Rocks and Minerals [EARTH SCIENCE]

• Rocks are classified in three types based on how they are formed: Igneous, Sedimentary, Metamorphic
• Igneous rocks are formed when molten rock (magma) from within Earth cools and solidifies. There are two types: intrusive igneous rocks solidify beneath Earth's surface; extrusive igneous rocks solidify at the surface. Examples: Granite, basalt, obsidian
• Sedimentary rocks are formed when sediment (bits of rock plus material such as shells and sand) gets packed together. They can take millions of years to form. Most rocks that you see on the ground are sedimentary. Examples: Limestone, sandstone, shale
• Metamorphic rocks are sedimentary or igneous rocks that have been transformed by heat, pressure or both. Metamorphic rocks are usually formed deep within Earth, during a process such as mountain building. Examples: Schist, marble, slate
• The Rock Cycle

  [1] Igneous rocks form from the cooling and crystallization of hot molten lava and magma. Igneous rocks undergo weathering and erosion to form sediments.

  [2] Sediments are deposited and lithified by compaction and cementation to form sedimentary rocks.

  [3] Sedimentary rock become buried by additional sedimentary deposition, and when they are deep within the Earth, they are subjected to heat and pressure which causes them to become metamorphic rocks.
• The Rock Cycle

  [4] With further burial and heating, the metamorphic rocks begin to melt. Partially molten metamorphic rocks are known as migmatite.

  [5] As melting proceeds with increasing temperatures and depths of burial, eventually the rock becomes molten and becomes magma, which cools and crystallizes to form plutonic igneous rock, or which is erupted onto the Earth's surface as lava, and cools and crystallizes to form volcanic igneous rock.
• Further complications within the rock cycle include (1) weathering of sedimentary and metamorphic rocks (in addition to igneous rocks), and (2) metamorphism of igneous rocks and repeated metamorphism of metamorphic rocks.
• WEATHERING – Breaking down of rocks brought about by either physial or chemical means giving rise to sediments or ther rock fragments
• EROSION - the process by which rock fragments and sediments are carried along by such agents as wind and running water
• DEPOSITION - the process by which rock fragments and sediments ae carried by agents of erosion are dropped or deposited in other places.
• COMPACTING – the process by which rock fragments and other materials that accumulated, usually at the bottom of a thick column of water, get cemented together and harden into rock
• METAMORPHISM - a change in constitution of a rock brought about by pressure, heat and chemical action resulting in a more compact and highly crystalline condition of the rock.
• Types of Weathering

  • Physical or Mechanical Weathering
  • Chemical Weathering
  • Biological Weathering
• Physical or mechanical weathering

  • Frost wedging - water expands when it freezes
• Physical or mechanical weathering

  • Exfoliation or unloading
  • rock breaks off into leaves or sheets along joints which parallel the ground surface;
  • caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial
• Physical or mechanical weathering

  • Thermal expansion
  • repeated daily heating and cooling of rock;
  • heat causes expansion; cooling causes contraction.
  • different minerals expand and contract at different rates causing stresses along mineral boundaries.
• Chemical weathering

  Rock reacts with water, gases and solutions (may be acidic); will add or remove elements from minerals.
• Chemical weathering

  • Dissolution (or solution)
  • Several common minerals dissolve in water: halite and calcite
  • Limestone and marble contain calcite and are soluble in acidic water
  • Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution. Note that the urn and tops of ledges are heavily weathered, but the inscriptions are somewhat sheltered and remain legible.
• Chemical weathering

  • Dissolution (or solution): Caves and caverns typically form in limestone
  • speleothems are cave formations
  • speleothems are made of calcite
  • form a rock called travertine: stalactites - hang from ceiling and stalagmites - on the ground
• Chemical weathering

  • Dissolution (or solution):
  • Karst topography forms on limestone terrain and is characterized by: caves/caverns, sinkholes, disappearing streams, springs
• Chemical weathering

  • Oxidation
  • Oxygen combines with iron-bearing silicate minerals causing "rusting"
  • Iron oxides are produced . Iron oxides are red, orange, or brown in color
  • Mafic rocks such as basalt (which may contain olivine, pyroxene, or amphibole) weather by oxidation to an orange color
  • "Georgia Red Clay" derives its color from the oxidation of iron bearing minerals
• Biological weathering

  Organisms can assist in breaking down rock into sediment or soil.

  1. Roots of trees and other plants
  2. Lichens, fungi, and other micro-organisms
  3. Animals (including humans)
• Minerals are solid, inorganic (not living) substances that are found in and on earth. Most are chemical compounds, which means they are made up of two or more elements. For example, the mineral sapphire is made up of aluminum and oxygen. A few minerals, such as gold, silver and copper, are made from a single element. Minerals are considered the building blocks of rocks. Rocks can be a combination of as many as six minerals.
• Many minerals, such as gold and silver, are very valuable because they are beautiful and rare. Limestone, clay and quartz are other examples of minerals.

