Geology mineral how to identify

Geology
• What is a mineral?
• What is a gem?
• What is a rock?
• How are these formed?

• Edited from WARDS
Geometry of Crystals Lab


Mineral Criteria
1. occurs naturally on Earth

Mining: Dig for Diamonds Park
2. is inorganically formed ( does not contain carbon)
Exceptions are diamonds and graphite = pure carbon


Criteria continued
3. a solid at standard temp and pressure
4. Elements that make up the substance are combined in
fixed proportions
5. The orderly arrangement of atoms that make up the
substance produce a definite and repeating geometric
pattern = characteristic crystal form


Minerals

Is an element (ex. Gold, Au)
or a compound (ex. Quarts or

pyrite FeS ) found naturally in
rocks and soils
2

There are more than 3500
minerals identified
New minerals are being
found every year


Periodic Table
• 8 elements account for 98%of Earth’s crust
• These elements combine to form minerals
Oxygen

Silicon

Iron

Calcium

Potassium

Magnesium

Aluminum
Sodium


Mineral Identification

• Based on physical and chemical traits
• Initially classified using one or more of the following
physical properties
• 1. color
5. specific gravity
• 2. luster
• 3. hardness
• 4. streak

6. cleavage or
fracture
7. crystal form


Color
• Least reliable trait
• Color can vary
• Color can change
when exposed to
moisture, high or low
temperatures or
weathering


Luster
• Physical appearance
of the unweathered
surface
• Way it reflects
sunlight

• Metallic or nonmetallic
(glassy, pearly, silky,
greasy or brilliant)


Hardness
• Is the resistance of a mineral’s smooth surface (face) to
being scratched by a point or an edge
• Mohs Hardness Scale has a range
• 1 (soft, easily scratched – ex. Talc) to
• 10 (hard – ex. Diamond)
• Any mineral can scratch a mineral with a lower hardness


• Tools for Testing Hardness
• You typically do not carry around a supply of the 10
minerals on the hardness scale. However, you can use
the following items to help estimate the hardness of a
mineral:
• Finger Nail (H = 2.5)
• Penny (H = 3)
• Knife Blade (H = 5.5)
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Streak
• Color of a mineral in its ground or powdered
form
• The powder is formed when the mineral is
rubbed across an unglazed porcelain plate
• More reliable than color

• Plate has a hardness of 7 and is white
• Scratch test cannot be used on minerals of the
same color or hardness



Specific Gravity (Relative Density)
• Specific gravity is the ratio of a mineral’s mass to the
mass of an equal volume water.
• Very reliable


Cleavage or Fracture
Cleavage: a break along a smooth, flat plane producing
smooth flat surfaces called faces
Ex. Galena

Fracture: a break that is not smooth but produces irregular
surfaces
Ex. Quarts


Crystal Form
• One of the most useful physical characteristics
• Crystal forms result from the internal atomic
arrangement of a mineral
• Repetition of these arrangements results in crystal
systems which produce the visible shape of the mineral

Pyrite : “Fool’s Gold”
• FeS2

• Is the result of many atoms of iron and sulfur forming a
crystalline solid with a definite internal arrangement


All crystals can be categorized into
1 of 6 crystal systems
• As defined by axes
• Axes are imaginary straight lines that pass through the
center of the crystal faces at right angles to these faces
and intersect at the center of a perfect crystal


Isometric or Cubic
Characteristics
• 3 equal exes intersecting
at right angles to each
other
• Same measure
• 6 faces
• Each face is a square
• Angle between two
adjacent sides is 90o
• Ex. Galena, halite, pyrite


Hexagonal
• Four axes

• 3 of the 4 intersect at 60o
angles to others in the
same plane
• 4th axis intersects the
other 3 at a 90o angle
• Each of the similar 6
faces join each other at
60o
• 2 more identical faces are
different form the other 6
• 2 identical faces form 90o
with the other 6
• Ex. Calcite, quartz and
apatite


Tetragonal
• All 3 axis intersect at
90o
• 2 of the 3 axis are of
equal length
• Four identical often
rectangular faces and
two square faces
• All adjacent faces hit
at 90o
• Ex. Chalcopyrite,
zircon

Orthorhombic

• 3 axis intersect at 90o
• None of the axis are
equal length
• Four of the faces form
similar rectangles
• 2 of the other faces
are similar rectangles
but different from the
first 4
• All adjacent sides
meet at 90o
• Ex. Olivine, silfur,
topaz


Monoclinic
• 3 axes none of equal
length
• Only 2 axes intersect at
90o
• 4 identical or similar faces
• 2 identical faces unlike
the first 4
• 6 sided prism
• Faces meet at 90o and
other angles
• Ex. Mica, gypsum,
orthoclase, hornblende

Triclinic
• 3 unequal axes
• None of the axes
intersect at 90o
• Four similar or identical
polygons and 2 larger
similar or identical
polygons different form
the first 4
• None of the faces
intersect at 90o
• Ex. feldspars


Gems
• A gemstone or gem, also called a precious or semiprecious stone, is a piece of attractive mineral, which —
when cut and polished — is used to make jewelry or
other adornments.[1] However certain rocks, (such as
lapis-lazuli) and organic materials (such as amber or jet)
are not minerals, but are still used for jewelry, and are
therefore often considered to be gemstones as well. Most
gemstones are hard, but some soft minerals are used in
jewelry because of their lustre or other physical
properties that have aesthetic value. Rarity is another
characteristic that lends value to a gemstone.
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Rocks (more info to come) 

Rocks are classified by mineral and chemical
composition, by the texture of the constituent
particles and by the processes that formed them.
These indicators separate rocks into igneous,
sedimentary and metamorphic. They are further
classified according to particle size. The
transformation of one rock type to another is
described by the geological model called the
rock cycle.
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