Belfast Naturalists’ Society
Workshop: minerals & rocks for
archaeologists
Dr. Rob Raine
(Geological Survey of Northern Ireland)
November 2015
Minerals
The ‘atoms’ or ‘building blocks’ of rocks: most rocks contain more than one mineral. There
are thousands of minerals, but only some 20 –30 are common rock-forming ones. Of these,
a small number (<10) are important in ‘everyday archaeology’.
How to identify minerals: each mineral has a specific chemical composition and crystal
structure (3D arrangement of its atoms). The latter controls its physical properties such as
hardness, presence or absence of cleavage(s), density, etc.
Important, common minerals in Archaeology:
Quartz,
Calcite
Feldspar
Mica
Oxides and other ores (malachite/ochre/haematite/galena)
Salt (halite)
But also the rare minerals ‘jade’, gold, lapis lazuli, etc.
Igneous Rocks
Solidification of magmas such as basalt and granite, generated by melting several 10s of km
beneath the Earth’s surface.
Solidification of lavas: fine-grained or glassy rocks (e.g. basalt, obsidian). Lava is magma
which has reached the surface.
Near-surface solidification in small bodies takes longer and the crystals grow slightly bigger
than those of lava into medium-grained rocks (e.g. dolerite).
Subsurface solidification in large masses: coarse-grained (e.g. gabbro, granite): individual
crystals can be seen clearly and identified.
Important factors: magma composition and cooling rate.
Most igneous rocks have a crystalline texture (c.f. sandstones) and lack a ‘mineral
orientation’ (c.f. schists).
Important igneous rocks in archaeology:
Basalt and dolerite
Obsidian (and pitchstone)
Granite
Tuff
Metamorphic Rocks
Solid state recrystallization of pre-existing rocks (no melting)
Important factors: original rock composition, temperature and pressure.
Two main types of metamorphism: contact (around igneous intrusions) and regional (large
areas).
Important metamorphic rocks in archaeology:
Porcellanite (contact type): originally red, iron- and aluminium-rich, weathered basalt.
Quartzite (regional type): originally quartz-rich sandstone
Schist or soapstone (steatite) (regional type): originally shale
Marble (contact and regional): originally limestone
Sedimentary Rocks and Fossils
Rocks formed of fragments and grains derived from other rocks or from the bodies of once
living organisms.
Sedimentary rocks can be divided into clastic (rock fragments) and chemical (precipitates).
Sedimentary rocks form in environments such as rivers, lakes and the sea and require some
pressure from overlying strata to expel water and lithify them (turn them from sediment
into rock).
Important sedimentary rocks in archaeology:
Limestone
Coal
Lignite (lithified peat) & Shale (laminated mudrock)
Flint, chert
But also the more rare sedimentary rocks amber (fossil tree sap) and jet (fossil monkeypuzzle wood)
Neolithic amber beads, Denmark
Jet necklace, Bronze age, Rasharkin, Co. Antrim.