The Mineral KAOLINITE

  • Chemistry: Al2Si2O5(OH)4, Aluminum Silicate Hydroxide
  • Class: Silicates
  • Subclass: phyllosilicates
  • Groups: The Clays and The Kaolinite Group.
  • Uses: In the production of ceramics, as a filler for paint, rubber and plastics and the largest use is in the paper industry to produce a glossy paper such as is used in most magazines.
  • Specimens

Kaolinite, which is named for its type locality, Kao-Ling, Jianxi, China; is a common phyllosilicate mineral. It lends it name to the Kaolinite Group, members of which also belong to the larger general group known as the Clays. Kaolinite's structure is composed of silicate sheets (Si2O5) bonded to aluminum oxide/hydroxide layers (Al2(OH)4) called gibbsite layers. Gibbsite is an aluminum oxide mineral that has the same structure as these aluminum layers in kaolinite. The silicate and gibbsite layers are tightly bonded together with only weak bonding existing between these silicate/gibbsite paired layers (called s-g layers). The weak bonds between these s-g layers causes the cleavage and softness of this mineral. The structure is very similar to the Serpentine Group and at times the two groups are combined into a Kaolinite-serpentine Group.

Kaolinite shares the same chemistry as the minerals halloysite, dickite and nacrite. The four minerals are polymorphs; meaning they have the same chemistry, but different structures. All four minerals form from the alteration (mostly weathering) of aluminum rich silicate minerals such as feldspars. Kaolinite is by far the most common and most clay deposits contain at least some kaolinite. In fact, clay deposits will frequently be nearly 100% kaolinite pure!

Kaolinite is important to the production of ceramics and porcelain. It is also used as a filler for paint, rubber and plastics since it is relatively inert and is long lasting. But the greatest demand for kaolinite is in the paper industry to produce a glossy paper such as is used in most magazines.

PHYSICAL CHARACTERISTICS:

  • Color is usually white, colorless, greenish or yellow.
  • Luster is earthy.
  • Transparency: Crystals are translucent.
  • Crystal System is triclinic; 1.
  • Crystal Habits include foliated and earthy masses. Crystals of any size are quite rare, usually microscopic.
  • Cleavage is perfect in one direction, basal.
  • Fracture is earthy.
  • Hardness is 1.5 - 2 (can leave marks on paper).
  • Specific Gravity is 2.6 (average).
  • Streak is white.
  • Other Characteristics: Clay like properties when water is added.
  • Associated Minerals include fluorite, microcline, pyrite, hemimorphite, augite, dickite, halloysite, montmorillonite, quartz, muscovite and other clays.
  • Notable Occurrences are spread around the world including the type locality of Kao-Ling, Jianxi, China as well as Cornwall and Devon, England; Haute-Vienne, France; Near Dresden, Saxony, Germany; Donets Basin, Ukraine; Huberdeau, Quebec, and near Walton, Nova Scotia, Canada and in the United States at Macon, Georgia; Dixie Clay Company Mine, South Carolina; near Webster, North Carolina; Arkansas; Mesa Alta, New Mexico and Sterling Hill, New Jersey.
  • Best Field Indicators are habit, softness, color, luster and clay like properties.
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