How Could the Same Minerals Form Different Rocks?
Rocks are essentially collections of minerals. Even if the same minerals are present, different proportions of each mineral can lead to vastly different rock types. The mineral composition of a rock reflects the physical environment and geologic history where a rock formed.
Rock form in a variety of geologic setting ranging from locations on or near the earth surface, deep underground, or even in outer space. Most of the rocks we see on the surface of the planet formed by processes that happened long ago, but we can see these processes actively taking place in many places.
Rapid rock formation can be seen happening such as lava cooling from a volcanic eruption in places like Hawaii or Iceland. However, most rocks we see around us form very slowly in settings that are not visible on the land surface.
Slow processes creating rocks can be inferred by observing reefs growing in the oceans, or sediments being carried by flowing water in streams or moved by waves crashing on beaches. We can see sediments being deposited, but we cannot see them turning into stone because the process may take thousand or even millions of years.
While rocks consist of aggregates of minerals, minerals themselves are made up of one or a number of chemical elements with a definite chemical composition. Minerals cannot be broken down into smaller units with different chemical compositions in the way that rocks can. More than two thousand three hundred different types of minerals have been identified. Luckily many are rare and the common rocks are made up of a relatively small number of minerals.
Pressure and Temperature Conditions: Different minerals are stable under different temperature and pressure conditions. While the same starting minerals might exist in a molten rock, changes in these conditions during cooling and crystallization can cause different minerals to form or existing minerals to recrystallize in new forms. Metamorphic rocks are a prime example of this; limestone (calcite) can be metamorphosed into marble (still calcite, but recrystallized) under heat and pressure.
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| Combinations of common minerals occur in different kinds of rocks. The kind of rock depends on the geologic setting where they form: igneous, sedimentary, or metamorphic. |
Formation Process: Rocks are formed through three primary processes: igneous (cooling of molten rock), sedimentary (accumulation and cementation of sediments), and metamorphic (alteration of existing rock by heat, pressure, and/or chemically active fluids). The same minerals can participate in different rock-forming processes, leading to distinct rock types. For example, quartz can be found in granite (igneous), sandstone (sedimentary), and quartzite (metamorphic).
Identifying the common minerals
These include features such as crystal form, hardness (relative to a steel blade or you finger nail), colour, lustre, and streak (the colour when a mineral is ground to a powder). More detailed explanations of these terms and other aspects of mineral identification may be found in field handbooks or textbooks. Generally the characteristics listed above can only be determined if the mineral grains are visible in a rock. Thus the identification key distinguishes between rocks in which the grains are visible and those in which the individual mineral components are too small to identify.
The six minerals olivine, quartz, feldspar, mica, pyroxene and amphibole are the commonest rock-forming minerals and are used as important tools in classifying rocks, particularly igneous rocks. Except for quartz, all the minerals listed are actually mineral groups. However, instead of trying to separate all the minerals which make up a group, which is often not possible in the field, they are dealt with here as a single mineral with common characteristics.
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