Collapse metastable structure

Loess, as noted earlier, owes its engineering characteristics largely to the way in which it was deposited since this gives it a metastable structure, in that initially the particles are loosely packed. The porosity of the structure is enhanced by the presence of fossil root-holes. The latter are lined with carbonate cement, which helps bind the grains together. However, the chief binder is usually the clay matrix. On wetting, the clay bond in many loess soils becomes soft, which leads to the collapse of the metastable structure. The breakdown of the soil structure occurs under its own weight. 

Magneli phases) is accommodated by the collapse of the structure in specific crystallographic directions. Decomposition of to form is a similar reaction. Reduction of by NH3 or a hydrocarbon gives a metastable phase of VOj in a B monoclinic structure similar to that of V O, which on cooling transforms to the stable monoclinic (B) structure [9]. In Figure 10.1.2 we show the reduction ofY O dispersed on TiO support to give metastable VO (B ). 

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