Conservation Science

214 Chapter 9

parts of their structures. Pyroxenes, for example, are long-chain minerals and
experience chemical weathering along the cleavage planes of the mineral.
Goldrich developed a simple ordering of minerals according to their relative
susceptibility to weathering, a sequence of mineral stability (Figure 1).
Sedimentary rocks are derived from the weathering products of other
rocks. These products are deposited in lakes or the ocean and form layers of
sediment. Sedimentary rocks can range from the very finely-bedded limestones
to coarsely-bedded and relatively unsorted sandstones. Limestones, such as
Portland Stone from the Isle of Portland, are the products of deposition in rela-
tively energy-low environments, such as lagoons. This limestone is composed
of microscale ooliths, rounded particles in a finer limestone matrix (micrite).
Bedding planes, formed by the different layers of sediment, are present, but they
can be relatively inconspicuous compared to the discontinuities produced by
changes in the environment within which sediment is deposited, e.g.change
from lagoonal to shallow sea. This means that certain types of sedimentary
rocks may have large discontinuities associated with them, but these can be
widely spaced and so be of little concern when the stone is in the building.
Indeed in the case of Portland Stone, such large structural divisions mark the
boundaries of the different building stone groups (Base, White and Roach
beds). Inherent within highly-bedded rocks, such as certain types of sandstones,
can be planes of structural weakness. These bedding planes represent more
distinct phases of deposition and can be important for the degradation of the
stone once it is in the building. Bedding planes can often act as sites for the
entry of fluids and as zones of weakness once the rock is stressed.

Olivine

Ca-plagioclase

Pyroxene

Ca-Na plagioclase

Amphibole Na-Ca plagioclase

Na-plagioclase

Biotite

K-feldspar, muscovite

Quartz

In

crea

sing

sus

pe

ctibilit

y to weathering

Figure 1Goldrich’s sequence of mineral stability