Natural Science in Archaeology

56 3 Exploitation of Mineral and Rock Raw Materials
that those who sought election whitened their togas with chalk, and Plato said wells
were dug to the chalk subsoil. Plato also indicated that in shipbuilding and house
construction chalk was used to draw lines (much as we do today). Strabo at least
twice referred to the city of Cameirus on the island of Rhodes as white with chalk.
Complete evaporation of seawater (in basins cut off from the sea) has concentrated
soluble salts such as halite in deposits of high purity. These deposits, referred
to as evaporates, include a great number of minerals. However, only gypsum, anhydrite,
and halite are of any importance in archaeomineralogy. Gypsum in sedimentary
deposits can form almost pure layers of considerable thickness. Deposits of
anhydrite and gypsum are strictly chemical deposits. These minerals are not biogenic
and only rarely form clastic deposits, although beds of gypsum and anhydrite
occur in many water-laid formations around the world. They are more common than
halite but much less abundant than limestone. Large evaporite deposits occur in the
Zechstein Basin in northern Europe.
Evaporite deposition may also occur from saline pore waters in sediments
exposed on arid supratidal flats and in playas. For example, gypsum and anhydrite
are currently being deposited on the south side of the Persian Gulf where the rate of
evaporation is high and seawater periodically floods extensive supratidal flats. At
least some ancient evaporite deposits probably originated this way. Sodium carbonate,
called soda ash from the ancient practice of obtaining sodium carbonate from
seaweed ash, is a related sedimentary mineral.
Travertines (“Sinter” in German) are freshwater carbonate deposits found in
caves, near the mouths of springs, as hot spring deposits, and as coatings on cliffs.
Impure and porous varieties are called calcareous tufa. The principal mineral is
calcite.
3.4.2 Terrigenous Sedimentary Rocks
Sandstones constitute a group of clastic rocks whose average grain size ranges from
1/16 to 2 mm. Sandstones are made up of a mixture of mineral grains and rock
fragments that are derived from the weathering and erosion of rocks of all kinds,
including older sediments. Their constituent mineralogy varies widely, but quartz
predominates. The average sandstone contains about 65% quartz. In addition to
quartz and feldspar, large detrital grains of muscovite, biotite, and chlorite are common
(but minor) constituents. The mineral grains are cemented predominantly by
silica (chalcedony, opal, or quartz), carbonates, or iron oxides.
Sandstone varies from poorly cemented rock with microfractures and joint planes
to compact orthoquartzites of immense strength and durability. Highly compact
quartzites are about as tough a lithic material as one can find. Late Pleistocene and
Holocene flaked stone artifacts from one rock shelter in the Australian desert are
dominantly a low grade orthoquartzite. (Smith 2006)
In ancient Egypt, sandstone was used for foundations, pillars, walls, and even
as roofing. Pharaonic stone monuments at Luxor and Karnak Temples in Luxor