Natural Science in Archaeology

3.3 Igneous Rock 51
When reading the obsidian literature or dealing with related artifacts, it is necessary
to become acquainted with ignimbrite, a related rock type. The term ignimbrite
refers to deposits from very hot pyroclastic flows, sometimes also called
ash-flow tuff or welded tuff. Most have a rhyolitic or dacitic composition and
contain feldspar, quartz, and biotite, along with glass. Phonolites are too low
in silica to contain quartz and are frequently porphyritic. Pitchstone is a glassy
volcanic rock, distinguished from obsidian by its higher water content. It occurs
as minor shallow intrusions and lava flows. Archaeologists have recorded pitchstone
artifacts from 101 archaeological sites in Scotland and northern England,
ranging in time from possibly Paleolithic to Bronze Age. Some pitchstone artifacts
have been found as far as 300 km from their source (Williams-Thorpe and
Thorpe 1984).
3.3.2 Intrusive Igneous Rocks
Wedepohl (1969) has estimated the relative abundance of intrusive (coarse-grained)
igneous rocks in the earth’s upper crust. His estimate is that 78% are of granite, granodiorite,
and quartz monzonite composition. From this we see the preponderance
of high-silica over low-silica, coarse-grained rocks. Granites contain both potassium
and sodium feldspars and from 20–40% quartz. Other minerals common in granites
are muscovite or biotite mica and one of the amphiboles, often hornblende. Syenites
are composed mainly of feldspars; quartz is absent or only a very minor constituent.
Diorites are low-silica, medium- to coarse-grained rocks in which plagioclase
is the predominant mineral with hornblende and biotite (dark mica) as the chief
mafic minerals. Monzonites are characterized by near-equal amounts of potassium
feldspar and plagioclase. Peridotites are high-magnesium ultramafic rocks composed
of olivine or its hydration product, serpentine [Mg 3 Si 2 O 5 (OH) 4 ]. Serpentinite
is an alteration product of a mafic igneous rock. It is important in archaeomineralogy
because it is soft enough to be carved, with a hardness of approximately 3.5
(see Chap. 6). The name comes from the Latin serpens in reference to the snake-like
patterns on some serpentine rocks.
Of the high-silica, coarse-grained, quartz-rich igneous rocks, granodiorites are
quantitatively the most important. Monuments and constructions composed of these
rocks are less subject to atmospheric weathering than low-silica igneous rocks. It
has been the author’s experience that many monuments, statues, and other artifacts
that are called granites are really granodiorites or sometimes diorites.
The term “granitic” is also used in a textural sense, implying that the rock is
granular, i.e., like a granite. Occasionally, the term “granitic rock” is used to refer
to the whole range of coarse-grained quartz/feldspar rocks. Granitic rocks are quite
hard and cohesive. They were used frequently for grindstones (Fig. 3.1). Quartz
monzonite was used as a construction material at many ancient sites around the
Mediterranean Sea including Ephesus on the Turkish Aegean coast (Birkle and
Satir 1994).