# Kaolinite

Kaolinite
General and classification
chemical formula Al 4 [(OH) 8 | Si 4 O 10 ]
Mineral class
(and possibly department)
Silicates and Germanates
System no. to Strunz
and to Dana
9.ED.05 ( 8th edition : VIII / H.23)
71.01.01.02
Similar minerals Dickite , nacrite , halloysite , allophane , imogolite
Crystallographic Data
Crystal system see crystal structure
Crystal class ; symbol see crystal structure
Space group see crystal structure
Lattice parameters see crystal structure
Formula units Z  = 1
Frequent crystal faces {001}
Twinning very rare
Physical Properties
Mohs hardness 2 to 2.5
Density (g / cm 3 ) measured: 2.61 to 2.68; calculated: 2.63
Cleavage completely after {001}
Break ; Tenacity uneven
colour white, also with a reddish, brown or blue tinge
Line color White
transparency translucent to opaque
shine earthy
Crystal optics
Refractive indices n α  = 1.553 to 1.563
n β  = 1.559 to 1.569
n γ  = 1.560 to 1.570
Birefringence δ = 0.007
Optical character biaxial negative
Axis angle 2V = 24 to 50 ° (measured); 44 ° (calculated)
Pleochroism very weak
Other properties
Chemical behavior not soluble in acids
Special features plastically deformable in water

The mineral kaolinite is a very common sheet silicate from the kaolinite serpentine group with the crystal chemical composition Al 4 [(OH) 8 | Si 4 O 10 ]. It is a typical representative of the two-layer clay minerals . Kaolinite crystallizes in the triclinic crystal system and develops mostly leafy, flaky, earthy or massive aggregates, but rarely also pseudo-hexagonal crystals of predominantly white color. Due to impurities, however, its color can also turn red, brown or bluish. Its line color is white. Kaolinite can occur as kaolin to form rock .

Kaolinite has a Mohs hardness of 2 to 2.5 and a density of 2.61 to 2.68 g / cm³. The mineral becomes plastically deformable in water .

## Etymology and history

The name kaolinite is derived from the rock kaolin , of which it is the main component. Kaolin, in turn, is derived from the first site, the Chinese place 高嶺 Gaoling (from Chinese: gāo lĭng = high hill).

Although the production of stone and clay is as old as the settling down of people, the production of porcelain from kaolinite and feldspar in a primitive form probably did not take place in China until the seventh century AD .

The mineral was used there as early as 105 as a filler mineral in paper manufacture. 600 years later it was used as a raw material for the Chinese ceramic and porcelain industry near the hill mentioned above. The development of this type of ceramic went hand in hand with the development of high-temperature kilns , which could provide a sufficiently high temperature of 1450 ° C for the vitrification of kaolinite and feldspar. China porcelain was initially one of the most important trade goods between Europe and China. The first kaolinite deposit for porcelain production in Europe was exploited near Meißen since 1707 .

## classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , kaolinite belonged to the mineral class of " silicates and germanates " and there to the department of " layered silicate minerals (phyllosilicates) ", where it gives it its name along with Dickit, Halloysit-7Å and Nakrit die Kaolinite group VIII / H.23 formed.

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), assigns kaolinite to the class of “ silicates and germanates ” and then to the department of “phyllosilicates”. However, this section is further subdivided according to the type of layer formation, so that the mineral, according to its structure, can be found in the sub-section "Layered silicates (phyllosilicates) with kaolinite layers, composed of tetrahedral or octahedral nets", where it is named together with dickite, nacrite and Odinit the "kaolinite group" with the system no. 9.ED.05 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns kaolinite to the class of "silicates and Germanates" and there to the "layered silicate minerals" section. Here it is together with dickite, nakrite, halloysite , endellite and odinite in the "kaolinite group" with the system no. 71.01.01 to be found within the subsection of " 71.01 Layered Silicates: Layers of six-membered rings with 1: 1 layers ".

## Crystal structure

Structure of kaolinite __ H der OH -   __ O 2−   __ Al 3+   __ Si 4+

The crystal structure of kaolinite consists of a tetrahedron layer linked to an octahedron layer. The former consists of tetrahedra that are linked (polymerized) via basal oxygen and are exclusively occupied with silicon. The octahedron layer, on the other hand, consists of edge-linked octahedra that are exclusively occupied with aluminum. This structure, consisting of tetrahedron and octahedron layers, is called a 1: 1 layer package.

