a. A colloidal-size, crystalline, hydroxyl silicate having a crystal
structure of the two-layer (7 Aa) type (kaolinite), or of the three-layer
(14 Aa) type (smectite), in which layers of silicon and aluminum ions have
tetrahedral coordination with respect to oxygen, while layers of aluminum,
ferrous and ferric iron, magnesium, chromium, lithium, manganese, and
other cations have octahedral coordination with respect to oxygen and to
hydroxyl ions. Exchangeable cations may attach to the silicate layers in
an amount determined by the excess negative charge within the composite
layers. These cations commonly are calcium and sodium, but may also be
potassium, magnesium, hydronium, aluminum, or others. The most common clay
minerals belong to the kaolinite, smectite, attapulgite, and illite
(hydromica) groups. Mixed-layer clay minerals are either randomly or
regularly interstratified intergrowths of two or more clay minerals.
See also: clay
b. Any mineral found in the clay fraction (less than 4 mu m) of a soil or
sediment; e.g., rock flour comminuted by glacial grinding.
c. Any kandite mineral of the kaolinite-serpentine group.
Dictionary of Mining, Mineral, and Related Terms