10 - Lecture notes for Clay Mineralogy

Required reading: Moore and Reynolds, 148-162

Suggested reading:
Brindley and Brown, pages 169-189
Newman, pages 48-84

Classification of hydrous layered silicates

2:1 with interlayer charge of 0.6 < X < 0.9

Dioctahedral


Illite is a 2:1 (10Å) structure similar to muscovite, buts differs by having more Si, Mg, Fe and H2 O and less K.

Often reported with a layer charge of 0.75. This an average value and it is likely that there is a range of layer charges within any one specimen.

K 0.75 Al 1.3 Mg 0.25 Fe+30.25 Si3.7Al0.3 10(OH )2

paragenesis - hydrothermal and burial diagenesis. Degraded muscovite (weathering).

Sericite is a descriptive textural term used in the field description of rocks or in thin section. In most cases the material extremely fine grain (i.e., beyond visual resolution. It is not a mineral name! Many times sericite, mineralogically is illite, mixed-layer illite/smectite or muscovitte.



Celadonite K (Mg,Fe+2)(Al,Fe+3) Si4O10(OH )2

Composed of a "tetrasilisic sheet", meaning that all the layer charge deficiency comes from the octahedral sheet.

paragenesis: Altered basalt - typical infilling of amygdules or lining of vesicular voids.

Glauconite - Appears to be like an Fe-rich illite, on average composition. Also some trioctahedral character to many glauconites.

K0.75(Al,Fe+3)1.3 (Mg , Fe+2)0.25 Si3.7Al0.3 O 10(OH )2

paragenesis: Occurs as aggregates and pellets in marine (but not solely) sediments. Appear to form at lower temperatures in reducing, Fe-rich environments. (e.g., inside fecal pellets). Slow sedimentation rates.

Sometimes the term glauconitic is used as a descriptive term to conotate the fact that a rock is marine in nature (e.g. glauconitic sandstone are often interpreted as marine entitites).

Trioctahedral and/or Dioctahedral

Vermiculite [Mg0.54H2O] (Mg,Fe+2)3Si 3 Al O10(OH )2

paragenesis: Alteration of pyroxene, biotite, chlorite, phlogopite in the hydrothermal and weathering environment.

Because vermiculite inherits its interlayer and structural chemistry and charge distribution from so many parent possibilities, it's properties can be quite variable.

Repeat distance for vermiculite with hydrated Mg in the interlayer site is about 14.5Å.

The fact that the layer charge comes from the tetrahedral sheet causes the hydrated layer to stay at this 14.5 Å thickness (as opposed to other 2:1 structures with lower layer charge that, as we will see, can expand to 18Å thickness).

Due to the high hydration energy of Mg in the interlayer site, the sphere of water is difficult to remove or it grabs free water readily.

Hydroxyl Al-interlayer varieties (incomplete chlorites) - do not undergo collapse upon heating to 300° C.

Operationally define as that which remains at 14.5Å after Mg saturation and glycol solvating. If expands to 18Å, then it is a lower charge 2:1 structure (smectite).

2:1 with interlayer charge of 0.2 < X < 0.6


Smectite Group Most diverse of the 2:1 groups. Very fined grained (no large crystals known to exist). Small layer charge allow exchange of interlayer cations. The weak bonding between structural units allow water molecules to coexist. The spacing between layers is therefore variable (10Å to 18Å) depending on the size of the 1) interlayer cation, 2) the hydration energy of the cation (function of size, electron shell configuration and valance state) and 3) the relative humidity.

Trioctahedral smectites

Saponite - positive octahedral layer charge and large (-0.66) tetrahedral layer charge.

M+0.33(Mg2.67Fe+30.33) Si3.33Al0.67O10(OH )2


paragenesis - Formed in alkaline lakes (Mg variety) and alteration of sea-floor basalts (Fe variety)

Hectorite - Li and F substituted M+0.33 (Mg2.67Li0.33) Si 4 O 10(OH,F )2


Sauconite - Zn-rich variety - associated with ore deposits.



Dioctahedral smectites

Montmorillonite - Tetrasilisic, therefore, layer charge derived from octahedral sheet.

"low-charge" Montmorillonite - Wyoming type (Na)

Na0.3Al1.7Mg0.3Si4O10(OH )2

"high-charge" Montmorillonite - Cheto type (Ca)

Ca0.3Al1.6Mg0.3Si4O10(OH )2

Beidellite - layer charge from tetrahedral sheet

M+0.3Al2Si3.7Al0.3O10(OH )2

Nontronite
M+0.3Fe+32Si3.7Al0.3O10(OH )2


Bentonite is a genetic rock term. It is not a mineral name! Bemtonites are usually a distinctive bed formed by the deposition and alteration of volcanic ash. Typically, they are composed of montmorillonite, but also can contain glass, mixed-layer clays, illite, kaolinite, quartz, zeolites, carbonates....