Transcript Document 7499008

Mineralogy
Carleton College
Winter 2003
The Crystalline state:
Matter
Gaseous
State
Liquid
State
Solid
State
The Crystalline state:
Matter
Gaseous
State
Liquid
State
Solid
State
Amorphous
(disordered)
Crystalline
(ordered)
Why study minerals?
Why study minerals?
• All field so earth science use mineral
knowledge one way or the another way.
For example:
–
–
–
–
Igneous or metamorphic petrologists
Sedimentologists
Geophysicists
Material science
Mineralogy requires an understanding of:
• Chemistry - elements, electronic structure,
bonding, chemical analyses; forms of
crystals
Mineralogy requires an understanding of:
• Physics - physical properties (density,
electrical properties, cleavage, hardness,
magnetic properties, etc.)
Mineralogy requires an understanding of:
• Crystallography - the structure of minerals internal & external; geometry and
trigonometry; symmetry
Mineralogy requires an understanding of:
• Mineral genesis - environments of
formation (e.g., ocean floor vs. subduction
zone)
Mineralogy requires an understanding of:
• Determinitive mineralogy - physical and
chemical testing; analytical equipment
• Classification schemes
• Interrelationships between humans and their
natural environment
Definition of a crystal
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Definition of a crystal
• Usually formed by inorganic processes (i.e.,
coal is not a mineral)
• Definite chemical composition (some
atomic substitution allowed)
• "ordered atomic arrangement" - eliminates
liquids and gases also eliminates amorphous
solids such as glass and obsidian
Composition of Earth
• Earth's crust:
• composition - 95% igneous rock (or it's
metamorphic equivalent) - e.g., basalt,
granite, granitic gneiss, schist...
• 8 most abundant elements in crust = (99
weight % and approx. volume) O, Si, Al,
Fe, Ca, Na, K, Mg
Composition of Earth
• Most common crustal minerals are:
– Silicates - e.g., quartz SiO2
– aluminosilicates - e.g., K-feldspar (orthoclase)
KAlSi3O8
– oxides - e.g., hematite Fe2O3
Composition of Earth
• Total earth:
– composition - 99% Fe, O, Si, Mg, Ni, S, Ca, Al
– - difference due to core and mantle
• Fe, Mg-silicates = mantle
• S, Ni, Fe = core
Definition of a crystal
• Crystal: A crystal is an anisotropic,
homogenous body consisting of a 3-D
periodic ordering of atoms, ions or
molecules.
Few properties that are
characteristic of crystals.
•
•
•
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Regular geometric shapes
Cleavage
Pleochroism
Hardness
Optical properties
Electrical Conductivity
Anisotropy/isotropy
Few properties that are
characteristic of crystals.
• Regular geometric
shapes
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Few properties that are
characteristic of crystals.
• Cleavage
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– When crystals of NaCl,
Calcite, and Biotite are
split, the resulting
fragments have similar
shapes with smooth
faces. This
phenomenon is called
cleavage.
Few properties that are characteristic of crystals.
Hornblende and Pyroxene
Few properties that are
characteristic of crystals.
• Pleochroism
– Color that appears to an
observer depends on the
optical absorption of the
crystal in that particular
direction. For example if
the crystal absorbs all
spectral colors of the white
light except blue then the
crystal appears blue.
Few properties that are
characteristic of crystals.
• Pleochroism
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– If you were to examine a
mineral of biotite under the
microscope in its two
different sections, as seen in
the following figure; you
will see two different
colors, as you rotate the
stage of the microscope.
Few properties that are
characteristic of crystals.
• Pleochroism
– When as in the above
case the absorption
differs in two or three
directions the crystal is
said to exhibit
pleochroism.
Few properties that are
characteristic of crystals.
Few properties that are
characteristic of crystals.
Few properties that are
characteristic of crystals.
• Hardness
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– When you took introduction to
geology you learned the
hardness of minerals, most
minerals can be identified by
their hardness. It is controlled
by the inter atomic forces or
what kind of bonding they
have. As you may recall there
is hardness scale of 1-10.
Few properties that are
characteristic of crystals.
• Hardness
– If you look at a crystal of
kyanite Al2SiO5, This mineral
if scratched parallel to its
length by a steel needle, a deep
indentation will be made in it,
while a scratch perpendicular
to the crystal length will leave
no mark. Thus the hardness of
the crystal in different in two
different directions. (4.5 to 6.5)
Few properties that are
characteristic of crystals.
• Optical properties
– In this slide, we have a
dot, and a clip for a
scale.
Few properties that are
characteristic of crystals.
• Optical properties
– Take a crystal of
calcite and glass and
see what happens.
Few properties that are
characteristic of crystals.
• Optical properties
Single row of dots
on a piece of paper
– With the glass and we
see only one dot.
Few properties that are
characteristic of crystals.
• Optical properties
– With calcite we see
two dots. What is
happening?
Few properties that are
characteristic of crystals.
Few properties that are
characteristic of crystals.
C
• Electrical conductivity
– If one face of a gypsum in
covered with a thin layer of
wax and heated with a metal
tip, the melting front in the
wax layer will be ellipsoidal
rather than circular showing
that the thermal conductivity is
greater in direction II than in
direction I.
a
Few properties that are
characteristic of crystals.
• Anisotropy/isotropy
– Such behaviors-different
values of physical properties in
different direction-is called
anisotropy.
– As in the glass where the
melting front is circular, which
imply that the thermal
conductivity is the same in all
directions, such behavior-the
same value of physical
property in all directions-is
called isotropy.
Few properties that are
characteristic of crystals.
• Anisotropy/isotropy
– So all of the above
properties that we discussed
tell us something about the
crystal. The origin of all of
the phenomena listed 1-6
lies in the internal structure
of the crystal. So in order
to understand this better let
us consider the various
aspects of a crystal.