in the department of " layer silicate minerals ( phyllosilicates ) ". This division, however, is further subdivided according to the type of film formation, so that the mineral according to its construction in the subsection " layer silicates ( phyllosilicates ) with kaolinite layers composed of tetrahedral or octahedral networks " is to find where it eponymous and together with dickite, nacrite Odinit the " kaolinite - group " with the system no. 9.ED.05 forms.

The mainly common in English-speaking classification of minerals according to Dana assigns the kaolinite in the class of " silicates and Germanates " and there in the department of " layer silicate minerals " one. Here he is with dickite, nacrite, halloysite, and Endellit Odinit in the " kaolinite " with the system no. 71.01.01 within the sub-division of " 71.01 phyllosilicates: with 1-1 layers of layers of six-membered rings " to find.

Education and Locations

Kaolinite consists of submicroscopic crystals with leafy habit (appearance ). It is generally a component of a clay mineral that the sediment, which is by definition associated with a grain diameter of less than two micrometers. The mineral is a ubiquitous aluminum silicate in the soil moist, warm regions and a typical product of the chemical weathering of other aluminum silicates by acid hydrolysis or partial, in particular of minerals of the feldspar group. Kaolinite is a component of various diagenetic sequences, and can be encountered as Füllmineral in pore spaces of sediments. It is formed at temperatures below 300 ° C, a low pressure and at a pH between 3 and 5 and at low concentrations of potassium. At higher concentrations, instead, the phyllosilicate formed illite.

The parent rock is mostly an acid such as magnetite to as granite or rhyolite. Output both minerals are feldspar and muscovite. The reaction of kaolinite with potassium feldspar to surface conditions will be referred to at a pH of less than 5 as acid hydrolysis or partial hydrolysis:

• 4 KAlSi3O8 ( s ) 6 H2O ( l) → Si4Al4O10 (OH ) 8 ( s ) 8 SiO2 ( s) 4K ( OH) ( aq)

Potassium must be removed, otherwise forms instead of kaolinite illite. Under tropical conditions with high rainfall rates, rapid drainage, low groundwater levels and adequate water flow to the removal of soluble components are granite and rhyolite easily weatherable to kaolinite and quartz. The immobile components are aluminum and silicon, while the other hand, the alkali metal and alkaline earth elements can be referred to as mobile. For even more intense weathering silicon is composed of kaolinite and gibbsite dissolved ( gibbsite ) is formed. Plagioclase weathering in general before K-feldspar and muscovite.

Quarzdruse with white calcite and kaolinite coating of St. Francisville, Clark County (Missouri ), USA ( geode Size: 4 "x 3 " x 2.5 ", equivalent to 10.16 x 7.62 x 6.35 cm)

Pseudomorphs of kaolinite after orthoclase twin ( Size: 4.5 x 2.6 x 1.1 cm )

SEM image of radialstrahlig - fibrous kaolinite from Hot Springs, Plumas County, California, United States

Crystal structure

The crystal structure of the kaolinite is composed of a tetrahedral layer, which is associated with an octahedron. The former consists of tetrahedra linked by basal oxygens ( polymerized) and are composed only of silicon. The octahedral other hand consists of edge-sharing octahedra that are occupied exclusively with aluminum. These, consisting of tetrahedral and octahedral structure is known as a 1:1 layer packet.

Kaolinite is the most common mineral of the kaolinite group. Dickite and nacrite, are Polytype of kaolinite, which means that they are chemically identical with the kaolinite, the stacking of the 1:1 layer packets are, however, at different regular sequences.

Dickite and nacrite For example, " double-layer " polytype, ie, the periodicity along the crystallographic c- axis is 2 times 7 angstroms, corresponding to two 1:1 layer packets. Halloysite, also referred to as 10 -Å halloysite, kaolinite is a hydrated, that is, there is an intermediate layer package of water molecules between the 1:1 layer packets. These may be the structure spontaneously or gradually by gentle warming, leave at room temperature or under vacuum conditions. Dewatered 10 Å halloysite is also referred to as 7 Å halloysite. Allophane and imogolite are poorly ordered hydrous aluminosilicates.

Structural transformations

The structure of kaolinite changes by thermal treatment in air at atmospheric pressure (calcination ). At 550-600 ° C for dehydration, which leads to amorphous metakaolin ( Al2Si2O7 ) begins. A dehydroxylation was observed up to 900 ° C. This phase is not a simple mixture of amorphous SiO2 and Al2O3, but a larger amorphous structure that has a certain order due to their hexagonal layers without being crystalline in the narrow sense:

Al2Si2O5 2 (OH ) 4 -> 2 H2O 4 Al2Si2O7

Upon further heating at 925-950 ° C, a low- silicon aluminum-silicon spinel formed ( Si3Al4O12 ), which is sometimes referred to as the γ -alumina type referred to:

2 Al2Si2O7 -> Si3Al4O12 SiO2

Upon calcination at 1050 ° C, the spinel phase is converted into mullite and cristobalite:

Si3Al4O12 3 -> 2 Si2Al6O13 5 SiO2

Use

Kaolinite respectively kaolin is mainly in the manufacture of porcelain, as a filler in paints and plastics, as a filler in paper making and finishing, as well as in brick manufacture and as a refractory material application. The main application area is the coating of paper, for which about 60% of the kaolin is spent nowadays.