Mineral Formation and Characteristics

• Mineral characteristics include being naturally occurring, inorganic, solid, crystalline with a specific chemical composition.

How Minerals Form

• Minerals form through precipitation from aqueous solutions with temperature change
• Minerals form through crystallization from magma with temperature change
• Biological precipitation by organisms can form minerals

Examples of Minerals

• Galena, Pyrite, and Halite are examples of minerals.

Crystallization from Solutions

• Minerals will form through precipitation when a solution becomes supersaturated
• Minerals can also crystallize when the solution evaporates

Crystal Form

• Halite always forms cubes
• Quartz forms six-sided crystals with double pointed ends

Crystallization of Magma

• When magma cools from a liquid state, crystals form
• Crystal size depends on the rate of cooling, the amount of gas in the magma, and the chemical composition of the magma

Physical Properties

• The streak of a mineral refers to the color of the powdered mineral
• The luster of a mineral is the way its surface reflects light
• Metallic luster is when the mineral reflects light with shiny faces
• Nonmetallic luster is when the mineral does not shine like a metal and can appear dull, pearly, waxy, silky, or earthy

Hardness – Mohs Scale

• Mohs Scale for hardness from softest to hardest: 1 Talc, 2 Gypsum, 3 Calcite, 4 Fluorite, 5 Apatite, 6 Feldspar, 7 Quartz, 8 Topaz, 9 Corundum, 10 Diamond

Hardness

• Hardness is a measure of how easily a mineral can be scratched
• Friedrich Mohs developed the Mohs Scale of Hardness, ranging from 1-10
• A mineral that can be scratched with a fingernail has a hardness of 2 or less
• A mineral that cannot be scratched with a fingernail and cannot scratch glass has a hardness between 2.5 and 5.5
• A mineral that can scratch glass has a hardness of 5.5

Other Properties

• Heft is how heavy a mineral feels in the hand, which is an informal sense of density
• Taste is definitive for halite, as well as a few other evaporite minerals

Cleavage and Fracture

• Cleavage describes how a mineral breaks along its plane of weakness
• Fracture occurs when a mineral breaks into pieces with rough or jagged edges
• Fracture is breakage that is not flat
• Conchoidal fractures are shell-shaped (smooth, curved breaks like glass), seen in quartz
• Uneven fractures are rough, irregular surfaces

Color

• Color is the most noticeable characteristic of a mineral, usually caused by the presence of trace elements
• Obsidian is black, while sulfur is yellow

Special Properties

• Special properties can be used for identification
• Double refraction is when light that travels through the mineral splits in two ways
• Effervescence occurs when a mineral reacts with hydrochloric acid
• Magnetism occurs between minerals that contain iron
• Iridescence is the play of colors caused by the bending of light rays
• Fluorescence is when minerals glow in the dark when exposed to ultraviolet light

Texture

• Texture refers to how a mineral feels to the touch
• Obsidian has a smooth texture, while talc has a greasy texture

Density and Specific Gravity

• Density is mass/volume and reflects the atomic mass and structure of the mineral
• Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water at 4°C
• Specific gravity is measured by picking up the mineral and determining how heavy it feels compared to how heavy it looks

Rock

• Rock is a naturally occurring aggregate of minerals and/or other rock fragments
• Rocks are classified into igneous, sedimentary, and metamorphic

The Rock Cycle

• Rocks are continually changing above and below the Earth's surface
• This continuous changing is called the Rock Cycle

Igneous Rocks

• Igneous rocks form when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks
• All igneous rocks form from magma
• The type of igneous rock formed depends on the chemical composition of the magma
• Magma is a slushy mix of molten rock, dissolved gases, and mineral crystals
• Lava is magma that flows out onto the Earth's surface, and its chemical composition may differ from magma due to gases escaping into the atmosphere
• Common elements in magma are the same as those in the Earth's crust: O, Si, Al, Fe, Mg, Ca, K, and Na
• Three main types of magma are classified by silica content: Basaltic (42-52%), Andesitic (52-66%), and Rhyolitic (66% and up)
• Magma forms from the melting of the Earth's crust or melting of rock in the mantle
• Four factors involved in the formation of magma are: temperature, pressure, water content, and mineral content
• Temperature increases with depth in the Earth's crust, known as the geothermal gradient
• Magma forms when the temperature is high enough to melt the rock
• Pressure increases with depth due to the weight of overlying rock
• Increased pressure on a rock will increase its melting point and require higher temperatures to melt
• 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
• Different minerals have different melting points
• Rocks rich in iron and magnesium melt at higher temperatures than rocks with higher levels of silicon
• Partial melting occurs when some minerals melt while others remain solid, changing the composition of the magma and rocks
• Fractional crystallization is the opposite of partial melting
• Minerals that melted last will crystallize first and are removed from the magma, unable to react and change its composition
• Bowen's Reaction Series shows the relationship between cooling magma and the formation of minerals that make up igneous rock
• Iron-rich minerals undergo abrupt changes; as minerals form, they react with the magma to become new minerals
• Feldspars undergo continuous changes, starting with calcium-rich compositions that slowly become sodium-rich as magma cools
• Intrusive rocks form under the Earth's surface with large grains due to slow cooling and large crystal formation
• Extrusive rocks form on top of the Earth's surface with small grains because the magma cools too quickly, resulting in small or no crystals
• Porphyritic rocks have two different crystal sizes due to cooling in different environments
• Vesicular rocks have a spongy appearance caused by gas bubbles

