Каолинит

Каолинит
Опште информације
Категорија	Phyllosilicates Kaolinite-serpentine group
Формула	Al 2(OH) 4Si 2O 5
Струнцова класификација	9.ED.05
Кристалне системе	Triclinic
Кристална класа	Pedial (1) (same H-M symbol)
Просторна група	P1
Јединична ћелија	a = 5.13 Å, b = 8.89 Å c = 7.25 Å; α = 90° β = 104.5°, γ = 89.8°; Z = 2
Идентификација
Боја	White to cream, sometimes red, blue or brown tints from impurities and pale-yellow; also often stained various hues, tans and browns being common.
Кристални хабитус	Rarely as crystals, thin plates or stacked, More commonly as microscopic pseudohexagonal plates and clusters of plates, aggregated into compact, claylike masses
Цепљивост	Perfect on {001}
Чврстина	Flexible but inelastic
Тврдоћа по Мосу	2–2.5
Сјајност	Pearly to dull earthy
Огреб	White
Специфична тежина	2.16–2.68
Оптичке особине	Biaxial (–)
Индекс преламања	nα = 1.553–1.565, nβ = 1.559–1.569, nγ = 1.569–1.570
2V угао	Measured: 24° to 50°, Calculated: 44°
Референце

Каолинит

Каолинит је минерал глине са општом молекулском формулом: Al₂Si₂O₅(OH)₄. Спада у групу алуминосиликата. У себи садржи 39,5% Al₂O₃, 46,5% SiO₂ и 14% H₂O. Познат је и под називом кинеска глина. Велике количине каолинита поседују: Бразил, Кина, Француска, Немачка, Енглеска, Индија, Аустралија и Јапан. Користи се у керамици, козметици, за производњу папира и као адитив за храну. Састојак је многих врста глина.

It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO
4) linked through oxygen atoms to one octahedral sheet of alumina (AlO
6) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.

The name kaolin is derived from Gaoling (), a Chinese village near Jingdezhen in southeastern China's Jiangxi Province. The name entered English in 1727 from the French version of the word: kaolin, following François Xavier d'Entrecolles's reports on the making of Jingdezhen porcelain.

Хемија

Нотација

The chemical formula for kaolinite as used in mineralogy is Al
2Si
2O
5(OH)
4, however, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is Al
2O
3·2SiO
2·2H₂O.

Structure

Kaolinite structure, showing the interlayer hydrogen bonds

Compared with other clay minerals, kaolinite is chemically and structurally simple. It is described as a 1:1 or TO clay mineral because its crystals consist of stacked TO layers. Each TO layer consists of a tetrahedral (T) sheet composed of silicon and oxygen ions bonded to an octahedral (O) sheet composed of oxygen, aluminum, and hydroxyl ions. The T sheet is so called because each silicon ion is surrounded by four oxygen ions forming a tetrahedron. The O sheet is so called because each aluminum ion is surrounded by six oxygen or hydroxyl ions arranged at the corners of an octahedron. The two sheets in each layer are strongly bonded together via shared oxygen ions, while layers are bonded via hydrogen bonding between oxygen on the outer face of the T sheet of one layer and hydroxyl on the outer face of the O sheet of the next layer.

• 
  View of the structure of the tetrahedral (T) sheet of kaolinite
• 
  View of the structure of the octahedral (O) sheet of kaolinite
• 
  Kaolinite crystal structure looking along the layers

A kaolinite layer has no net electrical charge and so there are no large cations (such as calcium, sodium, or potassium) between layers as with most other clay minerals. This accounts for kaolinite's relatively low ion exchange capacity. The close hydrogen bonding between layers also hinders water molecules from infiltrating between layers, accounting for kaolinite's nonswelling character.

When moistened, the tiny platelike crystals of kaolinite acquire a layer of water molecules that cause crystals to adhere to each other and give kaolin clay its cohesiveness. The bonds are weak enough to allow the plates to slip past each other when the clay is being molded, but strong enough to hold the plates in place and allow the molded clay to retain its shape. When the clay is dried, most of the water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile. If the clay is moistened again, it will once more become plastic.

