Transcript Mineral: Physical Properties Lecture Notes

Minerals: Physical Properties and Crystal Forms
From: http://webmineral.com/data/Rhodochrosite.shtml
Minerals: the building blocks of rocks
• Definition of a Mineral:
 naturally occurring
 inorganic
 solid
 characteristic crystalline structure
 definite chemical composition
• Definition of a Rock:
• A solid aggregate (mixture) of minerals
Mineral characteristics
•
Definition of a Mineral:
1.
2.
3.
4.
5.
naturally occurring
inorganic
solid
characteristic crystalline structure
definite chemical composition
steel plastic sugar
no, #1 no, #1 no, #1,2
table salt mercury ice
coal
YES!
no, #3 YES! no, #2
basalt obsidian mica gold
paper
chalk coral
no, #5
no, #4
YES! YES! no, #1,2 no, #2 no, #2
Mineral characteristics
•
Naturally formed
– No substance created artificially is a mineral.
examples: plastic, steel, sugar, paper
•
Inorganic
– Anything formed by a living organism and
containing organic materials is not a mineral.
examples: wood, plants, shells, coal
•
Solid
– Liquids and gases are not minerals.
examples: water, petroleum, lava, oxygen
Inorganic Calcite
CaCO3
calcium carbonate
Calcium carbonate, or CaCO3, comprises more than
4% of the earth’s crust and is found throughout the
world. Its most common natural forms are chalk,
limestone, and marble, produced by the sedimentation
of the shells of small fossilized snails, shellfish, and
coral over millions of years. Although all three forms
are identical in chemical terms, they differ in many
other respects, including purity, whiteness, thickness
and homogeneity. Calcium carbonate is one of the most
useful and versatile materials known to man.
Mineral characteristics
•
Characteristic crystalline structure
– must have an ordered arrangement of atoms
– displays repetitive geometric patterns in 3-D
glass not a mineral (no internal crystalline structure)
•
Definite chemical composition
– must have consistent chemical formula
examples: gold (Au), quartz (SiO2), orthoclase (KAlSi3O8)
basalt (like many other rocks) contains variable ratios
of different minerals; thus, has no consistent formula
Minerals are …
• Composed of ordered
arrangements of
atoms….
– Repeated patterns
– Uniform spacing
–atoms arranged in an
orderly, repeating,
3-D array.
e.g., Atomic Structure of
Diamond
How many minerals are there?
•
Nearly 4,000 types of minerals
– Only ~30 occur commonly
– Why not more?
•
Some combinations are chemically impossible
•
Relative abundances of elements don’t allow more
Mineral Formation
•
Crystallization from a magma
– quartz, feldspar, mica in granite
•
Crystal growth in the solid state
– mica, garnet, feldspar in schist
•
Precipitation from solution
– calcite in marine organism shells
– silica in agate
Quartz
Crystals
Element abundances in the crust
All others: 1.5%
Ions
•
When an atom loses or gains an electron to or
from another atom it is called an ion.
•
Positively charged ions (loss of electron) are
called cations.
•
Negatively charged ions (gain of electron) are
called anions.
•
Ions of opposite charge attract (net charge = 0):
– > Ionic Bonding
•
90% of all minerals are ~ ionic compounds.
Ionic Bonding
Cation (+)
Anion (-)
Ionic Compounds
Fig.3.4
- NaCl
- Halite
- Table Salt
Important Ions in Minerals
anions
O
Cl
S
charge
-2
-1
-2
cations
Si
K
Ca
Na
Al
Mg
Fe
charge
+4
+1
+2
+1
+3
+2
+2 or +3
Covalent Bonding
•
Electrons are shared between atoms.
•
Covalent bonds are much more stable and
stronger than ionic bonds.
Atomic
Structure of
Diamond
Crystal Structure
•
Anions are generally
larger than cations.
•
The structure of the
mineral is determined
largely by how the
anions are arranged,
and how the cations fit
between them.
Ionic Radius
and Charge
Crystal Structure
Perfect Crystals:
Predictable
Interface Angles
Polymorphs
•
Identical chemical compounds
•
Different atomic structure
•
Generally stable under different conditions.
“Polymorphs” (although different minerals)
Physical properties of minerals
• We know that minerals are composed of atoms,
arranged in a specific order, with a well defined
chemical composition.