Mineral Content

• Basaltic rocks are dark in color and have low silica content
• Granitic rocks are light in color and have high silica content
• Intermediate rocks are between basaltic and granitic in composition
• Ultrabasic rocks are very dark and rich in iron

Rocks as Resources

• Veins (also called "igneous intrusions") are small spaces containing valuable ores
• Pegmatites are veins of very large-grained minerals containing rare elements
• Kimberlites are a type of peridotite (ultrabasic rock) containing diamonds

Sedimentary Rocks

• Sedimentary rocks form through weathering and erosion, deposition and sorting, and lithification
• Weathering is the set of physical and chemical changes that break rocks into smaller pieces, ranging from microscopic to huge boulders
• Physical weathering causes rock fragments to break off
• Chemical weathering causes minerals in a rock to dissolve or chemically change
• Erosion is the removal and transport of sediment by glaciers, wind, water, and gravity
• Deposition and sorting occur when sediments settle on the ground or sink to the bottom of water, with larger grains settling first and finer grains on top
• Sorted deposits are influenced by water and wind

Lithificiation

• Most sediments end up at low points such as valleys or the bottom of the ocean basin
• As sediment builds up, pressure and temperature increase in the bottom layers
• Leads to compaction and cementation
• Compaction involves layers of sediment being pushed together, with some materials compacting better than others
• Cementation involves mineral growth that glues sediments together into solid rock
• The primary feature of sedimentary rock is horizontal layering called bedding, resulting from how sediment settles out of water or wind
• Ripple marks form when sediment is moved into small ridges by wind, wave action, or river current
• As sediment is transported, angular pieces knock into each other and become rounded, with harder materials becoming more rounded and further travel leading to more rounded shapes
• Fossils are preserved remains, impressions, or other evidence of once-living organisms, with parts of an organism replaced by minerals and turned into rock during lithification

Types of Sedimentary Rock

• Sedimentary rocks fall into 3 categories: clastic, chemical, and biochemical
• Clastic sedimentary rocks are formed from abundant deposits of loose sediments on the Earth's surface and are classified based on sediment size
• Coarse-grained rocks contain gravel-sized rock and mineral fragments
• Medium-grained rocks contain sand-sized rock and mineral fragments
• Fine-grained rocks contain silt and clay-sized particles
• Chemical sedimentary rocks form after the concentration of dissolved minerals reaches saturation, and crystal grains settle out
• Water evaporates, minerals concentrate, and lead to thick layers on the bottom of a body of water, usually in areas lacking precipitation
• Biochemical sedimentary rocks form from the remains of once-living organisms
• Most abundant is limestone made of calcite
• Many contain fossils

Metamorphic Rocks and Minerals

• Sometimes rock are subjected to pressure and heat but do not melt
• Instead, the mineral composition, texture, or chemical composition of the rock changes
• Metamorphic rock are rocks that change form while remaining solid
• Metamorphic mineral are minerals that form during metamorphism

Metamorphic Texture

• Foliated textures have layers and bands of minerals and form when pressure is applied in opposite directions
• Nonfoliated textures have blocky crystal shapes and form when pressure is not applied in opposite directions
• Porphyroblasts are mineral growth that results from the rearrangement of atoms during metamorphism

Grades of Metamorphism

• Low grade metamorphic rocks form under low temperature and pressure conditions
• Intermediate grade metamorphic rocks form under intermediate temperature and pressure conditions
• High grade metamorphic rocks form under high temperature and pressure conditions

Types of Metamorphism

• Regional metamorphism is caused by high temperature and pressure in large regions of the Earth's crust, and can range from high to low grade
• Regional metamorphism causes changes in mineral and rock types as well as folding and deforming of rock layers
• Contact metamorphism occurs when molten material comes in contact with solid rock, experiencing high temperatures and high to low pressure
• Effects of contact metamorphism decrease with distance
• Hydrothermal metamorphism occurs when very hot water interacts with rock, which can change original texture and mineral composition
• Ore deposits of gold, copper, zinc, tungsten, and lead form this way

Economic Importance

• Metallic resources such as gold, silver, copper, and other metallic minerals form through hydrothermal metal deposits
• Nonmetallic resources such as talc, asbestos, and graphite form through metamorphism

Explore the characteristics and formation of minerals, which include being naturally occurring, inorganic, solid, and crystalline with a specific chemical composition. Learn about mineral formation through precipitation from aqueous solutions, crystallization from magma, and biological processes. Examples include Galena, Pyrite, and Halite.