Contents

Argillaceous Rocks

These are the sources for SiO2, Al2O3 and Fe2O3

They include claystones, siltstones, shales, mudstones and even marls.

Claystone is a clastic sedimentary rock. It is composed of very fine particles (clay

sized, less than 1/256 mm) which have become cemented into a hard rock.

Claystone is distinguished from a mudstone by the mudstone’s softening upon

exposure to water. A claystone is also distinct from shale by splitting of layers.

Shale is a clastic sedimentary rock composed of silts, clays or muds that have

been compacted into distinct layers. Shales may be easily split along these layers.

Shale is a very common sedimentary rock, and often preserves fossils.

Mudstone is a clastic sedimentary rock. As it sounds, a mudstone is hardened

mud (silts and clays) and is mostly composed of very fine particles. A mudstone will

soften in water, while the related siltstones and claystones will remain hard.

Siltstone is intermediate in composition between sandstone and claystone. A

siltstone may have very fine particles but they are still large enough to feel gritty.

Marl or marlstone is a calcium carbonate mud or mudstone which contains

variable amounts of clays. Marl is mainly composed of 35-65% clay and 65-35%

carbonate. Marlstone is impure argillaceous limestone.

Granodiorite contains quartz, feldspars and mafic minerals.

All are subject to degradation (weathering into something else), except quartz.

Quartz, for all intents and purposes, does not weather and will survive in the system .

Orthoclase, for example, breaks down to form clay

Ca plagioclase goes into solution to form CaCO3.

Clay Mineral Structure

There are five polyhedra that are basic building blocks of crystals. Of these are the tetrahedral and

octahedral building units for the phyllosilicates. In the tetrahedral unit, the Si-O combination has a radius ratio of 0.30,

which means that the silicon ion fits nicely into a tetrahedral polyhedron. The Silicon ion shares its charge equally between the four oxygen ions, leaving each oxygen with an excess charge of -1 (SiO44- anion, orthosilicate anion). These anions are bound together forming sheet structure with apical oxygen.

The second basic building block of the phyllosilicates is an aluminum octahedral unit. The aluminum/oxygen radius ratio is 0.41.

Depending on conditions, aluminum can coordinate with either four (tetrahedral) or six (octahedral) oxygen ions. Within the phyllosilicate mineral structure, the aluminum ion is “more comfortable” in an octahedral coordination.

Aluminum might be said to share +0.5 of its charge with each of the surrounding oxygen ions, leaving each oxygen ion with a negative -1.5 charge.

In a matrix of these octahedral units each oxygen will be bonded to two aluminum ions, leaving it with a remaining-1 charge.

The charge can be satisfied by attaching a proton (hydrogen ion) and when this type of structure is continued in three dimensions we have the mineral GIBBSITE (Al2 OH6). The octahedral coordination can be filled by Al, Mg or Fe equidistant from six oxygens or hydroxyls. The structure is called gibbsite (Al2 OH6) when Al is filling the octahedral co-ordinations and BRUCITE (Mg3OH6) when Mg is present.

Kaolinite (1:1) structure

The sheet of silicon tetrahedral units with apical oxygen ions still having an unbalanced charge.

The two sheets (tetrahedral and octahedral) can be brought together with the apical oxygen ions of the tetrahedral layer by being in the octahedral layer. As a result, the charge on these oxygen

ions is balanced by bonding to one silicon ion (in the tetrahedral sheet) and two aluminum ions (in the octahedral sheet). This is the basic structure KAOLINITE.

The kaolinite mineral is actually made up of many micelles piled one atop the other.

Since the surface on one micelle contains hydrogen ions and the other surface only oxygen ions there is a tendency for hydrogen bonds to form between micelles.

While individual hydrogen bonds are very low energy, the bonding energy is additive and the sum of the many hydrogen bonds between micelles results in the micelles being very strongly bonded together and nearly impossible

to separate. Thus, we speak of kaolinite as being a nonexpanding phyllosilicate.

Since each micelle is constructed of a layer of silicon tetrahedral units and a layer of octahedral units, kaolinite is called a 1:1 clay

mineral. Thus, kaolinite is a 1:1 nonexpanding clay mineral.

Kaolinite Structure

Montmorillonite (2:1 structure)

The hydrogen ions in the kaolinite structure can be replaced by tetrahedral units,

and the structure of many clay minerals result (Montmorillonite and illite). These minerals consist of two silicon tetrahedral layers and one aluminum octahedral layer and are called 2:1 clay expandable minerals.

Chlorite (2:2 structure)

In certain situations we find that a 2:1 clay mineral has been crystallized in an environment containing an excess of aluminum. Under these conditions an extra aluminum octahedral layer (Gibbsite) will form between the micelles. Minerals having this characteristic belong to the CHLORITE group. This structure is referred to 2:2 minerals (2 tetrahedral layers and 2 octahedral layers) The Gibbsite layer has the effect of binding the micelles tightly(nonexpanding).

DIOCTAHEDRAL vs TRIOCTAHEDRAL MINERALS

As we have already seen, the octahedral layer consists of two planes of oxygen ions, each ion of which is bonded with two of the smaller aluminum ions which are located in the space created between the larger oxygen ions when they come together in this close packed structure. If the octahedral layer contains divalent ions in all the possible sites, it is a known as a trioctahedral mineral. If it contains trivalent ions in two of every three possible sites, it is known as a dioctahedral mineral. Therefore, the mineral GIBBSITE (Al2 OH6) is dioctahedral mineral. The mineral BRUCITE (Mg3OH6) is a trioctahedral.

SOURCE OF PHYLLOSILICATE CHARGE

There are other ions in our environment which fit the tetrahedral and octahedral positions .There are a variety of ions which can fit into either tetrahedral or octahedral polyhedra. Aluminum is unique in that it falls right on the line between the two. A second source of charge on the minerals is the broken bonds found at the mineral edges. The structure cannot extend infinitely, so at some point there will be oxygens without all charges satisfied by associating with cations. In these cases a hydrogen ion from solution will normally satisfy the requirement. Whether this can occur will, however, depend on the solution pH. Therefore, these charges are called either pH‑ dependent charge or variable charge.

Clay mineral Identification

Kaolinite is chemically compose of: SiO2 46.54 %, Al2O3 39.50 % and H2O 13.96 %. Al

replacement is rare by titanium, iron.

Al2Si2O5(OH)4 = Al2O3 + 2SiO2 + 2H2O

Small endothermic peak at about 100 °C for the moisture water

Sharp endothermic peak at about 550 to 580 °C, due to thedehydroxylation (crystalline water)

Exotheric peak in the range (950 – 1000) °C due to the formation of mullite

SEM of Kaolinite

Montmorillinite is chemically composed of: SiO2 66.7%, Al2O3 28.3% and H2O 5 %. Al and/or P may replace Si in the tetrahedral sheet, however Mg, Fe, Zn, Ni and Li can replace Al in the octahedral sheet. If Mg filled the octahedral positions totally, we have saponite. When Fe it is nontronite.

Large endothermic peak system (two) are due to the loss of both adsorbed and inter-layer water (among units) at (150 – 180) °C. The Shape and intensity is changed based on the nature of the exchangeable cations (Ca, Na, K, Sr, NH4,….) among units.

Endothermic peak (medium to small) at about 700 °C, due to the dehydroxylation of (crystalline water)

Small endothermic-exothermic peak system (S-shape “two”) at about 950 °C. The small endothermic peak is due to the completion ofdehydroxylation (complete destruction of the clay), whereas the exothermic peak is due to the formation of a new phase.