• We might expect then that the microscopic
variations in bond environment will also be
manifested in macroscopic physical and
chemical properties.
• This is indeed the case.
The Physical Properties of Minerals
•
•
•
•
•
•
Color
Streak
Luster
Hardness
External Crystal Form
Cleavage
The Physical Properties of Minerals (cont.)
•
•
•
•
•
Fracture
Specific Gravity
Special Properties
Other Properties
Chemical Tests
Important Physical Properties
• Color - Although an obvious feature, it is often
unreliable to use to determine the type of mineral.
– Color arises due to electronic transitions, often of trace
constituents, in the visible range of the EM spectrum.
For example, quartz is found in a variety of colors.
• Color of a mineral may be quite diagnostic for the
trace element and coordination number of its
bonding environment.
Color
Some minerals have more than one color for example; purple amethyst and yellow
citrine are both varieties of quartz. In contrast, yellow is the only color of sulfur
and is therefore a useful tool in identifying this mineral.
Amethyst
Citrine
Crystal Quartz
Smokey Quartz
Agate, Calcedony, Jasper
Quartz Gems
Important Physical Properties
• Streak - The color of a mineral in its
powdered form; obtained by rubbing the
mineral against an unglazed porcelain plate.
– Useful for distinguishing between minerals
with metallic luster.
Streak - The color of a mineral in its
powdered form;
Hematite may look black, but it will always produce a RED/BROWN
streak on a streak plate. Care must be taken if the mineral being tested is
harder than the porcelain, the result will be a powder produced by the
porcelain plate being scratched and will always be white.
Important Physical Properties
• Luster - This property describes the
appearance of reflected light from the
mineral's surface. Nonmetallic minerals
are described using the following terms:
vitreous, pearly, silky, resinous, and
earthy.
Luster
a.
b.
•
•
•
•
•
•
•
Metallic
Non-metallic
Vitreous
Resinous
Dull
Pearly
Silky
Greasy
Glassy
Hope Diamond: 44.5 carats
http://www.nmnh.si.edu/minsci/hope.htm
Hardness:
We measure the hardness
of a mineral by how easy
we can scratch it using
different tools like finger
nails, piece of glass and
piece of copper (usually a
penny).
MOH’s
hardness scale
External Crystal Form
• Crystal form
• Minerals are grouped into systems according
to their crystal symmetry (regularity of form).
•
The figure below shows the six main systems.
Cleavage
Cleavage occurs when a mineral
has a preferred direction of breakage.
Cleavage like most other properties
is a function of the crystal structure
and the nature of the bonding. When
a mineral is broken, it breaks into
pieces that resemble one another,
then it is said to have perfect cleavage.
Perfect cleavage is due to a higher order
of symmetry and is more prevalent in
minerals with strong ionic (therefore
weak) bonding. Cleavage surfaces
usually look almost polished and are
very flat. The angles between cleavage
surfaces is related to the crystal structure
and hence diagnostic of the particular
mineral.
Planer Cleavage in Mica
Weak Bonding Yields Planer Cleavage
Fracture
• Fracture is the
tendency of a
mineral to break
along curved
surfaces without a
definite shape.
These minerals do
not have planes of
weakness and
break irregularly.
Mineral
Type of Breakage
Halite
CLEAVAGE
Cleavage in three directions at right
angles (90o). Cubic cleavage.
Calcite
CLEAVAGE
Cleavage in three directions not at
right angles (120o and 60o).
Rhombohedral cleavage.
Mineral
Type of Breakage
Quartz
FRACTURE
Mineral does not exhibit cleavage, it
breaks or fracture in an irregular manner.
Feldspar
CLEAVAGE
Cleavage in two directions at right
angles.
Rhombohedral Cleavage in Calcite
Conchoidal Fracture in Glass
Density and Specific Gravity
• Density - Defined as the mass divided by the
volume and normally designated by the Greek
letter, rho, 
 mass/volume; SI units: kg/m3 or kg m-3, but
geologists often use g/cm3 as the unit of choice.
• Specific Gravity - Ratio of the mass of a
substance to the mass of an equal volume of water.
Note that water = 1 g cm-3. S.G. is unitless.
