MATERIALS OF THE EARTH'S CRUST: MINERALS
> the earth's crust is comprised of 5 kinds of materials, all of which are critical to earth
processes
1) rocks
2) aggregates (i.e., loose particles and soil)
3) water and ice
NOTE: 1, 2, and 3 are made up (mostly) of minerals
4) gases
5) life (i.e., organic material)
This section will deal with MINERALS, which make up most of 1, 2 and 3.
First, let's ask: WHAT DETERMINES THE BEHAVIOUR OF ROCKS AND
AGGREGATES?
(a) the properties (and proportions) of the minerals that make up the rock/aggregate
(b) the bulk properties of the rock/aggregate
e.g., the nature of fractures, how the minerals are stuck together, the porosity and
permeability
MINERALS
> to be a mineral, you must have:
(1) a specific structure (i.e., be crystalline)
(2) a specific composition
For example, ICE is a mineral: it has a specific composition (H2O) and is crystalline (you
can see its crystal structure in the hexagonal form of snow-flakes)
What determines the external properties of a mineral (e.g., its crystal form; its hardness;
the way it breaks)?
Answer: the internal atomic structure
>> so, we need to look at why and how atoms bond together
(This study will allow us to answer: Why is it cheaper to build a road on sand soil than on
clay soil? (Where most sand soils are made mostly of quartz, and clay soils are made
mostly of the mineral clay.)
Atoms bond to reach a lower overall energy state.
> to understand the bonding process, we must consider the OCTET RULE:
>> atoms are more stable if they have their outer electron completely filled (which,
except for the innermost shell with two electrons, is filled when there are eight electrons:
hence, the OCTET rule)
>> this is what leads to bonding
(Note: the mineral that forms must be electrically neutral)
(Note: the inert elements don't tend to bond because their outer shells are already filled:
e.g., Argon with 8 electrons in the outer shell)
We can consider four types of bonds:
(1) IONIC: there is an exchange of electrons, and then the two atoms are attracted
electrically
> e.g., Na gives up its one electron in the outer shell to Cl that has originally only seven
in the outer shell
>> this makes a plus one Na ion and a minus one Cl ion that are thus attracted, and they
bond to form the mineral halite, NaCl (commonly known as salt)
>> (the actual crystal form will further depend on how the atoms can be most efficiently
packed; for equal numbers of Na and Cl atoms, where the Na atoms are very small, and
the Cl atoms are large, the most efficient way produces a cubic crystal form)
> in minerals, this ionic bond is a moderately strong bond but not as strong as the
covalent bond (discussed next); for example, the ionic bonds in halite are fairly easily
broken in water (salt dissolves readily)
(2) COVALENT: there is a sharing of electrons
> for example, in the mineral diamond (C), there is the covalent bonding of carbon atoms
>> each carbon atom has four electrons in the outer shell, so each atom surrounds itself
by four other carbon atoms
>> each atom shares an electron with each other atom, thus making each atom "think"
that its outer shell is filled with eight electrons, creating the bonding in the process
>> this is clearly electrically neutral
> in minerals, covalent bonding is strong (e.g., diamond is the hardest known mineral: it
is used on the end of diamond-drills to provide a strong cutting surface that can cut
through rocks)
(3) METALLIC: an electron cloud forms around the atoms to effect the bonding
> for example, Cu atoms have one electron in the outer shell
>> in the pure Cu metal, all the atoms give up their outer electron to satisfy the octet rule,
but the electron hang around as a cloud between the atoms
>> the whole thing is electrically neutral (because no electrons have been gained or lost)
>> it makes metals generally good electrical conductors, as they electrons in the cloud
can whiz around easily
>> metals are generally soft and malleable because of the relative weakness of this kind
of bond
(4) INTERMOLECULAR: the weakest bond, formed by weak electrostatic
charges (positive attracted to negative) on molecules
>> there is a weak positive charge on one side of the molecule, and a weak negative
charge on the other side
>two examples:
a) hydrogen bond in ice
>> the 104o bond angle between the two hydrogens and the oxygen in water means that
the oxygen side has a higher positive charge, and the hydrogen side has a higher negative
charge (the water molecule is still electrically neutral)
>> it is a BIPOLAR water molecule
>> when ice forms, the negative oxygen ends and the positive hydrogen ends are weakly
attracted to form the HYDROGEN BOND
>> this leads to a hexagonal shape (think of snowflakes!)
