Igneous Rocks
Composition and Texture
Understanding igneous rocks requires that we examine two things about them:
composition and texture.
Compositionally igneous rocks are strongly dominated by the few minerals covered
in an earlier presentation: olivine and other ferromagnesian minerals, feldspar, and
quartz, with lesser amounts of mica and tiny amounts of other minerals. All of
these minerals are silicates – minerals whose anion is a combination of silicon (Si)
and oxygen (O). Even though the ratio of Si to O varies in the minerals all are made
of the same Si-O building block called a Si-O tetrahedron. We will first examine
that structure and see how it makes the various minerals in igneous rocks.
Texture in igneous rocks is not about how the rock “feels”, though some of the
textures have a smoother feel than others, but rather about the crystal size. That
is, “texture” in the igneous sense is something you gauge with your eyes, not your
fingers.
1. Composition
The Si-O tetrahedron is made of one Si atom and 4 O atoms. The O atoms are stacked
into a triangular pile (making a 4-sided pyramid with triangular faces) with the Si nestled
between them.
Si has the atomic number 14; O has the atomic number 8. The silicon-oxygen
tetrahedron has the formula SiO4. Let’s think about the electrical charge of this
structure.
A shell model for Si would look like this:
The electron dot models are therefore:
& for O like this:
:Si:
&
.
:O:
.
The Si has four e- to contribute to a bond (leaving it with a net +4 charge), the O atoms
each need 2 e- to fill their shells (giving them a -2 charge). You would think that SiO2
(quartz) would be the perfect solution ([+4] for Si and 2x [-2] = [-4] for O will balance the
charge), but in this case it is not so straightforward. Even quartz starts with this SiO4
configuration. It is a very stable structure.
So electrically the charges in the tetrahedron are [+4] for Si and 4 x [-2] = [-8] for O.
[+4] = [-8] =[-4] so this structure behaves as a -4 anion.
Let’s look at some characteristics of the tetrahedron before we go on.
“Tetrahedron” translates “four faces”. Imagine placing a sheet of
paper cut into appropriately sized isosceles triangles on each
triangle of O atoms. You would get a shape like the one at left –
four triangles with their edges joined to form a pyramid. You
could roll this into any position with any of the triangles as the
base and the shape would be identical.
We sometimes
symbolize the
tetrahedron as just
a triangle, like this.
The -1 in the center
indicates the
fourth O is above
the structure.
Omitting it
suggests the O is
below.
We can also leave
the -1’s out
-1
altogether.
The four negative charges left after the atoms form
this structure are not spread evenly over the surface.
Instead, one -1 charge remains associated with each
O, at the corners of the pyramid.
-1
-1
-1
-1
-1
-1
-1
There are two ways to deal with the dangling -1 charge at a tetrahedral corner.
Si-O ratio is 2:7
Si-O ratio
is 1:4
(or 2:8)
Sharing O’s drives
the proportion of Si
up. As more and
more of the corner
O’s are shared that
trend continues.
We say that the
“silica” content is
increasing.
1
2 shared O
2 shared O
2
2 shared O
3
3 shared O
No shared O
Progressive sharing of O makes for more and more complex structures. There are 5
important ways that SiO4 tetrahedra are joined in igneous rocks.
3 shared O
4
5
In the silicates the tetrahedra are bonded with strong covalent bonds. The atoms
share a common valence shell in these bonds. Shared O atoms mean that the
structure created by the sharing is also covalently bonded.
On the other hand, the bonds that hold the structures together, for example the +2
cations that hold isolated tetrahedra or chains together, are ionic bonds, and are
usually weaker. Here the valence e- are transferred rather than shared and only the
resulting electrical difference holds the atoms together.
The minerals therefore break more easily along paths of ionic bonds and resist
breaking across covalently bonded structures.
See if you can figure out what this has to do with cleavage.
1- No preferred
direction of stronger
bonds. NO CLEAVAGE.
2- Preferred direction
of weaker bonds
parallel to page and
along planes indicated.
Chains are equally high
and wide, so there are
2 CLEAVAGE
DIRECTIONS AT 90°.
4- All bonds in plane of
page are equally strong
covalent bonds. Each
sheet is bound to the
next with weaker ionic
bonds. 1 CLEAVAGE
DIRECTION PARALLEL
TO PAGE. Produces
sheets. What mineral
group is this?
3- Preferred direction
of weaker bonds at 60°
to page intersecting
along lines indicated.
Chains are wider than
high and wide, so there
are 2 CLEAVAGE
DIRECTIONS AT 60° and
90°.
5 - No preferred
direction of weaker
bonds. NO CLEAVAGE.
1- OLIVINE
3- AMPHIBOLE
4- MICA
2- PYROXENE
5- QUARTZ and FELDSPAR
1- OLIVINE
3- AMPHIBOLE
4- MICA
2- PYROXENE
5- QUARTZ and FELDSPAR
Except in the feldspars, which form independently of the other minerals in an
igneous rock, the sharing of O atoms increases as the temperature drops during
cooling. This is because of the energy stored in the bonds, which is low when the
liquid is hot (more energetic) and greater when the liquid is not as hot (less
energetic).
The silicate structures form in sequence during cooling, each one falling apart to be
replaced by the next (using the same atoms) as long as there is any liquid magma.
It’s like musical chairs – the one forming when the last crystal is formed (when the
minimum temperature is reached and the last liquid freezes) is the one that stays.
