Stone and Geological Collections
Introduction
Many of the most famous landmarks and elements of the world’s cultural heritage are made of stone. Buildings, monuments and statues including the pyramids and Sphinx in Egypt, the temple city of Angkor Wat, the Taj Mahal, the statue of David and even the Sydney Opera House exemplify the significance of structures made from stone. While many of these structures have existed for millennia they will continue to deteriorate unless maintained and protected from various elements of decay.
Rocks are essentially aggregates of different minerals. Some rocks, such as limestone and quartzite, are composed of only one mineral, calcite and quartz respectively. Once cut and dressed, a rock is then referred to as a stone. Rocks are classified into three major groups:
- igneous;
- sedimentary; and
- metamorphic.
The distribution of rocks in the first 16 kilometres of the earth's crust is about 95 % igneous, 4 % clay rocks, 0.75 % sandstone and 0.25 % limestone. Of the rocks outcropping on the earth's surface (including the underwater surface) 5 % are igneous rocks, 4 % are metamorphic and 75 % are sedimentary with the rest being covered by ice.
Igneous rocks are formed as magma cools. They are either glassy such as obsidian and pumice (frothy glass), or crystalline such as granite, porphyry and dolerite. Igneous rocks are primarily silica (35 – 80 %) with smaller amounts of alumina (up to 25 %), potash, soda, lime, magnesia and iron oxides also present.
Sedimentary rocks are formed following cycles of weathering, sedimentation and lithification. Transportation of the sediments is followed by settling and then lithification (transformation into rock). All rock types (igneous, sedimentary and metamorphic) are subject to weathering. Sedimentary rocks are generally stratified, fine grained and heterogeneous. They may contain pebbles, sand, clay, shells, mineral grains and fragments of older rocks.
Sandstone is formed from sand particles cemented by secondary silica or calcite. Shale is formed from clay minerals plus mica and quartz. Limestones, which represent 25 to 35 % of all sedimentary rocks, are formed from either the precipitation of fine calcite (CaCO3) or from the build-up of fossilised shells. Soapstone is formed by the weathering products of talc (steatite) while alabaster is an aggregate of gypsum formed by chemical sedimentation.
Metamorphic rocks are formed when existing rocks are transformed by intense pressure, stress or heat. Their mineral components are recrystallised in the solid state, without melting or solution. All rock types, igneous, sedimentary and metamorphic, may undergo metamorphosis. For example as a result of metamorphic processes, granite is transformed to gneiss, sandstone to quartzite, limestone to marble and shale to slate.