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.