Identifying rocks and minerals This powerpoint outlines the three common rock types – igneous, sedimentary and metamorphic rocks – and also establishes the difference between rocks and minerals.

Rocks Naturally occurring aggregate of minerals Lithosphere (Earth’s crust made of rocks) Three main types igneous, metamorphic and sedimentary Petrology is the scientific study of rocks. Does not have a fixed chemical composition. These are the general characteristics of rocks – more detail about three major rock types is provided in following slides.

Igneous Rocks Crystallise from molten material either above (volcanic) or below (plutonic) the Earth’s surface Characterised by interlocking crystals Volcanic rocks fine-grained Plutonic rocks coarse-grained Image sources: http://www.fs.fed.us/r2/gmug/permits/grand_valley/gv_rock.shtml http://www.beg.utexas.edu/mainweb/publications/graphics/granite.htm When rocks melt they become molten. Rocks may be pulled beneath the surface of the Earth due to tectonic processes. For example, at subduction zones oceanic crust is pulled under continental crust as it is more dense. As the rocks are pulled down, the heat from deep inside the Earth melts the rocks. Molten material on the surface of the Earth is called lava. As this material cools and crystallises it produces fine-grained rocks called volcanic rocks. The rocks are fine-grained because it is cool at the surface of the Earth, therefore the lava crystallises quickly. The minerals do not have a long time to grow large, rather, as many minerals are crystallising at the same time, they grow in any spare space around them. Molten material beneath the Earth’s crust is call magma. If this material cools and crystallises underground the rocks are called plutonic rocks. Plutonic rocks have large crystals as the molten material has cooled slowly, giving individual seed crystals time and space to grow large crystals. The photo shows a coarse-grained rock. If molten material spends some time beneath the surface cooling, and then is quickly brought to the surface, the resultant rock has some large crystals in a finer-grained matrix. This type of rock is called a porphyry. The porphyritic texture indicates two separate stages of solidification. In the first phase the phenocrysts (large crystals) form in the molten mass below the ground; in the second, the molten mass itself crystallizes into a solid. Porphyritic texture is especially common in extrusions.

Metamorphic Rocks Formed as a result of heat and pressure being applied to pre-existing rocks Characterised by foliation Structure where crystals align perpendicular to the direction of pressure Classified by texture and mineral/chemical assemblage. Image Source : http://www.cropsoil.uga.edu/soilsandhydrology/Important%20Rocks%20&%20Minerals.htm Metamorphic rocks form when a rock is exposed to temperature and pressure conditions that change the physical appearance and chemical constituents of the rock. The original rock that undergoes changes is called the parent rock. The temperature and pressure is often applied as a result of tectonic forces. Metamorphic rocks are classified based on their mineral assemblages and their texture. Metamorphism takes place through heat, pressure, and chemically active fluids, but there are many combinations of heat, pressure, and chemically active fluids and so there are many different responses – therefore many different rocks produced. Any rock type (igneous, sedimentary or metamorphic rock) can be the parent rock, so the resultant rock is also influenced by the parent rock, as well as the pressure, temperature and fluid conditions. Different parent rocks that undergo the same kinds of metamorphism (Temp, pressure and fluid conditions) can end up looking the same. Some common texture names given to metamorphic rocks include schist and gneiss. A schist is a type of layering or foliation caused because minerals large enough to be seen by eye tend to line up all in the same direction. Schistose rocks include Greenschist (chlorite dominated), Blueschist (glaucophane dominated, plus many other minerals), and just plain Schist (biotite or muscovite, quartz, and feldspar dominated), and each of these form under different tectonic conditions. To add confusion some rocks with a schist-like texture do not include the term in the name, like amphibolite. Gneissic rocks can be produced from both sedimentary and metamorphic parents, and not always under the same metamorphic conditions.

