12 - Lecture notes for Clay Mineralogy

Required reading: Moore and Reynolds, 117-125, 155-168
Brindley and Brown, pages 305-360

XRD identification of discrete clay minerals

Vermiculite - Operationally defined as those layers that expand to 14.5Å after Mg-saturation and solvation with glycerol (not ethylene glycol - both polar organic compounds). Smectites will expand to 18Å. If ethylene glycol is used, then smectite expands to about 17Å.

Vermiculite accepts one layer of glycerol and smectites accept two layers of glycerol.

(1) Mg-vermiculite

Distinguishing vermiculite from smectite is not always foolproof. In reality, both mineral groups exhibit a range of layer charges. Those vermcilulites with higher layer charge (e.g., 0.8) will readily collapse, while those with lower layer charge (e.g., 0.6) will readily expand.

Also, the presence of hydroxy-interlayers (HIV) will prop the structures open and the XRD characteristics begin to resemble those of chlorite.

Vermiculites and smectites are best diagnosed by performing a number of different cation saturations, solvations and heatings. See page 12 of handout. Mg, Ca vs. Na vs. K saturations.

(2,3) Mg -vermiculite vs. Fe-vermiculite Ratio of the (003)/(002)

Smectites - Operationally defined as those layers that expand to 17 Å upon ethylene glycol solvation.

(4, 5, 6, 7) Interlayer water.

Why does talc (with no layer charge) have no interlayer water?

Recall that for 2:1 structures, the overall intralyer structure is negatively charged and the interlayer cation is there to compensate the charge.
The hydration energy of the cation (function of charge density and radius) attracts water.
The presence of water pushes the 2:1 layers apart.
A monovalent cation has a lower hydration energy than an equivalent sized divalent cation. Therefore, monovalent cations do not hold the layers apart as well as divalent cations.
Divalent cations have higher hydration energies. The presence of a strongly held hydration sphere better holds the layers apart.
The response of the interlayer distance is then going to be a result of the balance of forces necessary to dehydrate the cation and the forces of attraction between the layers and the cation.
Talc has no interlayer cation therefore, no water to be drawn into the structure.

Mixtures of discrete clay minerals

Most often in nature we encounter mixtures of minerals (i.e., a mineral assemblage). How do we discriminate mixtures.

For example we can have a range of hydration states within a smectite alone. The extreme (and not realistic) example is a 50/50 mixture of hydrated and dehydrated smectite.

Another example (maybe more realistic is a 50/50 mixture of 1-water and 2-water hydration states.

What would happen if the 1-water and 2-water layers were randomly stacked? This brings to our attention the concept of a mixed-layer clay mineral. Below is an example of a 50/50 randomly mixed layer mineral.

What would happen if the 1-water and 2-water layers were stacked in an ordered fashion? This again brings to our attention the concept of a mixed-layer clay mineral. Below is an example of a 50/50 ordered mixed layer mineral.

Click here for a table that shows changes in low angle reflection (00l) of discrete clay minerals with ethylene glycol and thermal treatments.

Palygorsite and Sepiolite structures.

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