Conservation Science

216 Chapter 9

Crystal lattice breakdown refers to the removal of ions from the minerals
of which the stone is composed. Removal of ions requires that the chemical
bondsbetween molecules are broken. This requires an energy input, the
strength of which will vary depending on the strength of the bonds and the
presence of catalysts. Crystal lattice breakdown requires the presence of a
substance capable of reaction, as well as a mineral capable of reacting with it.
Any chemical reaction involved is an exchange rather than a straightforward
removal of ions. There are two basic exchanges, exchange of a proton and
exchange of an electron. The former can involve acid solutions, while the lat-
ter is the redox (reduction and oxidation) reaction. For removal, it is essential
that the reactants can react, that there is sufficient energy to overcome the
bonds within the mineral and that the products of reaction are removed from
the surface where the reaction occurred. Delays in removal of reaction prod-
ucts means that a layer of unreactive material can build up on a surface,
restricting further reactions and so slowing weathering. Where products are
removed, such as in flowing water, the surface reaction may be reduced in its
effectiveness as the two reactants may not be in contact long enough to
permit the reaction to occur.
Brittle fracture refers to the breakdown of stone when stress exceeds the
capacity of the stone to deform (strain). The general relationship between stress
and strain is illustrated in Figure 2. As stress increases, stone is a material that
tends to deform relatively slowly, and by a relatively small amount,and if the
stress is removed, the stone suffers no permanent deformation. A point
is reached, however, where deformation is permanent, the yield point. Stress
and resultant strain beyond this point causes the stone to fracture. There is

Stre

ss

Strain

Yield point or

elastic limit

Elastic or

recoverable

deformation

Plastic or non-recoverable

deformation

Figure 2Stress-strain behaviour of rock. Initially any deformation can be recovered when
stress is removed. When yield point is reached further stress produces non-
recoverable deformation of the rock, usually fracturing