KAOLINITE
Kaolinite is a common & important clay mineral.
“Clay” has more than one meaning in geology, which is unfortunate. Clay
refers to a group of silicate minerals that result from chemical weathering of
other silicate minerals. Clay also refers to very fine-grained sediment
(each grain is less than 1/256 of a millimeter in size). Clay minerals
include kaolinite, montmorillonite/smectite, diaspore, illite, etc.
The 1st specimen shown below is an earthy mass of
millions of tiny kaolinite crystals. In this form, kaolinite appears
deceivingly like chalk or diatomite. But, kaolinite won't bubble in acid
as chalk does. Kaolinite is whitish, soft, and powdery. It has an
earthy feel & an earthy smell. When wet, the earthy smell is stronger
and kaolinite becomes noticeably sticky.
Kaolinite is an aluminum hydroxysilicate - Al2Si2O5(OH)4.
Under electron microscopes, kaolinite crystals are seen to be thin,
hexagonally-shaped sheets. Kaolinite forms from weathering or significant
hydrothermal alteration of aluminosilicate minerals.
Famous localities for kaolinite (a.k.a. kaolin)
include Cornwall & Devon in southwestern Britain, where hydrothermal
metamorphism has completely altered the feldspars of granite batholiths.
This material has been used to make English China.
When heated over 500º C, the tiny crystal plates of kaolinite curl up as the
hydroxyls (OH-) are driven away in the form of water. The tiny
curled plates hook together to make ceramic. Ceramic remains in a
hard state because the molecules won't take back the water to make kaolinite
again.
Kaolinite is also moderately common in the Southern
Appalachians of America (for example, in South Carolina and Georgia). In
these areas, a kaolinite-rich clay occurs in Cretaceous-aged onlap
deposits. The kaolinite comes from chemical decomposition of feldspars in
sands produced by erosion of the ancient Appalachians during Triassic and
Jurassic times.
Kaolinite (4.6 cm across) from a kaolinite quarry in Twiggs County, central
Georgia, USA. Massive kaolinite in this area occurs in lenses in
paleodeltaic sands of the Tuscaloosa Formation (Upper Cretaceous).
Kaolinitized agate - this is a bizarre, but fascinating, specimen of an
agate (quartz) nodule replaced by whitish, powdery kaolinite.
Locality:
Salta Jacui, Rio Grande do Sul State, far-southern Brazil.