Structural transformations

Kaolinite group clays undergo a series of phase transformations upon thermal treatment in air at atmospheric pressure.

Milling

High-energy milling of the kaolinite results in the formation of a mechanochemically amorphized phase similar to metakaolin, although, the properties of this solid are quite different. The high-energy milling process is highly inefficient and consumes a large amount of energy.

Drying

Види још: Buell dryer

Below 100 °C (212 °F), exposure to dry air will slowly remove liquid water from the kaolin. The end-state for this transformation is referred to as "leather dry". Between 100 °C and about 550 °C (1.022 °F), any remaining liquid water is expelled from kaolinite. The end state for this transformation is referred to as "bone dry". Throughout this temperature range, the expulsion of water is reversible: if the kaolin is exposed to liquid water, it will be reabsorbed and disintegrate into its fine particulate form. Subsequent transformations are not reversible, and represent permanent chemical changes.

Metakaolin

Endothermic dehydration of kaolinite begins at 550–600 °C producing disordered metakaolin, but continuous hydroxyl loss is observed up to 900 °C (1.650 °F). Although historically there was much disagreement concerning the nature of the metakaolin phase, extensive research has led to a general consensus that metakaolin is not a simple mixture of amorphous silica (SiO
2) and alumina (Al
2O
3), but rather a complex amorphous structure that retains some longer-range order (but not strictly crystalline) due to stacking of its hexagonal layers.

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

Појава

Kaolinite is one of the most common minerals; it is mined, as kaolin, in Malaysia, Pakistan, Vietnam, Brazil, Bulgaria, Bangladesh, France, the United Kingdom, Iran, Germany, India, Australia, South Korea, the People's Republic of China, the Czech Republic, Spain, South Africa, Tanzania and the United States.

Mantles of kaolinitic saprolite are common in Western and Northern Europe. The ages of these mantles are Mesozoic to Early Cenozoic.

Kaolinite clay occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates—for example in tropical rainforest areas. Comparing soils along a gradient towards progressively cooler or drier climates, the proportion of kaolinite decreases, while the proportion of other clay minerals such as illite (in cooler climates) or smectite (in drier climates) increases. Such climatically-related differences in clay mineral content are often used to infer changes in climates in the geological past, where ancient soils have been buried and preserved.

In the Institut National pour l'Étude Agronomique au Congo Belge (INEAC) classification system, soils in which the clay fraction is predominantly kaolinite are called kaolisol (from kaolin and soil).

In the US, the main kaolin deposits are found in central Georgia, on a stretch of the Atlantic Seaboard fall line between Augusta and Macon. This area of thirteen counties is called the "white gold" belt; Sandersville is known as the "Kaolin Capital of the World" due to its abundance of kaolin. In the late 1800s, an active kaolin surface-mining industry existed in the extreme southeast corner of Pennsylvania, near the towns of Landenberg and Kaolin, and in what is present-day White Clay Creek Preserve. The product was brought by train to Newark, Delaware, on the Newark-Pomeroy line, along which can still be seen many open-pit clay mines. The deposits were formed between the late Cretaceous and early Paleogene, about 100 to 45 million years ago, in sediments derived from weathered igneous and metakaolin rocks. Kaolin production in the US during 2011 was 5.5 million tons.

During the Paleocene–Eocene Thermal Maximum sediments deposited in the Esplugafreda area of Spain were enriched with kaolinite from a detrital source due to denudation.

Литература

• Deer WA, Howie RA, Zussman J (1992). An introduction to the rock-forming minerals (2nd изд.). Harlow: Longman. ISBN 0582300940. 
• Hurlbut CS, Klein C (1985). Manual of mineralogy – after J. D. Dana (20th изд.). Wiley. стр. 428–429. ISBN 0471805807. 
• Breck DW (1984). Zeolite molecular sieves. Malabar, FL: R. E. Krieger Publishing Co. стр. 314—315. ISBN 0898746485. 

Спољашње везе

• CDC – NIOSH Pocket Guide to Chemical Hazards