THE CLAY MINERAL GROUP

The Clay Minerals are a part of a general but important group within the phyllosilicates that contain large percentages of water trapped between their silicate sheets. Most clays are chemically and structurally analogous to other phyllosilicates but contain varying amounts of water and allow more substitution of their cations. There are many important uses and considerations of clay minerals. They are used in manufacturing, drilling, construction and paper production. They have great importance to crop production as clays are a significant component of soils.

It is the physical characteristics of clays (more so than chemical and structural characteristics) that define this group:

  • Clay minerals tend to form microscopic to sub microscopic crystals.
  • They can absorb water or lose water from simple humidity changes.
  • When mixed with limited amounts of water, clays become plastic and are able to be molded and formed in ways that most people are familiar with as children's clay.
  • When water is absorbed, clays will often expand as the water fills the spaces between the stacked silicate layers.
  • Due to the absorption of water, the specific gravity of clays is highly variable and is lowered with increased water content.
  • The hardness of clays is difficult to determine due to the microscopic nature of the crystals, but actual hardness is usually between 2 - 3 and many clays give a hardness of 1 in field tests.
  • Clays tend to form from weathering and secondary sedimentary processes with only a few examples of clays forming in primary igneous or metamorphic environments.
  • Clays are rarely found separately and are usually mixed not only with other clays but with microscopic crystals of carbonates, feldspars, micas and quartz.
Clay minerals are divided into four major groups. These are the important clay mineral groups:
The Kaolinite Group
This group has three members (kaolinite, dickite and nacrite) and a formula of Al2Si2O5(OH)4. The different minerals are polymorphs, meaning that they have the same chemistry but different structures (polymorph = many forms). The general structure of the kaolinite group is composed of silicate sheets (Si2O5) bonded to aluminum oxide/hydroxide layers (Al2(OH)4) called gibbsite layers. The silicate and gibbsite layers are tightly bonded together with only weak bonding existing between the s-g paired layers.

Uses: In ceramics, as a filler for paint, rubber and plastics and the largest use is in the paper industry that uses kaolinite to produce a glossy paper such as is used in most magazines.

The Montmorillonite/Smectite Group
This group is composed of several minerals including pyrophyllite, talc, vermiculite, sauconite, saponite, nontronite and montmorillonite They differ mostly in chemical content. The general formula is (Ca, Na, H)(Al, Mg, Fe, Zn)2(Si, Al)4O10(OH)2 - xH2O, where x represents the variable amount of water that members of this group could contain. Talc's formula, for example, is Mg3Si4O10(OH)2. The gibbsite layers of the kaolinite group can be replaced in this group by a similar layer that is analogous to the oxide brucite, (Mg2(OH)4). The structure of this group is composed of silicate layers sandwiching a gibbsite (or brucite) layer in between, in an s-g-s stacking sequence. The variable amounts of water molecules would lie between the s-g-s sandwiches.

Uses: Are many and include a facial powder (talc), filler for paints and rubbers, an electrical, heat and acid resistant porcelain, in drilling muds and as a plasticizer in molding sands and other materials.

The Illite (or The Clay-mica) Group
This group is basically a hydrated microscopic muscovite. The mineral illite is the only common mineral represented, however it is a significant rock forming mineral being a main component of shales and other argillaceous rocks. The general formula is (K, H)Al2(Si, Al)4O10(OH)2 - xH2O, where x represents the variable amount of water that this group could contain. The structure of this group is similar to the montmorillonite group with silicate layers sandwiching a gibbsite-like layer in between, in an s-g-s stacking sequence. The variable amounts of water molecules would lie between the s-g-s sandwiches as well as the potassium ions.

Uses: A common constituent in shales and is used as a filler and in some drilling muds.

The Chlorite Group
This group is not always considered a part of the clays and is sometimes left alone as a separate group within the phyllosilicates. It is a relatively large and common group although its members are not well known. These are some of the recognized members:
  • Amesite (Mg, Fe)4Al4Si2O10(OH)8
  • Baileychlore (Zn, Fe+2, Al, Mg)6(Al, Si)4O10(O, OH)8
  • Chamosite (Fe, Mg)3Fe3AlSi3O10(OH)8
  • Clinochlore (kaemmererite) (Fe, Mg)3Fe3AlSi3O10(OH)8
  • Cookeite LiAl5Si3O10(OH)8
  • Corundophilite (Mg, Fe, Al)6(Al, Si)4O10(OH)8
  • Daphnite (Fe, Mg)3(Fe, Al)3(Al, Si)4O10(OH)8
  • Delessite (Mg, Fe+2, Fe+3, Al)6(Al, Si)4O10(O, OH)8
  • Gonyerite (Mn, Mg)5(Fe+3)2Si3O10(OH)8
  • Nimite (Ni, Mg, Fe, Al)6AlSi3O10(OH)8
  • Odinite (Al, Fe+2, Fe+3, Mg)5(Al, Si)4O10(O, OH)8
  • Orthochamosite (Fe+2, Mg, Fe+3)5Al2Si3O10(O, OH)8
  • Penninite (Mg, Fe, Al)6(Al, Si)4O10(OH)8
  • Pannantite (Mn, Al)6(Al, Si)4O10(OH)8
  • Rhipidolite (prochlore) (Mg, Fe, Al)6(Al, Si)4O10(OH)8
  • Sudoite (Mg, Fe, Al)4 - 5(Al, Si)4O10(OH)8
  • Thuringite (Fe+2, Fe+3, Mg)6(Al, Si)4O10(O, OH)8

The term chlorite is used to denote any member of this group when differentiation between the different members is not possible. The general formula is X4-6Y4O10(OH, O)8. The X represents one or more of aluminum, iron, lithium, magnesium, manganese, nickel, zinc or rarely chromium. The Y represents either aluminum, silicon, boron or iron but mostly aluminum and silicon.

The gibbsite layers of the other clay groups are replaced in the chlorites by a similar layer that is analogous to the oxide brucite. The structure of this group is composed of silicate layers sandwiching a brucite or brucite-like layer in between, in an s-b-s stacking sequence similar to the above groups. However, in the chlorites, there is an extra weakly bonded brucite layer in between the s-b-s sandwiches. This gives the structure an s-b-s b s-b-s b sequence. The variable amounts of water molecules would lie between the s-b-s sandwiches and the brucite layers.

Uses: No industrial uses.

Some minerals listed above (specifically chlorite, pyrophyllite and talc) as belonging to one of the clay groups are often excluded by some minerologists. Usually the reason is that their crystal size and character do not consistently conform to the parameters that define a clay. Such minerals are listed here more for their structural similarities. However, all three minerals are quite often found associated with and do behave like clays occasionally.
Popular Members of the Silicates Class
 

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