Kaolinite
Kaolinite is a clay mineral with the chemical composition Al2Si2O5(OH)4. It is a layered
silicate mineral, also known as china clay or kaolin.
Source
The name is derived from Gaoling or Kao-Ling ("High Hill") in Jingdezhen, Jiangxi province,
China. Kaolinite was first described as a mineral species in 1867 for an occurrence in the Jari
River basin of Brazil.
Kaolinite is one of the most common minerals on earth. It is mined, as kaolin, in Brazil,
France, United Kingdom, Germany, India, Australia, Korea , the People's Republic of China
and the USA.
Kaolinite is a soft, earthy, usually white mineral, produced by the chemical weathering of
aluminum silicate minerals like feldspar. In many parts of the world, it is colored pink-orangered by iron oxide, giving it a distinct rust hue. Lighter concentrations of iron oxide yield
white, yellow or light orange colors. Alternating layers of colored clay are sometimes found,
as at Providence Canyon State Park in Georgia, USA.
Our white kaolin clay is from deposits in Texas, USA and is heat treated to remove some
portion of its hydroxyl (OH) composition. Calcined kaolin is produced by heating ultrafine
natural kaolinite to high temperatures in a kiln. The calcination process increases whiteness
and hardness and alters the size and shape of the kaolin particles.
Kaolin-type clays undergo a series of transformations upon heating in air at atmospheric
pressure. Dehydration begins at 550-600 C to produce disordered metakaolin, Al2Si2O7, but
continuous hydroxyl loss (-OH) is observed up to 900 C. Due to disagreement concerning
the nature of metakaolin, research has led to general consensus that metakaolin is not a simple
mixture of amorphous silica (SiO2) and alumina (Al2O3), but rather a complex amorphous
structure that retains some longer-range order (but not strictly crystalline) due to stacking of
its hexagonal layers.
Between 100-200 C, clay minerals lose most of their adsorbed water. Between 500-800 C
kaolinite becomes calcined by losing water through dehydroxilization. At this temperature
range, kaolinite becomes metakaolin, with a two-dimensional order in crystal structure. In
order to produce a pozzolan nearly complete dehydroxilization must be reached without
overheating. This produces an amorphous, highly pozzolanic state, whereas overheating can
cause sintering, to form the dead burnt, nonreactive refractory, called mullite.
Uses
Calcined kaolin clay is used as a functional extender in paint. For example, calcined kaolin
clay has proven to be an excellent extender for titanium dioxide (white) pigment in paint. This
is because kaolin clay is chemically inert, has a high covering power, gives desirable flow
properties and reduces the amount of expensive pigments required. Its hardness also improves
physical durability of the paint film and its burnish resistance.
Kaolin clay is also used in grounds, such as for painting and for gilding. The kaolin clay can
be used in recipes calling for "pipe clay" and mixed with starch or flour to make a thick cream
and applied to wood panel or canvas supports for painting. The ground dries quickly and
several very thin layers applied one after another to form a ground typical of 19th century
paintings.
Physical Properties
Median Particle Size (microns):
1.5
Hegman Grind:
5.5
Residue 325 Mesh (%)
(ASTM C 325-81 or C 371-77):
0.05
Specific Gravity
(ASTM C-329-75):
2.58
Bulk Density:
Loose (lbs/cu ft):
23
Compacted (lbs/cu ft):
33
Mohs Hardness:
2.0-2.5
Oil Absorption
(Gardner-Coleman):
60 g
pH Value
(ASTM D 1208-78):
6.0
Brightness (Colorimeter):
89
Color:
White
Refractive Index:
1.62
Moisture Content (max. %)
(ASTM C 323-56 or C 234-82):
0.4
Loss on Ignition (%)
(ASTM C 323-56):
0.3
Chemical Analysis
Chemical Formula:
Al2Si2O5(OH)4
Silicon Dioxide (SiO2):
57.00%
Aluminum Oxide (Al2O3):
39.00%
Iron Oxide (Fe2O3):
1.00%
Titanium Dioxide (TiO2):
0.70%
Calcium Oxide (CaO) + Magnesium Oxide
(MgO):
0.35%
Potassium Oxide (K2O) + Sodium Oxide
(Na2O):
1.70%
WARNING: The product contains crystalline silica - quartz, which can cause silicosis (an
occupational lung disease) and lung cancer.
