ROCKS & MINERALS UNIT...Metamorphic Rocks and Minerals •Sometimes rock are subject to pressure and...
Transcript of ROCKS & MINERALS UNIT...Metamorphic Rocks and Minerals •Sometimes rock are subject to pressure and...
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ROCKS & MINERALS UNIT 8th Grade Earth & Space Science
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Characteristics of Minerals
8th Grade Earth & Space Science Class Notes
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Mineral Characteristics
• Naturally occurring – formed by natural processes
• Inorganic – not alive; does not come from anything living
• Solid, crystalline structure – atoms arranged in in a regular geometric pattern
• Specific composition – unique chemical formula (can be an element or compound)
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Examples of Minerals
Halite (NaCl)
Galena (PbS)
Pyrite (FeS2)
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How Minerals Form
• Minerals form during crystallization
– Process by which atoms are arranged into a specific, repeating structure
• There are two ways minerals can form:
– Crystallization of magma
– Crystallization of materials dissolved in solution
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Crystallization of Magma
• When magma cools from a liquid state crystals form.
– Size of the crystals depends on:
• Rate of cooling
• Amount of gas in the magma
• Chemical composition of the magma
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Crystallization from Solutions
• When a solution becomes supersaturated minerals will form through precipitation.
• Minerals can also crystallize when the solution evaporates.
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Identifying Minerals
• All minerals have unique characteristics.
• You must use a combination of characteristics to correctly identify a mineral!
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Crystal Form
• Some minerals form distinct crystal shapes.
Halite always forms cubes. Quartz forms six-sided crystals with double pointed ends.
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Luster
• The way a mineral reflects light
– Metallic luster – shiny faces that reflect light
– Nonmetallic luster – does not shine like a metal; can be dull, pearly, waxy, silky, or earthy
Metallic luster Nonmetallic luster (earthy)
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Hardness
• Measure of how easily a mineral can be scratched
– Friedrich Mohs developed the Mohs Scale of Hardness
– See page 91 in book
– Scale goes from 1 to 10
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Hardness
– Mineral that can be scratched with fingernail = 2 or less
– Mineral that cannot be scratched with fingernail but cannot scratch glass = Between 2.5 – 5.5
– Mineral that can scratch glass = 5.5
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Cleavage and Fracture
• Cleavage – describes the way a mineral breaks along its plane of weakness
• Fracture – when a mineral breaks into pieces with rough or jagged edges
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Cleavage Examples
One directional cleavage Two directional cleavage Three directional cleavage
Cubic cleavage (halite) Octahedral cleavage
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Fracture Examples
Conchodial Fracture (arch like patterns)
Non conchodial fracture (no archlike patterns)
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Color
• Most noticeable characteristic of a mineral; usually caused by presence of trace elements
• Examples: obsidian – black; sulfur - yellow
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Special Properties
• Can sometimes be used for identification
• Double refraction – light that travels through the mineral is split in two ways
• Effervescence – reacts with hydrochloric acid • Magnetism – occurs between minerals that contain
iron • Iridescence – play of colors caused by the bending of
light rays • Fluorescence – glows in the dark when exposed to
ultraviolet light
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Examples of Special Properties
Double refraction in Calcite
Iridescence in Franklinite
Fluorescence in Fluorite
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Streak
• Color of the mineral when it is broken up and powdered
• Example: Sulfur – white; hematite - red
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Texture
• How a mineral feels to the touch
• Examples: obsidian – smooth, talc - greasy
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Density and Specific Gravity
• Sometimes, two minerals of the same size have different weights (result of density)
• Density = mass/volume; reflects the atomic mass and structure of the mineral
• Specific gravity – ratio of the mass of a substance to the mass of an equal volume of water at 4 C. – We measure by picking up the mineral and
determining how “heavy it feels” compares to how “heavy it looks.”
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Formation of Igneous Rocks
8th Grade Earth & Space Science
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Igneous Rock Formation
• All igneous rocks form from magma.
• The type of igneous rock that forms depends on the chemical composition of the magma.
• Magma – slushy mix of molten rock, dissolved gases, and mineral crystals.
• Lava – magma that flows out onto the Earth’s surface – Chemical composition may be different from magma
because gases escape into the atmosphere
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All About Magma
• The common elements in magma are the same that are common in the Earth’s crust.