Crystallographic data of the kaolinite polytype
Surname Kaolinite-1A Kaolinite-1M
Crystal system triclinic monoclinic
Crystal class 1 not defined
Space group C 1 not defined
Lattice constants of the unit
cell
a  = 5.15  Å
b  = 8.94 Å
c  = 7.39 Å
α = 91.9 °
β = 105.0 °
γ = 89.8 °
a  = 5.16 Å
b  = 8.93 Å
c  = 7.39 Å
β = 104.5 °
Number of formula units in the unit cell 1 1

Kaolinite is the most common mineral of the kaolinite group. Dickite and nakrit are polytypes of kaolinite, which means that they are chemically identical to kaolinite, but the 1: 1 layer packages are stacked in different regular sequences.

Dickit and Nakrit, for example, are “double-layer” polytypes, that is, the periodicity along the crystallographic c-axis is 2 times 7 Angstroms, corresponding to two 1: 1 layer packages. Halloysite , also known as 10 Å halloysite, is a hydrated kaolinite, which means that between the 1: 1 layer packages there is an intermediate layer package of water molecules. These can leave the structure spontaneously or gradually through careful heating, already at room temperature, or under vacuum conditions. Dehydrated 10 Å halloysite is also known as 7 Å halloysite. Allophane and imogolite are poorly ordered, water-containing aluminosilicates.

## Structural transformations

The structure of kaolinite changes through thermal treatment in air at atmospheric pressure (calcination). At 550–600 ° C, dehydration begins , which leads to amorphous meta-kaolinite (also metakaolin ) with the empirical formula Al 2 Si 2 O 7 (oxide formula Al 2 O 3 · 2SiO 2 ). A dehydroxylation was up to 900 ° C observed. This phase is not a simple amorphous mixture of SiO 2 and Al 2 O 3 , but a larger amorphous structure, which, due to its hexagonal layers, has a certain order without being crystalline in the strict sense:

${\ displaystyle {\ ce {2 Al2Si2O5 (OH) 4 -> 2 Al2Si2O7 + 4 H2O}}}$

Further heating to 925–950 ° C produces a low-silicon aluminum-silicon spinel (Si 3 Al 4 O 12 ), which is sometimes referred to as the γ-aluminum oxide type:

${\ displaystyle {\ ce {2 Al2Si2O7 -> Si3Al4O12 + SiO2}}}$

When calcined to 1050 ° C, the spinel phase is converted into mullite and cristobalite :

${\ displaystyle {\ ce {3 Si3Al4O12 -> 2 Si2Al6O13 + 5 SiO2}}}$

So far, meta-kaolinite is only known synthetically and has been demonstrated in coal-burning experiments and in ceramics, among other things. It is believed, however, that the connection can also arise in natural surroundings.

## Education and Locations

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

The parent rock is mostly an acidic magmatite such as granite or rhyolite . The starting minerals are both feldspars and muscovite . The conversion of potassium feldspar into kaolinite under surface conditions is called acid hydrolysis or partial hydrolysis at a pH value below 5:

${\ displaystyle {\ ce {4 KAlSi3O8 (s) + 6 H2O (l) -> Si4Al4O10 (OH) 8 (s) + 8 SiO2 (s) + 4K (OH) (aq)}}}$
Potassium feldspar reacts with water to form kaolinite, quartz and potassium hydroxide solution.

Potassium has to be removed, otherwise illite will form instead of kaolinite. Under tropical conditions with high precipitation rates, rapid drainage, low groundwater levels and adequate water flow to remove the soluble components, granite and rhyolite are easily weatherable to kaolinite and quartz. The immobile components are aluminum and silicon, while the alkali and alkaline earth elements can be described as mobile. In the case of even more intensive weathering, silicon is dissolved from kaolinite and gibbsite (hydrargillite) is formed. Plagioclase generally weather before potassium feldspar and muscovite.

## use

Kaolinite or kaolin is mainly used in the manufacture of porcelain , as a filler material in paints and plastics, as a filler material and finish in paper production , as well as in brick production and as a refractory material. The most important area of ​​application today is the coating of paper, for which around 60% of the kaolin is used.

## literature

• Steffen Guggenheim, A. Alietti, VA Drits, Milton LL Formoso, Emilio Galan, HM Köster, H. Paquet, T. Watanabe u. a .: Report of the Association Internationale Pour L'Étude des Argiles (AIPEA) Nomenclature Committee for 1996 . In: Clays and Clay Minerals . tape 32 , no. 3 , September 1997, p. 493–495 , doi : 10.1180 / claymin.1997.032.3.11 (English).
• Haydn H. Murray, Wayne M. Bundy, Colin C. Harvey: Kaolin Genesis and Utilization. Special Publication No. 1 . The Clay Minerals Society, 1993, ISBN 978-1-881208-38-9 , pp. 341 ff ., doi : 10.1346 / CMS-SP-1 (English).
• Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Villager Nature ). Edition Dörfler im Nebel-Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 257 .