>> this is a very weak bond (ice has the lowest melting point of all the minerals that we
study: 0o C)
b) Van der Waal's bond
>> for example, in the mineral graphite, carbon atoms bond covalently to form sheets that
are electrically neutral, but one side of the sheet has a slight concentration of electrons,
and the other side a deficit
>> this allows the sheets to be WEAKLY attracted together
>> hence, graphite is one of the weakest minerals we know
>> it is used for pencil points, because, when we write, we can actually split apart the
Van der Waal's bonds between the sheets, and make a smear on the paper of graphite
particles
NOTE: diamond (C) and graphite (C) have the same chemical composition, but
we have to give them different names because their crystalline form is different
>> minerals with the same chemical composition but with different crystal structures are
called POLYMORPHS
(Quite a difference between diamond and graphite: the hardest mineral and one of the
softest, respectively!!}
MINERALS IN THE EARTH'S CRUST:
> there are ca. 2200 different crustal minerals
BUT, eight elements comprise greater than 99% of the earth's crust (O, Si, Al, Fe, Ca,
Na, K, Mg in order of weight %)
>> this means that there are only twenty major minerals, and less than ten extremely
abundant ones
>> that's why you don't need to learn very many minerals in the lab
(NOTE: virtually all ORE minerals are in the scarce category; more on that later)
> since O and SI comprise 74% by weight (and 98% by volume) of the crust
>> therefore, the most common minerals are a 3-dimensional linking of Si and O
>> the atoms of the other elements are contained in regularly spaced "holes" between the
Si and O
>> these minerals are called SILICATES
There are two broad groups of minerals: SILICATES and NON-SILICATES
> SILICATES make up about 97% of the earth's crust, and we will talk about them first
SILICATE MINERALS:
> the basic building block of all silicate minerals is the SILICA TETRAHEDRON (also
known as the silicon-oxygen tetrahedron)
>> it consists of one silicon atom bonded to four oxygen atoms
>> the silicon starts with four electrons in the outer shell, and each oxygen has six
electrons in the outer shell
>> by combined covalent-ionic bonding, the silicon gets satisfied according to the octet
rule (8 electrons in the outer shell), but each oxygen only ends up with seven, not eight
>> thus, the silica tetrahedron is not stable on its own, as it needs four more electrons to
satisfy the oxygens
>> the silica tetrahedron, with a valency of -4, is thus able to bond to other things
Silica tetrahedron: (SiO4)-4
(NOTE: it is called a tetrahedron because the most efficient way for nature to bring
together four large oxygen atoms with one small silicon atom creates a pyramid-shaped
structure)
> the strong mixed covalent/ionic bonds in the silica tetrahedron makes the silicates
generally strong minerals (but within the class of silicates there are different strengths
based on how the silica tetrahedra are bonded together)
There are FIVE different ways that silica tetrahedra can be stuck together to satisfy the
octet rule for the oxygens
> this leads to the five SILICATE-STRUCTURE TYPES
TYPE 1: Single (or Isolated) Tetrahedral (e.g., olivine; garnet)
TYPE 2: Framework Structure (e.g., quartz; feldspar)
TYPE 3: Single Chain Structure (e.g., pyroxene)
TYPE 4: Double Chain Structure (e.g., amphibole)
TYPE 5: Sheet Structure (e.g., mica; clay; chlorite)
(Note: 2, 3, 4 and 5 involve POLYMERIZATION (we will talk about this later)
Each of these structures represents a different way that the (SiO4)-4 building blocks can be
put together
> we want to look at these five different ways, and see how the different ways lead to
different external properties for the minerals
(1) SINGLE (or ISOLATED) TETRAHEDRAL STRUCTURE:
> olivine is a good example of this
>> single tetrahedra (4 oxygens; 1 silicon) are held together by Fe and/or Mg atoms (so
the silica tetrahedra don't touch each other; they have a "glue" of Fe (or Mg) in between.