– O, Si, Al, Fe, Mg, Ca, K, Na
• There are three main types of magma and they are classified based on their silica content.
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Types of Magma Magma Type Silica Content Example
Basaltic 42 – 52% Hawaiian Islands
Andesitic 52 – 66% Cascades
Rhyolitic 66% and up Yellowstone
Basaltic
Andesitic
Rhyolitic
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How Does Magma Form?
• Forms from the melting of the Earth’s crust or melting of rock in the mantle.
• Four factors are involved in the formation of magma:
- temperature
- pressure
- water content
- mineral content
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Temperature and Magma Formation
• Temperature increases with depth in Earth’s crust.
• This temperature increase is known as the geothermal gradient.
• For magma to form, the
temperature must be
high enough to melt the
rock.
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Pressure and Magma Formation
• Pressure also increases with depth as a result of the weight of overlying rock.
• The increased pressure on a rock will increase the rock’s melting point and require higher temperatures to melt.
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Water Content and Magma Formation
• Rocks and minerals often contain small amounts of water, which changes the melting point of the rocks.
• As water content increases, the melting point decreases so rock will melt at a lower temperature.
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Mineral Content and Magma Formation
• Different minerals have different melting points.
• Rocks that are rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon.
Melting point of hematite = 1565 °C Boiling point of muscovite = 1250 °C
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Partial Melting
• Some minerals will melt while others remain solid.
• This changes the composition of the magma and rocks.
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Fractional Crystallization
• Opposite of partial melting
• Minerals that melted last will crystallize first
– These get removed from the magma and cannot react with the magma and change its composition
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Bowen’s Reaction Series • Shows the relationship between cooling
magma and the formation of minerals that make up igneous rock
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Reactions of Iron-Rich Minerals
• Undergoes abrupt changes
– As minerals form they react with the magma to become new minerals
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Reactions of Feldspars
• Undergo continuous changes
• Start with calcium-rich compositions that slowly become sodium-rich as magma cools
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Igneous Rock Classification
8th Grade Earth & Space Science - Class Notes
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Formation and Texture
• Intrusive – forms under the Earth’s surface
– Large grain magma cools slowly and large crystals form
• Extrusive - forms on top of the Earth’s surface
– Small grain magma cools too quickly and small or no crystals form
Extrusive Intrusive
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Formation and Texture
• Porphyritic – two different crystal sizes
– Caused by cooling in different environments
• Vesicular – spongy appearance
– Caused by gas bubbles
Porphyritic texture Vesicular texture
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Mineral Content
• Basaltic – dark color, low silica content
• Granitic – light color, high silica content
• Intermediate – in between basaltic and granitic
• Ultrabasic – very dark, only iron-rich minerals
Basaltic
Granitic
Intermediate
Ultrabasic
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Rocks as Resources
• Veins – also called “igneous intrusions”; small spaces that contain valuable ores
• Pegmatites – veins of very large grained minerals that contain rare elements
Gold Vein Beryl Crystal in Pegmatite
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Rocks as Resources
• Kimberlites – type of peridotite (ultrabasic rock) that contains diamonds
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Formation of Sedimentary Rocks
8th Grade Earth & Space Science
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Sedimentary Rocks Form Through a Process
1. Weathering and Erosion
2. Deposition and Sorting
3. Lithification
Let’s look at each step of this process!
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Weathering and Erosion
• Weathering – the set of physical and chemical changes that breaks rocks into smaller pieces
• Size can range from microscopic to huge boulders.
• Physical weathering rock fragments break off
• Chemical weathering minerals in a rock are dissolved or are chemically changed
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Weathering and Erosion
• Erosion – the removal and transport of sediment
• Four Main Agents: – Glaciers – Wind – Water – Gravity
• For these reasons eroded sediment will eventually wind up downhill
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Deposition and Sorting
• Occurs when sediments settle on the ground or sink to the bottom of water (deposition)
• Usually large grains will settle to the bottom and finer grains on top (sorting)
• Sorted deposits water and wind
• Unsorted deposits glaciers and mudslides
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Lithification
• Most sediments wind up at low points (valleys or bottom of ocean basin)
• As sediment builds up, pressure and temperature increase in bottom layers
• This leads to compaction and cementation!
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Lithification Steps in Lithification:
1. Compaction – layers of sediment are pushed together
– Some materials compact better than others
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Lithification
2. Cementation – mineral growth glues sediments together into solid rock
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Sedimentary Features • The primary feature of sedimentary rock is
horizontal layering called bedding.