Sedimentary Rocks Formed when sediments are cemented together at or near the surface of the Earth Sediments formed as a result of chemical or mechanical weathering, erosion, transportation and deposition. Commonly form layers as sediment is deposited following erosion and transportation May contain structures to indicate way-upness! Image source: http://www.edupic.net/geo.htm Any rock on the surface of the Earth is undergoing weathering. The weathering may be in the form of mechanical or chemical weathering. Mechanical weathering is the breakdown of rocks without changing the chemical composition of the minerals within the rock. This is most commonly caused by temperature changes. For example, during the day water may find its’ way into small cracks in a rock. Overnight, if the temperature decreases enough to freeze the water, the ice that forms will expand and crack the rock further. If this happens a number of times, the crack becomes larger, eventually breaking parts of the rock off. This increases the surface area of the rock exposed to chemical weathering. Chemical weathering is the breakdown of rock by chemical reaction, which changes the materials in a rock into particles that can be easily carried away. Air and water are commonly involved in these reactions. Rock particles in the form of clay, silt, sand, and gravel, and dissolved chemicals produced by chemical weathering, are transported by the agents of erosion (usually water, and less frequently by ice and wind) to new locations and redeposited in layers, generally at a lower elevation. These deposited particles eventually become compacted and cemented together, forming sedimentary rocks.

Classifying Sedimentary Rocks Breccia Conglomerate Sandstone Halite Limestone Coal Sedimentary rocks are classified into three groups, clastic, chemical precipitate and biochemical sedimentary rocks. Clastic sedimentary rocks are formed from clasts (discrete fragments) of materials from other rocks. These clasts form when other rocks are exposed to weathering (by chemical or biological means) and erosion. Transported sediments (by wind, water or other means) are deposited and this assemblage of material is cemented together to form a sedimentary rock. Clastic sedimentary rocks are classified based on the size of the clasts. The finest particles (<0.002mm) form shale, silstone has particles between 0.002 – 0.063mm, sandstone particles are 0.063 – 2mm, and rocks are classified as conglomerates or breccias if the particles are 2-263mm in size. The difference between a conglomerate and a breccia is that conglomerates have rounded clasts, while breccias have angular clasts. Biochemical or organic sedimentary rocks contain materials generated by living organisms. Examples include carbonate minerals such as limestone which are formed by the calcite remains of coral, molluscs and foraminifera that cover the ocean floor. Coal and oil shale are the product of the remains of tropical plants buried and subjected to pressure. Chemical sedimentary rocks form when mineral solutions, such as sea water, evaporate. Examples include the evaporite minerals halite (rock salt) and gypsum.

Minerals Naturally occurring substance formed through geological processes Have a characteristic chemical composition, a highly ordered atomic structure and specific physical properties Range in composition from pure elements and simple salts to very complex silicates with thousands of known forms Study of minerals is called mineralogy Can be mined and processed to obtain constituent metals Image Source: http://en.wikipedia.org/wiki/Mineral A mineral is any naturally-occurring, homogeneous solid that has a definite chemical composition and a distinctive internal crystal structure. Minerals are usually formed by inorganic processes. Synthetic equivalents of some minerals, such as emeralds and diamonds, are often produced in the laboratory for experimental or commercial purposes. While most minerals are chemical compounds, some such as copper and gold occur naturally as elements. The composition of a mineral can be defined by its chemical formula. The common mineral groups are native elements, sulfides, sulfosalts, oxides and hydroxides, halides, carbonates, nitrates, borates, sulfates, phosphates, and silicates. Silicates are the most commonly occurring minerals because silica is the most abundant constituent of the Earth's crust (about 59 percent). A mineral crystallizes in an orderly, three-dimensional geometric form, so that it is considered to be a crystalline material. Along with its chemical composition, the crystalline structure of a mineral helps determine such physical properties as hardness, color, and cleavage. Minerals combine with each other to form rocks. For example, granite consists of the minerals feldspar, quartz, mica, and amphibole in varying ratios. Rocks are thus distinguished from minerals by their heterogeneous composition. A mere 100 of the several thousand known types of minerals constitute the main components of rocks. Can use this interactive key to help identify minerals based on their physical properties…. Interactive key: http://geology.csupomona.edu/alert/mineral/minerals.htm

Rocks v Minerals- What’s the difference? A mineral is made up of one substance that under normal circumstances would form crystals. Rocks are made up of minerals.  Most rocks have several types of minerals in them, but some have only one type of mineral.  For example, sandstone is mostly quartz.  Examples: Minerals:  Quartz, feldspar and mica. Rocks: Granite, pegmatite, shale, slate, gneiss, schist (all made of quartz, feldspar and mica) Mineral:  Calcite    Rock: Limestone and marble (made of calcite) Minerals: Plagioclase feldspar and augite Rock: Basalt (oceanic volcanic rock made of plagioclase and augite) Source: http://borntoexplore.org/ryan/rocksvs.htm

Task Identify the samples provided as either rocks or minerals, justifying your response for each.