Natural Science in Archaeology

5.4 Carbonate and Sulfate Minerals 111
Muscovite, [KAl 2 (AlSi 3 )O 10 (OH) 2 ], has a hardness of 2.5 parallel to its cleavage
sheets and 4.0 perpendicular to the sheets. Its density can vary from 2.77 to
2.88. It has a pearly luster and is colorless to yellow or pale brown. It was used in
ancient Egypt for beads during the Predynastic Period. Its use expanded to pendants
and attachments during the Middle Kingdom, particularly by the Kerma culture in
ancient Nubia (Andrews 1991).
Slate is a fine-grained metamorphosed shale. Slate is highly siliceous and occurs
in a wide range of colors, including gray, black, green, purple, red and brown. It
cleaves in sheets. The ancient Egyptians of the Early Dynastic Period manufactured
rings and bangles of green slate from the Eastern Desert (Andrews 1991).
5.3.3 Glassy
Obsidian is a volcanic glass whose color is usually brown or black. According
to Pliny, obsidian obsianus was named for Obsius who discovered the material
in Ethiopia. This widely distributed material was prized in prehistoric times
in Europe, Asia, and North America. Although used most commonly for tools
because it produced a sharp cutting edge, it was also used to manufacture decorative
and ceremonial objects, statues, beads, and gems. The ancient Egyptians
used obsidian as early as Predynastic times. It was not locally available and so
was imported, probably from Abyssinia (Lucas 1989). Obsidian from El Chayal,
Guatemala, was used by the Maya. In North America, “Apache tears” were valued
obsidian ornaments. Pliny lists sources for obsianus lapis as Ethiopia, India,
Italy, and Spain (N.H. 36.196–197; 37.177). Obsidian is treated more extensively
in Chap. 4.
5.4 Carbonate and Sulfate Minerals
5.4.1 Coarse-Grained
Calcite, CaCO 3 , has a hardness of 3 and a density of 2.71. Its crystals are colorless or
white when pure. However, impurities often produce various shades of gray, yellow,
brown, red, green, blue, and black. Its name comes from Latin calx (“lime”). Calcite
has perfect rhombohedral cleavage and large calcite crystal masses will cleave into
clear gem-like rhombs. These were admired far back into antiquity and have been
recovered in excavations worldwide. A transparent, colorless form of crystalline
calcite known as “Icelandic Spar” is found in the Egyptian Eastern Desert. Small
beads of this material are known dating from the New Kingdom and Third Intermediate
Period (Andrews 1991). Calcite crystals were recovered from German silver
mines as far back as classical times.