That's why we call them SINGLE or ISOLATED)
>> the Fe (or Mg), with a valency of +2, provides an electron for each of the oxygens in
the two adjacent silica tetrahedra
>> this results in a formula like Mg2SiO4
Note: in olivine, either Fe or Mg can act as the glue
>> this is because they have the same valency, and very similar ionic radii (0.66
Angstroms for Mg2+ and 0.74 Angstroms for Fe2+ )
>> this means you can have an olivine with any range of Fe to Mg ratio
>> this is called a SOLID SOLUTION, and we express this idea by writing the formula
for olivine as (Mg, Fe)2 SiO4
> olivine is hard, because of the strong bonds, and it has no cleavage because there is no
continuous direction along which weak bonds can be broken
(Remember: cleavage is the defined as the splitting of a mineral along planes of
weakness)
The remaining silicate structures involve variable degrees of POLYMERIZATION:
> polymerization involves the joining together of silica oxygen tetrahedra by sharing of
oxygen atoms
>> for example, two silica tetrahedra can be joined by sharing an oxygen atom between
them
>> that shared oxygen atom will have its octet rule satisfied, but the remaining six
oxygens still need an electron each
Let's look at how the other four silicate structures incorporate polymerization:
(2) FRAMEWORK STRUCTURE:
> in the framework structure, there is COMPLETE polymerization
>> every oxygen is shared, and all silica tetrahedra are joined (no other "glue" is needed
to hold them together)
> there are two important examples of this: quartz and feldspar
e.g., QUARTZ:
> all silica tetrahedra are touching by sharing oxygens
>> since each oxygen does double duty (it is shared between tetrahedra), the formula for
quartz is: SiO2
> quartz is the hardest common silicate mineral because all of the bonds in it are the
strong mixed covalent/ionic bonds of the silica tetrahedra
>>furthermore, there is NO CLEAVAGE because all the bonds are of identical strength,
and if you try to split it there will be no plane of weaker bonding to split along
So, why didn't nature simply make the whole crust out of quartz? Because there was not
enough Si for all that O to make nothing but quartz
>> since Al is third in abundance in the crust (after O and Si), nature made feldspar
(which has Al in it) to accommodate all that oxygen
e.g., FELDSPAR:
> feldspar is the most common mineral in the earth's crust
> like in quartz, all silica tetrahedra are touching by sharing oxygens
>> however, compared to quartz, some of the Si4+ ions in the silica tetrahedra are replace
by Al3+ in the feldspar
>> this replacement of Al for Si can happen because they have similar ionic sizes (0.39 A
for Si ions, and 0.51 A for Al ions)
NOTE, however: although the Si and Al ions have similar size, they have different
valencies, and that is why other ions (K, Na, and Ca) have to be added to feldspars to
make them electrically neutral
> there are two kinds of feldspar: a. potassium feldspar, and b. plagioclase feldspar
Potassium Feldspar:
> every fourth Si is replaced by an Al
>> the charge imbalance (of 4-plus Si versus 3-plus Al ions) is countered by adding K+1
ions
>>> this results in the formula KAlSi3O8
> potassium feldspar is hard (not as hard as quartz) due the main bonds being the O-Si
bonds in the tetrahedra
>> but there IS cleavage (two directions at right angles), as the K occurs in the structure
at specific points, resulting in planes of weaker bonds where the K is
Plagioclase Feldspar:
a) in Na-rich plagioclase, every fourth Si is replaced by an Al
>> the charge imbalance is countered by adding a Na+1 ion
>>> the formula is therefore NaAlSi3O8
b) in Ca-rich plagioclase, every other Si is replaced by Al
>> since this leads to a double charge imbalance, Ca2+ is added
>>> the formula is therefore CaAl2Si2O8
> the hardness and cleavage of plagioclase will be the same as for potassium feldspar
NOTE: the concept of SOLID SOLUTION is valid for plagioclase
>> since Ca and Na ions have very similar size (0.99 versus 0.97 Angstroms,
respectively), they can readily replace each other
>> thus, we can find in nature plagioclase feldspar with compositions anywhere between
pure Na-plagioclase and pure Ca-plagioclase
(Pure Na-plagioclase is white in colour; Ca-rich plagioclase is darker (even black))
NOTE: there can not normally be solid solution between plagioclase feldspar and
potassium feldspar because of the vast difference in ionic radii of K (1.33 Angstroms)
compared to Na and Ca.
