Basic Geology about mineral

Basic Geology about mineral
From :

http://www.geo.utep.edu/pub/ortega/minerals.PDF

Minerals
What is a mineral?
What is a rock?
Minerals
What is a mineral?
· Naturally occurring (not man-made)
· Inorganic (not part of or a product of a (once) living organism )
· Solid (not made of liquid or gas) with a definite chemical structure which
give it unique physical properties.

Minerals vs. Rocks
· There are nearly 4000 known minerals - but most rocks are formed by
only a few dozen minerals.
· Rocks are aggregates (mixtures) of minerals. So . . . minerals are the
building blocks of rocks.
A question for the future: How do minerals come together to form a rock?
Composition and Structure of Minerals
To understand how minerals form, we need to understand the characteristics of
elements and atoms.
Chemical Elements
Elements are the basic building blocks of minerals. There are over 100 known
elements.
Composition and Structure of Minerals
Atoms - smallest particle of matter that exhibits all the characteristics of an
element.
Atoms are made up of:
· Nucleus, which contains Protons (atomic number)- positive electrical
charges and Neutrons - neutral electrical charges (P +N = atomic mass)
· Shells which surround the nucleus and contain Electrons - negative
electrical charges
· The weight (density) of an element depends on the number of protons
and neutrons in the nucleus.
Combining Elements to Form Minerals
Elements combine with each other to form a wide variety of minerals (chemical
compounds)
Atoms combine chemically by gaining, losing, or sharing and electron with other
atoms. (opposites attract)
The new mineral (compound) will have very different physical properties from
the elements that combined to form it.
Since rocks are mixtures (not chemical combinations) of minerals, minerals keep
their physical properties within a rock.
Is it possible for two different minerals to have the same chemical
composition?
YES! Both diamond and graphite are made of carbon. The difference between
these two minerals is the way in which the carbon atoms are arranged.
Mineral Properties
Minerals have lots of different properties that help us identify them.
Crystal form, Luster, Color,
Streak, Hardness, Cleavage,
Fracture, Specific gravity,
Taste, Smell, etc.
Rock Forming Minerals
The most common rock forming minerals are composed of 8 elements:
Oxygen (O), Silicon (Si),
Aluminum (Al), Calcium (Ca), Sodium (Na), Potassium (K),
Iron (Fe), and Magnesium (Mg)
There are just a few dozen minerals that we call the rock-forming minerals
Composition of the Crust
The most abundant elements in Earth's crust are:
Oxygen (46.6% by weight)
Silicon (27.7% by weight)
Silicate Minerals
Silicate minerals, minerals built primarily from silicon-oxygen tetrahedrons, are
the most common rock-forming minerals.
Silicate minerals are grouped according to how the tetrahedrons are arranged in
the mineral.
Silicate Mineral Groups
Olivine - independent tetrahedrons (Mg, Fe)
Pyroxene group - tetrahedrons are arranged in chains (Mg, Fe)
Amphibole group - tetrahedrons are arranged in double chains (Mg, Fe, Ca)
Micas - Tetrahedrons are arranged in sheets
Two common types of mica:
Biotite (dark) (K, Mg, Fe, Al),
Muscovite (light) (K, Al)
Silicate Mineral Groups
Feldspars - Three-dimensional network of tetrahedron
Two common types of feldspars:
Orthoclase (K, Al)
Plagioclase (Ca, Na)
Most plentiful mineral group
Quartz - three-dimensional network of tetrahedrons (SiO2)
Mineral Color
Mineral color is often governed by presence of Mg, Fe (dark) or absence of these
elements (light).
Non-Silicate Minerals
Major groups:
Oxides (FeO2), Sulfides (PbS), Sulfates (CaSO4),
Halides,
"Native" elements (gold),
Carbonates (limestone, marble) (CaCO2)
Mineral Resources
Ore - rock that contains useful metallic minerals that can he mined at a profit.
Since 98% of the Earth's crust is made up of 8 elements, an element/mineral has
to be concentrated at levels above normal for profitable mining.
How do minerals "mix" together to form rocks?
What is the rock cycle (Chapter 2)?
Igneous Rocks
Importance of Igneous Rocks
