Natural Science in Archaeology

11.6 Weathering and Decomposition 277
Clastic sedimentary rocks are composed of the reworked products of weathering,
so it is important to know whether these rocks are the end products or intermediate,
immature, products of weathering. This is important because further weathering of
immature rocks is deleterious in building and monumental stone.
The durability of igneous and metamorphic rocks under atmospheric weathering
depends on their mineral constituents. Minerals that formed under high-temperature,
anhydrous conditions are generally not stable under atmospheric conditions. The
most unstable minerals are olivine, pyroxene, and calcic plagioclase. Hornblende,
biotite, and the other feldspars are only of modest stability, as are the sedimentary
carbonates. Quartz, muscovite mica, and the clay minerals are stable under normal
atmospheric conditions.
The Goldich Stability Series, shown in the chart below, describes the order in
which silicate minerals weather and gives an indication of the ease with which
igneous rock minerals break down Goldich (1938).
Olivine Calcic plagioclase
Augite
Calcic-alkalic plagioclase
Hornblende Alkali-calcic plagioclase
Alkalic plagioclase
Biotite
Potassium feldspar
Muscovite
Quartz
Minerals at the bottom of the chart are most stable under the atmospheric conditions
at the earth’s surface. Mafic igneous rocks (gabbro, basalt) are composed of
the mineral constituents near the top of the chart; as a result they break down when
exposed to the atmosphere. Felsic igneous rocks (granite) are composed of the
more stable minerals near the bottom of the chart, and these rocks break down more
slowly. In the United States the National Institute of Standards and Technology has
constructed a stone test wall to study the performance of stone subjected to weathering.
It contains 2352 individual samples of stone, of which 2032 are domestic
stone from 47 states. There are 320 stones from 16 foreign countries. More than 30
rock types are represented. Common rock types used in building, such as marble,
sandstone, limestone, and granite, are represented by many varieties.
The deterioration of sandstone building materials is directly affected by their
chemical composition; those which contain carbonate as the natural cementing
agent are very susceptible to attack by acid rain. The loss of just a small amount
of the carbonate deprives the sand grains of their cementation. As with other rock
types, moisture is also a major destructive agent. Coarse-grained and porous sandstones
usually withstand freezing and frost action better than fine-grained ones
because water escapes more readily. However, some porous sandstones are also
poorly cemented and therefore friable. Figure 11.10 illustrates the extensive damage
caused by wind-blown sand and silt on poorly cemented sandstone building blocks.
The Republican and Early Imperial monuments in Rome were constructed
(largely) of tuffs quarried from local pyroclastic deposits. Over the centuries,