(3) SINGLE CHAIN STRUCTURE:
> this structure has polymerization, but the lowest degree of polymerization
>> it consists of silica tetrahedra polymerized by sharing two oxygens in every silica
tetrahedron
>> this forms a chain (like a strand of spaghetti!)
>> chains are joined to each other with the "glue" of ions of Mg, Fe, and/or Ca (with
valencies of two-plus)
e.g. pyroxene: (Mg,Fe,Ca)SiO3
>> it is hard, and there are two directions of cleavage at right angles
>>> the mineral splits along the length of the chains where the Mg/Fe/Ca atoms are
bonded to the chains (it can't be split ACROSS the chains)
(4) DOUBLE CHAIN STRUCTURE
> the double chain structure is more polymerized than the single chain
>> each double chain is nothing more than two single chains that are bonded together by
sharing oxygens between silica tetrahedra (i.e. they are POLYMERIZED together)
>>> the double chain is like a strand of fettuccini (double-width pasta)
>> the double chains are "glued" together by Ca, Mg, Fe, Na, OH (e.g. amphibole:
Ca2(Mg,Fe)5 (Si4 O11 )2 (OH)2
>>> the cleavage of amphibole is two directions (along the chains) at 56o / 124o
(It is NOT at ninety degrees, like in pyroxene, because of the fatter (fettuccini) chains)
(5) SHEET STRUCTURE:
> this structure is highly polymerized (but not as high as the framework structure)
>> it consists of sheets of polymerized silica tetrahedra (imagine bringing together
double chain after double chain and bonding them together by sharing oxygens
>>> the sheets are like sheets of lasagna pasta
>now, each sheet still has some oxygens that need electrons to satisfy the octet rule
>> this is taken care of by bringing in atoms like K or Fe or OH to provide the electrons
and to "glue" together the sheets in the process
>>> e.g., in the micas and chlorite
> in the sheet structure minerals, there is one perfect direction of cleavage parallel to the
sheets
>> they can be easily split along this direction
Clay: clay is a sheet silicate that is VERY weak
> this weakness arises because it is made up of "sandwiches" of Si-O and Al-O-OH
sheets that are bonded together by oxygen sharing such that each "sandwich" is
electrically neutral, BUT the "sandwiches" are weakly held together by Van der Waal's
bonds
>> that is why clay can be so easily molded: the Van der Waal's bonds are easily broken
>>> furthermore, BIPOLAR water molecules can enter between the "sandwiches" along
where the Van der Waal's bonds are
>>>> this can greatly expand the clay when it is wet, and cause it to shrink when it dries
up (EXPANSIVE, or SHRINK-SWELL, clays)
>>>> such EXPANSIVE clays result in more money lost annually in N. America from
foundation damage than from all other natural disasters combined
SO: Why is sand soil better to build on than clay soil?
Sand Soil:
> most usually it is made up of quartz
>> quartz has a strong framework structure to support the load
> quartz in sand soil usually has rounded grains, which leaves space between grains to
allow good drainage
Clay Soil:
> made up mostly of the mineral clay with a sheet structure
>> the mineral clay has a weak sheet structure and can't support a heavy load
> clay soils have poor drainage since the clay grains pack tightly together
>> also, the clay may suffer from shrink-swell behaviour (i.e., be expansive), and thus the
road foundation could break up during wetting and drying of the clay
NON-SILICATE MINERALS and SOURCES OF METAL
> non-silicate minerals are minerals that don't contain Si
>> non-silicates make up approximately 3% of the earth's crust
Non-silicates are formed of:
a. Small metallic atoms (e.g., Fe, Cu, Pb, etc.)
