9 - Lecture notes for Clay Mineralogy

Required reading: Moore and Reynolds, 148-162
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


Illite is a 2:1 (~10Å) structure similar in composition to muscovite, buts differs by having slightly more Si, Mg, Fe and H2O 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 sample.

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

Paragenesis:   Illite forms authigenically during hydrothermal alteration and during burial diagenesis of smectite-rich shales (including bentonites). Degraded muscovite (formed during weathering) also takes on the XRD characteristics of illite.  See notes that follow on XRD identification of illite.

Sericite is a textural term used in the description of rocks in the field or in thin-section (denote by relatively high birefringence). In most cases, sericite is extremely fine grained (i.e., beyond visual resolution). Sericite is not a mineral name! Mineralogically, sericite is usually either (1) illite, (2) mixed-layer illite/smectite, or (3) muscovite.

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

0.75(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 snd P-rich environments. (e.g., inside fecal pellets). Slow sedimentation rates. Bacteria may induce the formation of this mineral.

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 entitities).

Trioctahedral and/or Dioctahedral

[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 possible parent sources, 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. Vermiculite grabs free water readily and is often used as a soil amendment to help hold moisture.

Hydroxyl Al-interlayer (HIV) varieties are "incomplete" chlorites where the brucite-like interlayer sheets may be discontinuous - They do not undergo collapse upon heating to 300° C.

Operationally defined as that which remains at 14.5Å after Mg saturation and glycol solvating. If the lattice 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), 3) associated organic molecules, and 4) the relative humidity.

Trioctahedral smectites

- 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 O10(OH,F )2

Paragenesis: formed from altererd tuffs and hot spring activitiy.

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

Dioctahedral smectites

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

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

0.3Al1.7Mg0.3Si4O10(OH )2

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

0.3Al1.6Mg0.3Si4O10(OH )2

Beidellite - layer charge from tetrahedral sheet

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

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

Bentonite is a genetic rock term. Bentonite is not a mineral name! Bentonites are usually found as distinctive beds 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....