• Understanding potential hazard represented by volcanic activity and how to
prevent loss of life/property.
n Kilauea (Hawaii)
n Montserrat, Mount St. Helens
• Many mineral resources are associated with igneous rocks
n Copper
n Gold
Types of Igneous Rocks
• There are two main types of igneous rocks:
n Extrusive (volcanic)
Ø form at the surface
Ø molten rock "pushed out" of the earth
Ø cool quickly
Ø fine-grained (small crystals)
n Intrusive
Extrusive (Volcanic) Rocks
• What happens to material pushed out onto the Earth's surface due to volcanic
activity?
n Depends on the kind of magma:
Ø Mafic (Lo Si ; Hi Mg, Fe) lava flows more easily than Felsic lavas (Hi Si;
Low Mg, Fe).
Ø Mafic lavas also let dissolved gas, water out of the magma easily.
n Two main types: lava flows and explosive material.
Lava Flows
• Lava Flows (Gentle) - molten rock flows along surface and solidifies.
n Aa (colder, slower moving flows)
n Pahoehoe (warmer, faster moving flows)
n Obsidian (glassy)
n Vesicular (gas bubbles)
Explosive Material
• In thick gas-rich magmas, release of the gas will cause explosion of material
out of the volcanic vent.
n Pyroclastics ("fire fragment") - any fragments of volcanic rock ejected into
the air
Ø Characterized by size: ash, lapilli, cinders, bombs, blocks
n When the hot sticky fragments come to rest, they stick together to form
rock:
Ø pumice, tephra, tuff
Styles of Explosive Eruptions
n Eruption column
n Lateral blast
n Pyroclastic flow
Controls on Eruption Style
• Temperature
n High temperature --> low viscosity
Ø Hot - runny
Ø Cool(er) - syrupy
• Composition
n High Silica = gooier = high viscosity
Ø Food analogy: water vs. syrup vs. honey
n Felsic magmas have more silica than mafic magmas.
• Gas content
n Dissolved gases provide explosive force to propel material from a volcano
n Analogy: splattering spaghetti sauce
Types of Volcanoes
• Shield Volcanoes
n Built of basaltic lava flows and a small amount of pyroclastic material.
n Has the shape of a broad dome (roughly resembles a warrior's shield).
• Cinder Cones
n built of ejected lava fragments.
n Usually small (< 1000 ft high), have steep slopes, and often form near
bigger volcanoes.
• Stratovolcanoes
n Composite volcanoes built of interbedded lava flows and pyroclastic
deposits.
• Fissure eruptions
n Liquid flows (usually) from elongated fractures or cracks on the slopes of a
volcano.
• Calderas
n Large bowl-shaped crater that form by the collapse of a volcanic cone after
an eruption.
Dangers Associated with Volcanoes
• Pyroclastic flows
• Poison gases
• Tephra
n Materials of all types and sizes that are erupted from a crater or volcanic
vent and deposited from the air.
• Tsunamis (tidal waves)
n Huge (~ 50 ft high) seawave produced by a volcanic eruption, an
underwater landslide, or an earthquake.
n Waves carry a lot of a material with them.
• Mudflows (Lahars)
• Property damage
• Earthquakes
n large earthquakes are associated with volcanic activity and cause lots of
damage
Intrusive Rocks
• Form underground
n Molten rock "enters" rock from below,
n Cool slowly,
n Coarse-grained (large crystals)
Intrusive Rock Textures
• Textures
n Appearance of rock based on size and arrangement of crystals
Ø Coarse-grained - large crystals (visible to the naked eye)
Ø Porphyritic - large crystals embedded in matrix of small crystals.
Composition
• The kind of mineral that crystallizes out of a magma depends on magma
temperature and chemistry.
• Bowen's Reaction Series
Bowen's Reaction Series
n Discontinuous Branch - As magma cools slowly mineral formed at higher
temperature reacts with remaining magma to form the next mineral (e. g.
olivine will recrystallize into pyroxene)
n Continuous Branch - Same mineral forms, but has continuously changing
chemical composition calcium-rich plagioclase becomes more sodium-rich
as temperature drops).
n Exceptions: a) quick cooling; b) crystal settling (chemical composition of
magma changes)
Composition
Geometry of Plutons
• Massive bodies
n stocks
n batholiths
n laccoliths
• Tabular bodies
n dikes - vertical
n sills - horizontal
n volcanic necks (inside of a cinder cone)