  
  

  Gems are minerals or pearls that have been cut and polished. They are used as ornaments, such as jewelry. Precious stones are the most valuable gems. They include diamonds, rubies and emeralds
• Rocks are naturally-occurring aggregates of minerals and mineraloids. There are three main classifications of rocks based on how they were formed: igneous, sedimentary, and metamorphic
• Igneous rocks (from Latin word ignis meaning “fire”) are formed when molten material cools and solidifies.

  When igneous rocks formed below the surface of the Earth, they are called intrusive igneous rocks or plutonic rocks. When they form on the surface, they are called extrusive igneous rocks or volcanic rocks.
• igneous rocks

  Intrusive and extrusive rocks can generally be distinguished using the size of their mineral grains. Intrusive rocks have bigger or coarser grain crystals, while extrusive rocks have smaller or finer crystals. This is because higher temperatures beneath the Earth’s surface slow down the cooling rate of minerals, giving more time for larger crystals to form.

  The composition of igneous rocks largely depends on what type of magma or lava they form from. The composition of magma is dependent on the amount of silica (SiO2), which affects its viscosity, and the temperature.
• igneous rocks

  Komatiite is a very rare type of extrusive igneous rock which forms when extremely hot lava cools rapidly and was common during the Archean eon. However, current surface conditions do not allow komatiite to form anymore.

  Other more common types of extrusive rocks are obsidian (formed when lava rapidly cools; also known as volcanic glass), bombs (rounded solidified lava fragments), blocks (angular solidified lava fragments), and volcanic ash
• Sedimentary rocks are formed from loose material called sediments that have been eroded in a process called weathering and then buried and compacted in a process called diagenesis.

  The sediments that make up sedimentary rocks can come from pre-existing rocks and materials or from the remains of living things. Because of this, there are two main classifications of sedimentary rocks: clastic sedimentary, non-clastic sedimentary
• clastic sedimentary rocks

  
  

  Sediments come from pre-existing rocks. Clastic sedimentary rocks are classified based on the characteristics of their clasts such as size, angularity/roundedness, and sorting.

  
• Non-clastic sedimentary rocks

  Chemical sedimentary rocks are formed when water evaporates, leaving behind dissolved minerals. Common examples include halite or rock salt, rock gypsum, flint, chert, travertine, umber, and some limestone rocks.

  
• Non-clastic sedimentary rocks

  Another type called biochemical or organic sedimentary rocks are composed of the remains of living things (shells, bones, plant fragments, etc.). Common examples include some fossiliferous limestone (contains fossils), chalk (composed of very tiny marine organisms called coccolithophores and foraminifera), coquina (composed of >2 mm shell fragments and grains), and coal (altered rock from remains of plant life).

  
• When a rock is subjected to certain chemical (addition or removal of chemicals) or physical (change in temperature or pressure) processes that alter its chemical composition, mineralogy, and/or texture, a metamorphic rock is formed.

  The original rock or “parent rock” that was altered is called a protolith. Metamorphic rocks are divided into two types based on their texture: foliated and nonfoliated rocks
• The first type is characterized by the appearance of planar arrangement of mineral grains called foliation or foliated rocks. Foliation in rock is the result of deformation and the more foliated a rock, the higher the grade of metamorphism.

  
• Nonfoliated rocks usually develop in environments where deformation is minimal and other factors such as chemically-active fluids play a larger part in altering the rock. Some common examples of non foliated rocks are marble, hornfels, quartzite, metapelite, and others.

  Rocks continually go through changes that alter its characteristics, ultimately changing it into a different rock.

  
• Rock Cycle

  
• Minerals are building blocks of rocks. To be considered a mineral, it must be the following:

  • Naturally-occurring. Man-made materials such as synthetic diamonds cannot be considered as real minerals.
  • Inorganic. Organic materials such as pearls or sugar are not minerals.
  • Homogeneous solid. Minerals should be crystalline solids. Water is not mineral, whereas ice is considered a mineral. Mercury occurs as a liquid in its natural state and is considered as a mineraloid.
• Minerals are building blocks of rocks. To be considered a mineral, it must be the following:

  • Has definite chemical composition. You should be able to describe a mineral’s composition using a chemical formula.
  • Ordered crystalline structure. Atoms in a mineral are placed in a repetitive and orderly manner. Substances that lack this kind of atomic structure such as obsidian (volcanic glass) or plastic are not considered as minerals.
• Minerals come in all sorts of appearances. In order to identify different minerals, certain properties are observed in hand specimens. Here are some of the most commonly used properties in describing a mineral’s appearance:

  • Color. It refers to the wavelengths of light reflected by the minerals. While it can be tempting to identify a mineral based on its color, it is the least useful property because a lot of minerals can occur in different colors.
• Minerals come in all sorts of appearances. In order to identify different minerals, certain properties are observed in hand specimens. Here are some of the most commonly used properties in describing a mineral’s appearance:

  • Luster. It describes how light is reflected from the mineral’s surface. A mineral could have a metallic luster or nonmetallic luster similar to pearls (pearly), glass (vitreous), resin (resinous), silk (silky), or others. Brilliantly cut gems are described to have an adamantine luster