Geology- Minerals

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What is a mineral?
• “A naturally occurring, inorganic solid that possesses
an orderly crystalline structure and a definite
chemical composition”
1. Naturally occurring—NOT man made
• Synthetic materials (such as cubic zirconia) don’t count

2. Inorganic—plant and animal products don’t count
• For example, coal is NOT a mineral
• Some exceptions—inorganic secretions by marine organisms
(shells) may count as a mineral if preserved in rock

3. Solid—must be solid in nature, cannot be liquid or gas!

4. Orderly crystalline structure—has an internal structure
of organized atoms that repeats itself
• All minerals have a crystal shape
• Nonexample: volcanic glass (obsidian)—NO internal structure

5. Definite chemical composition (substitutions are okay)
• Quartz—SiO2
• Halite (salt)—NaCl
• But substitutions are allowed—Olivine (Mg, Fe)2 SiO4
• Nonexample: Opal (too much variation in chemical formula)

Rocks are composed of minerals!
• Minerals are the “building blocks” of rocks
• A rock can be composed of many different minerals (granite) or
mostly one mineral (limestone)
Some useful analogies:
• A rock is to a mineral as a
cookie is to flour, sugar,
chocolate chips, etc.
• Minerals are the ingredients of
• However some rocks are only
composed of one “ingredient”
or mineral
• One color of Lego vs. many
colors of Lego
Many mineral properties are
determined by the way atoms
• All atoms want 8 electrons in
their outermost shell
• Atoms bond with other atoms
to achieve this
• Three types of bonding:
Ions are atoms with a charge
• Losing an electron—POSITIVE charge
• Gaining an electron—NEGATIVE charge
• What happens? Opposites attract.
• Minerals with ionic bonds: halite (salt) NaCl
Atoms (with no charge) SHARE
• Minerals with covalent bonds:
quartz SiO2; diamond (C)
• **Most minerals utilize BOTH ionic
and covalent bonds!**
Metallic- metals give up all
their outer shell electrons
• “sea of electrons”
• Gives metal their unique properties
(conductivity, malleability)
• Only occurs when metals bond
with themselves or other metals
• Minerals with metallic bonds:
gold, silver, native copper
Bonding creates compounds (a group of 2+ elements)
• Most minerals are compounds (quartz, halite, etc.) but
some minerals only contain one type of atom (gold,
copper, diamond)
Over 4000 known minerals! But only
a few are common
• Main groups: SILICATES vs.
• Silicates are by far the most
common rock-forming minerals
• 90% of Earth’s crust made up of
Silicates: contain silicon + oxygen
• All silicates contain an important
compound: the silicate
tetrahedron (SiO4
-4 )
Since silicates are the most common mineral group–the
most common elements on Earth’s crust are silicon and
Since there are so many silicates, we divide them into two
basic groups:
• Nonferromagnesian (light) silicates
• Ferromagnesian (dark) silicates
Nonferromagnesian (light) silicates:
• Light in color (pink, white, gray) and light in density
• Contain sodium (Na), potassium (K), or aluminum (Al)
Ferromagnesian (dark) silicates
• Dark in color (black, brown, green)
• Denser than nonferromagnesian silicates
• Contain iron (Fe) or magnesium (Mg)
There are many important nonsilicates as well—many
useful for economic purposes
• For full list, Table 3.1 on pg. 85
• Carbonates
• All contain compound CO3
• Fizz with acid
• Calcite, dolomite, micrite
• These minerals are found in limestone
(the most common rock in Texas!)
How do you identify them?
• Each mineral has a specific set of
properties that can be used to
identify it
• Optical properties (luster, streak, color,
• Mineral strength (hardness, fracture,
• Density and specific gravity
• Random other properties (magnetic,
fizziness, saltiness, etc.)
Optical Properties
• Luster-how light reflects off a mineral
• Is it shiny? What kind of shiny?
Words commonly used to describe nonmetallic luster:
• Vitreous (glassy)
• Waxy
• Dull
• Earthy
• Silky
• Pearly
Color—not very diagnostic, lots of exceptions
Olivine is almost always green however!
Streak is the color of the
powdered mineral
• Scratch on porcelain plate and you
get streak color
Crystal shape or habit—some minerals grow in unique
shapes (when they have the space to grow that way)
Hardness—resistance to
abrasion or scratching
• What will scratch the mineral?
• Mohs hardness scale (Figure
3.16 pg. 76)
• Cleavage—minerals break along
planes of weakness, so cleavage
describes how a minerals breaks
• Not all minerals have cleavage
• Those with cleavage break along in
distinct patterns
• Cleavage described by its direction and
angle (i.e. biotite has perfect cleavage
in one direction)
• NOT the same as crystal shape!
Fracture—when a mineral
breaks in uneven patterns (all
minerals will fracture, not all
have cleavage)
• Conchoidal fracture- breaks like
glass (curved, sharp edges)
• Quartz notorious for its conchoidal
Specific gravity- how heavy
it is compared to water • Ex.: If specific gravity is 3.0, it’s 3x
heavier than water
• Density- mass/volume (g/
• Generally, ferromagnesian
silicates are more dense than
nonferromagnesian silicates
• Metallic minerals often very dense • Density is NOT the same thing as
Some minerals have other random
specific properties that will help us
identify them:
• Double refraction (calcite)
• Fizziness in acid (calcite)
• Magnetic (magnetite)
• Soapy feel (talc, kaolinite)
Striations—”hairline” grooves alongside a mineral face
• Can be seen along cleavage planes
• Common in: plagioclase, pyrite
• Exsolution lamellae—
• Slight changes in mineral composition that look like banding (light/
dark stripes)
• Common in: potassium feldspar (NOT plagioclase)
Categories: Geology