BONDED TO
b. Larger non-metallic atoms (e.g., O, Si, Cl, etc.)
Non-silicate minerals are grouped according to the NON-METALLIC atoms
> there are four main groups
(1) OXIDES: metals are bonded by oxygen
e.g., magnetite (Fe3 O4)
e.g., chromite (FeCr2O4)
(2) SULPHIDES and SULPHATES: metals are bonded by sulphur
e.g., pyrite (FeS2 )
e.g., galena (PbS)
e.g., gypsum (CaSO4 . H2 O)
(3) CARBONATES:
e.g., calcite (CaCO3 )
e.g., dolomite ((Ca, Mg) CO3 )
(4) CHLORIDES: metals are bonded by chlorine
e.g., halite (salt) (NaCl)
e.g., sylvite (potash) (Kcl)
Bonding in non-silicate minerals is almost always weaker than the bonding in the
silicates.
Let's talk about METALS AS A RESOURCE:
> we extract METALS (e.g., Fe, Pb, Zn, etc.) from minerals for use in our technological
world
Why are metals important?
> they are tough, ductile, malleable, and fusible, so that we can readily melt them, pour
them into forms, work them into shapes we want, and use their strength for support of
structures (e.g. Fe in steel)
> they have high thermal and electrical conductivities
>> some metals are used in their pure form
>> some are ALLOYED with other materials to achieve special properties (e.g., STEEL
= iron (Fe) plus other metals)
WHERE DO METALS OCCUR IN THE EARTH ?
> they RARELY occur in their elemental state (e.g., pure Au)
> MOST occurs bonded to other atoms in minerals
It helps to subdivide metals into two groups:
(1) Abundant Metals (each is greater than 0.1% by weight of the earth's crust)
> there are six
>> Si (28%), Al (8%), Fe (5.6%), Mg (2.3%), Ti (0.57%), Mn (0.1%)
(2) Scarce Metals (each is less than 0.1% by weight of the earth's crust)
> e.g., Cu, Pb, Zn, Cr, Mo, etc.
Where do ABUNDANT METALS (e.g., Si, Fe, Al, Ti, Mg) occur?
> they occur as ESSENTIAL components in both silicate and non-silicate minerals
e.g. Iron in olivine (silicate) and iron in pyrite (non-silicate)
Olivine: Fe2SiO4 : a silicate, and Pyrite: FeS2 : a non-silicate
Where do SCARCE METALS (e.g., Cu, Pb, Zn, etc.) occur?
a. Scarce metals occur MAINLY by ATOMIC SUBSTITUTION in common silicate
minerals
>> for example, Pb can substitute for K in potassium feldspar (KALSi3O8), but only to a
small extent (at parts-per-million or parts-per-billion levels)
>> in fact, about 99.99% of any scarce metal occurs this way
b. Scarce metals occur RARELY as ESSENTIAL components in non-silicate minerals
>> for example, Pb in galena (PbS), where Pb is 83% by weight
>> only about 0.01% of any scarce metal occurs this way (i.e., it is a VERY RARE
OCCURRENCE)
NON-SILICATE minerals make up only 3% of the earth's crust:
BUT, despite this low abundance, we obtain almost all of our metals (both ABUNDANT
and SCARCE) from NON-SILICATE minerals
WHY IS THAT???
> there are 3 reasons:
(1) Metals (both abundant and scarce) are concentrated in non-silicate minerals
compared to silicate minerals
e.g. a scarce metal like Pb:
KALSi3O8 (potassium feldspar) at p.p.m. Pb versus PbS (galena) at 83% Pb
e.g., an abundant metal like Fe:
Fe2 SiO4 (olivine) at 55% Fe versus Fe3O4 at 72% Fe
(2) It takes less energy, and therefore costs less, to extract metals from nonsilicate minerals compared to the silicate minerals
(This is the process of actually breaking apart the bonds and freeing up the metals:
it is called the smelting process)
> why does it take less energy?