• Examples - quartz (SiO2) has a S.G. of 2.65 while
galena (PbS) has a S.G. of 7.5 and gold (Au) has a
S.G. of 19.3.
Steps to determine the density of a mineral.
1) Use a balance. In this example the balance to be used is a
triple beam balance.
2) Place the specimen in the weighing
pan.
3) Record the weight of the specimen,
in this case 155.8 grams.
4) Record the level in a graduated cylinder before you put the
specimen in. In this case 900ml.
5) Record the level after the
specimen was placed under water. In
this case 920ml.
Density =
155.8grams
/ 20 cc
6) Divide 155.8/20 =
7.79 g/cc. (in this case
20ml = 20cc, because
the amount of units
displaced are
equivalent). So, the
density of the minerals
is 7.79g/cc.
7) The closest mineral having this density is galena with a density of 7.60 g/cc.
(In order to get a precise density sophisticated equipment with a tolerance of five
to seven decimals is used).
Special and Other Properties
• Striations - Commonly found on plagioclase
feldspar. Straight, parallel lines on one or more
of the cleavage planes caused by mineral
twinning.
• Magnetism - Property of a substance such that it
will spontaneous orient itself within a magnetic
field. Magnetite (Fe3O4) has this property and it
can be used to distinguish it from other nonmagnetite iron oxides, such as hematite (Fe2O3).
• Double Refraction - Seen in calcite crystals.
Light is split or refracted into two components
giving rise to two distinct images.
Plagioclase
striations
Calcite Double Refraction
Important Mineral Groups
Name
Important constituents
(other than O)
Olivine
Pyroxene
Amphibole
Micas
Feldspars
Carbonates
Evaporites
Si, Fe, Mg
Si, Fe, Mg, Ca
Si, Ca, Mg, Fe, Na, K
Si, Al, K, Fe, Mg
Si, Al, Ca, Na, K
C, Ca, Mg
K, Cl, Ca, S
Color and Density
•
Two broad categories are ferromagnesian and nonferromagnesian silicates,
which simply means iron and magnesian bearing or not. The presence or absence of
Fe and Mg strongly affects the external appearance (color) and density of the
minerals.
•
Ferromagnesian silicates - dark color, density range from 3.2 - 3.6 g/cc
–
Olivine - high T, low silica rocks; comprises over 50% of upper mantle
–
Pyroxenes - high T, low silica rocks
–
Amphiboles - esp. hornblende; moderate T, higher silica rocks
– Mica - esp. biotite; moderate T, higher silica rocks
–
Garnet - common metamorphic mineral
•
Nonferromagnesian silicates - light color, density close to 2.7 g/cc
–
Mica - exp. muscovite; moderate T, higher silica rocks
–
Feldspars - plagioclase and orthoclase; most common mineral in crust; form
over a wide range of temperatures and melt compositions
–
Quartz - low T, high silica rocks; extremely stable at surface, hence it tends to
be a major component in sedimentary rocks.
–
Clay - esp. kaolinite; different types found in different soils
Important Silicates
The Olivine Group is made up of two "end
members" one with iron (Fe) and the other with
magnesium (Mg). In the real world there is
probably never a pure olivine, most are
combinations of the two end members. Fe2SiO4
( Fayalite ) and Mg2SiO4 (Foresterite).
A combination formula might look like this:
(Mg,Fe)2SiO4
Olivine occurs in the crust, ocean crust, and
upper mantle and is not rare, except as clean
crystals. It normally grows in small granular
forms. As a gem it is known as peridot.
The Garnet Group is another
with the isolated silicate
structure. As with olivine it is
made up of different
substitution patterns, but
unlike olivine it has many "end
members" and thus a variety of
different chemistries.
The general formula is
A3B2(SiO4)3
Where A can be any of the following: Mg+2, Fe+2,
Ca+2, and Mn+2. The B elements can be any of the
following: Al+3, Fe+3, Cr+3 or a mixture thereof.
Because it has a wide variety of possible chemical
formulas, it can be found in a rainbow of colors.
One of the important uses for garnet is the
manufacture of wood sandpaper.
Grossular (Ca,Al) tsavorite, rhodinite,
pyrope
Andradite (Ca,Fe) Almandine (Fe,Al)