Environmental Engineering Reference
In-Depth Information
glossary on stone deterioration patterns published by ICOMOS International
Scientific Committee on Stone [VER 08].
8.2. Intrinsic degradation factors
8.2.1
. Mineralogical composition
As the first degradation factor is water, we may consider as a rough estimation
that stones composed of minerals with higher resistance to water will deteriorate at a
slower rate than those that are hydrophilic. Table 8.1 provides the dissolution
rates of different rock minerals. Quartz is clearly more resistant than calcite. Stones
rich in quartz, micas and feldspars are consequently very resistant to dissolution
by rainwater run-off. Limestone, having calcite as a major component (solubility:
0.014 g/l) is far less resistant.
Mineral
Formula
Durability (years)
3.4 x 10
7
Quartz
SiO
2
6.9 x 10
5
Muscovite (mica)
KAl Si
3
O
10
(OH)
2
6.0 x 10
5
Biotite (mica)
K(Mg,Fe)
3
Al Si
3
O
10
(OH)
2
4.4 x 10
5
Orthoclase (feldspar)
KAl Si
3
O
8
2.1 x 10
5
Albite (feldspar)
NaAl Si
3
O
8
1.9 x 10
5
Anorthite (feldspath)
CaAl
2
Si
2
O
8
6.7 x 10
3
Diopside (pyroxene)
CaMg Si
2
O
6
3.1 x 10
3
Forsterite (olivine)
Mg
2
Si O
4
Dolomite
CaMg (CO
3
)
2
1.75
Calcite
CaCO
3
0.6
Table 8.1.
Approximate durability of a 1 mm thick crystal of different minerals
in pure water at pH 5 at 25°C [LAS 84]
An extremely bad stone in this respect is alabaster, which is exclusively
composed of gypsum, with solubility in water (2.4 g/l) 20 times higher than calcite.
This material is used mainly to make artworks meant to be protected against the
rain, but occasionally it was used as an outdoor building or decorating material
(Knossos Palace in Crete and Jacques d'Amboise Palace in Cluny, France, for
example).