>> because the bonds are WEAKER in the non-silicates
(3) There are occasional LOCAL ABUNDANCES of non-silicate minerals
e.g., calcite (CaCO3 ) concentrated in the rock limestone
e.g., chalcopyrite (CuFeS2) concentrated in veins in a granite porphyry (where
you get a granite with up to 1.5% Cu in it)
> a MINERAL DEPOSIT is defined as a local accumulation (above the background,
average, value of the earth's crust) of a specific mineral
> an ORE DEPOSIT is a mineral deposit that can be mined PROFITABLY
>> how does a mineral/ore deposit form?
>>> there has to be an input of energy in order to "sort" materials and concentrate certain
ones
>>>> this can be internal OR external energy (or both!)
Therefore, the ROCK CYCLE that summarizes the effects of internal and external energy
can be used to consider how the formation of different rocks can also result in
mineral/ore deposits
>> when we discuss the 3 different rock types, we will consider how ore deposits of
metal form in each case
> how do we find ore deposits efficiently?
a. by knowing how they form (the geological setting)
e.g., in what rock type and composition will we find chromite ?
b. by knowing their geophysical and geochemical signatures
>> this helps us find them if they are buried (e.g., a buried magnetite ore body might be
found because of its strong magnetic character (geophysical), or a buried deposit of
chromite might be found because the soil above may be enriched in Cr (geochemical)
Now, suppose you have found a MINERAL deposit. What will determine if it is valuable
enough to be an economic ORE deposit?
>> there are a number of factors, such as:
1. The price people are willing to pay for the metal or material
2. The grade of the deposit (i.e., the % of the metal or material present)
3. The size of the deposit
4. The depth of the deposit in the earth's crust
5. The geographic location of the deposit (is it close or far away from market)
Once you have determined that a mineral deposit is an economic ORE deposit, then you
can mine it
>> if it is a metal you are extracting, there are three steps:
(Let's use the example of veins of chalcopyrite in a granite porphyry)
Step 1. Mine (i.e., dig out) the ORE and concentrate the ORE MINERAL:
>> the ORE is the enriched rock, and to get it you may need to dig out some waste rock
around it
>> also, the ORE itself will include a lot of waste rock (mostly silicate minerals) that you
need to get rid of (e.g. in the granite porphyry, there may be only ca. 1% chalcopyrite
surrounded by 99% feldspar and quartz); these silicate wastes are called GANGUE
>>> you dig out and crush the ore, and then separate and concentrate the ORE
MINERAL (e.g., chalcopyrite) using PHYSICAL PROPERTIES (such as density
differences, as chalcopyrite is denser than the common silicate minerals)
>>>> once you have concentrated the chalcopyrite, you now have a material with 30%
Cu in it
Step 2. Smelt the ore to extract the metal from the mineral:
>> this is very energy intensive, as you need to disintegrate stable chemical compounds
>>> this is most often done by heating the mineral (e.g. chalcopyrite) until the bonds are
broken and it melts
>>> you then refine the melt to purify the Cu out of it (to get 100% Cu)
Step 3. Take care of all the waste material:
>> the GANGUE (silicate and some non-silicate wastes) have to be dumped somewhere
>>> these are called MINE TAILINGS
>>>> because the tailings commonly contain sulphides mixed in with the silicates, there
is the danger of rain water dissolving sulphur to make sulphuric acid that can run-off into
streams (such tailings are called ACID MINE TAILINGS, and they have to be managed
carefully)
>> also, when you melt sulphides like chalcopyrite, the S may go up the chimneys
(stacks) as sulphur dioxide
>>> this can combine with rainwater to make ACID RAIN
>>> this requires "scrubbers" in the stacks to try and trap down as much of the sulphur as
possible
Last revision: 10 October 2000
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