– results from the way sediment settles out of water or wind
Graded – coarser particles towards the bottom
Cross – inclined layers of sediment
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Sedimentary Features
• When sediment is moved into small ridges by wind or wave action or by a river current, ripple marks form.
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Sedimentary Features • As sediment is transported, pieces that began
with an angular shape knock into each other and become rounded as their edges are broken off.
• Harder materials more rounded
• Further distance traveled more rounded
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Sedimentary Features • Fossils are the preserved remains,
impressions, or any other evidence of once-living organisms.
• During lithification, parts of an organism can be replaced by minerals and turned into rock.
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Types of Sedimentary Rocks
8th Grade Earth & Space Science
Class Notes
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Types of Sedimentary Rocks
• Sedimentary rocks fall into one of three categories:
– Clastic
– Chemical
– Biochemical
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Clastic Sedimentary Rocks • Formed from the abundant deposits of loose
sediments that accumulate on the Earth’s surface – most common type
• Classified based on the sediment size:
– Coarse-grained
– Medium-grained
– Fine-grained
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Clastic Sedimentary Rocks
Coarse-grained Rocks – contain gravel sized rock and mineral fragments
Conglomerate • Rounded particles
Breccia • angular particles
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Clastic Sedimentary Rocks Medium-grained Rocks – contain sand sized rock and mineral fragments
Sandstone • Contains quartz and rock • High porosity so often contains oil, gas, and ground water
Arkose • Contains quartz, potassium feldspar and rock
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Clastic Sedimentary Rocks
Fine-grained Rocks – contains silt and clay sized particles
Siltstone • Quartz and clay • Fine-grained
Shale • Quartz and clay • Very fine-grained
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Chemical Sedimentary Rocks • Forms after the concentration of dissolved
minerals reaches saturation and crystal grains settle out
• Water evaporates and the minerals become more concentrated leading to thick layers on the bottom of a body of water. – Usually forms in areas that lack precipitation
• See Table 6.1 in your textbook for examples and classification
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Chemical Sedimentary Rock Examples
Rock Salt Microcrystalline Gypsum
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Biochemical Sedimentary Rocks
• Form from the remains of once-living organisms
• Most abundant is limestone (made of calcite)
• Many contain fossils
• See Table 6.1 for examples and classification
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Biochemical Sedimentary Rock Examples
Limestone
Coquina
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Metamorphic Rocks
8th Grade Earth and Space Science
Class Notes
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Metamorphic Rocks and Minerals
• Sometimes rock are subject to pressure and heat but do not melt.
• Instead the mineral composition, texture, or chemical composition of the rock changes
• Metamorphic rock – rocks that changes form while remaining solid
• Metamorphic mineral – minerals that form during metamorphism
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Metamorphic Rocks and Minerals
Granite Gneiss forms from Granite
Some example metamorphic minerals. These minerals are stable at different conditions than other minerals!
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Metamorphic Textures
• Foliated – layers and bands of minerals
– Forms when pressure is applied in opposite directions
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Metamorphic Textures
• Nonfoliated – blocky, crystal shapes
– Forms when pressure is not applied in opposite directions
Example – marble from limestone
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Porphyroblasts • Mineral growth that results from the
rearrangement of atoms during metamorphism
Garnet growing in quartzite
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Grades of Metamorphism
• Low grade low temperature and pressure
• Intermediate grade in-between
• High grade high temperature and pressure
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Types of Metamorphism
• Regional – caused by high temperature and pressure in large regions of the Earth’s crust
• Can range from high to low grade
• Changes in mineral & rock types • Folding & deforming of rock
layers
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Types of Metamorphism
• Contact – when molten material comes in contact with solid rock
• High temperatures • High to low pressure • Metamorphic effects decrease
with distance
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Types of Metamorphism
• Hydrothermal – when very hot water interacts with rock
• Original texture and
mineral composition can change
• Ore deposits of gold, copper, zinc, tungsten, and lead form this way
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Economic Importance
• Metallic resources – gold, silver, copper and other metallic minerals form through hydrothermal metal deposits
• Nonmetallic resources – Talc, asbestos, and graphite form through metamorphism
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The Rock Cycle • Rocks are continually changing above and
below the Earth’s surface.
• This continuous changing is called The Rock Cycle.