Here is the order, starting with the highest temperature form:
Isolated tetrahedra (no shared )O ----------------------------------- Olivine
Single chains (2 shared O) --------------------------------------------- Pyroxene
Double chains (2 or 3 shared O) ------------------------------------- Amphibole
Sheets (3 shared O) ----------------------------------------------------- Biotite/Muscovite
Frameworks (all 4 O shared) ----------------------------------------- Quartz
The plagioclase feldspars do something similar, also related to temperature. When the first
(highest temperature) plag crystal forms it will only fit Ca ions in its structure. As the
temperature drops progressively less Ca and more Na goes into the structure. This changes the
composition gradually until the very last plagioclase to crystallize can only fit Na into its structure.
(Assuming of course that the liquid has lasted that long).
The first Ca plag crystal forms at about the temperature that olivine is dissolving back into the
melt and pyroxene is beginning to form instead. It continues to be mostly Ca plag for as long as
the pyroxene is stable.
Once the temperature is low enough for the pyroxene structure to fall apart and for amphibole to
form in its place, the plag is crystallizing with roughly half Ca and half Na atoms in its structure.
We call this “mixed plagioclase”. It is white, like Na plag.
When the temperature falls to the stability field of biotite Na is almost the only or entirely the
only atom going into the plagioclase. (If the Fe is used up while the mica stability temperature is
still in effect then muscovite forms instead).
At lower temperatures K feldspar will form, after all the Ca and Na are used up or during the last
phases of Na plag formation.
Any remaining liquid will have only SiO tetrahedra in it and it will crystallize as quartz.
BOWEN’S REACTION SERIES
“SERIES” IS PLURAL HERE
First to crystallize
Last to Crystallize
Red arrows indicate minerals that crystallize at about the same temperature/time.
2. Texture
The texture (crystal size) is also related to the cooling of the magma that made the
rock, but instead of the actual temperature it is the rate at which the temperature
changes that matters.
Consider this picture:
1) Notice the coarse texture to the bucket – the large darker and (particularly) lighter
squarish patches. What are these?
2) Why do blacksmiths dunk the objects they are making into water (or oil)? (Hint:
they still will not pick the object up barehanded for a long time.)
3)
(Also, incidentally, notice that the horseshoe is red hot and think back to stars …)
You can see the crystals of galvanized steel in the bucket
because they are quite large.
An object with large crystals is
easier to break than an object
made of the same stuff but with
smaller crystals because the
boundaries between crystals are
relatively weak. A crack that forms
between two crystals then is able
to propagate across the object,
particularly of many of the crystal
boundaries are aligned.
This doesn’t matter so much with a
bucket because the stresses
ordinarily put on buckets are pretty
small. Going to the trouble of
making the crystals smaller is an
unnecessary expense that would
only raise the cost of the bucket.
Quenching the horseshoe forces rapid crystallization by dropping the temperature
quickly. The iron (and whatever else is there) forms many, many crystals all at once
and none of them is able to grow to a large size. Smaller crystals make the shoe less
likely to break when a half ton of horse drops its feet on them.
Now, compare these two rocks:
(This one has a weathering “rind” on it that is essentially rust.)
What can you infer about the way they cooled?
(The one on the right cooled at a much higher temperature because of the minerals in it. That’s not the
topic of the moment though.)
Because of the crystal size you can infer that the rock on the left cooled much more
slowly than the one on the right, giving the crystals plenty of time to grow to a large size.
What controls the rate of cooling of a rock?
How do we go
about controlling
the rate at which
we cool down?
We “cover up”.
Actually, the better analogy
would be, “how do we speed up
the rate at which we cool
down?”
We uncover, of course.
The key to controlling the rate
of heat loss of a thing is to
insulate or un-insulate it as
necessary.
The insulator in the case of igneous rocks is the Earth
itself. Magma originates within the Earth (not,
relatively speaking, very deep) and as long as it is
underground it will ordinarily cool very slowly
because the surrounding rock is a good insulator. If it
drops below its crystallization temperature still in the
ground it will have coarse crystals (except in
extraordinary cases).
On the other hand, if the magma
is erupted (or “extruded”) from a
volcano it will cool much more
quickly because it is no longer
well insulated from heat loss.
There are several different igneous textures but we will only examine four of
them, and of those four these two are the most important: crystals that are
uniformly visible to the naked eye (“coarse crystals” and crystals that are
microscopic (“fine crystals”). The technical names are “phaneritic” (Greek for
“visible”) and “aphanitic” (Greek for “not visible”). (Pay attention to the spelling
and sound them out).
Coarse crystals (phaneritic)
Fine crystals (aphanitic)
One of the two additional textures
we’ll look at is shown here.
Notice that there are large white
crystals of plagioclase (an almost
pure Ca plag, by the way – it takes a
fair bit of sodium to make it gray or
black). These large, obvious crystals
are “phenocrysts” – with the same
root as “phaneritic”.
Surrounding this is a dark finely
crystalline (aphanitic) “groundmass”.
With a microscope we can see that
it is a mix of Ca/Na plagioclase
(but well over half Ca) and
pyroxene. This fine material is the
rock’s “groundmass”.
How would such a texture form?
The coarse plagioclase
phenocrysts indicate that the rock
formed its first (and highest
temperature) crystals while
cooling slowly, still within the
Earth.
However, before the magma
could crystallize completely it was
extruded from a volcano and the
fine groundmass crystallized
there, uninsulated from heat loss
to the atmosphere (or water, if
the eruption was submarine).
The other texture to examine is called “glassy” texture. The rock (obsidian) is always black and looks like
broken glass – conchoidal fracture and all.
Your book, and most introductory textbooks, suggest that this rock forms when the cooling is too rapid for
any crystals to form at all. That’s what “glass” is – a non-crystalline solid.
In fact, with a normal magma it is impossible to cool the magma fast enough to make glass. Even eruptions
at the ocean floor, in water 2-4°C below freezing produce rocks with small crystals.
This texture forms only from magmas that are extraordinarily thick (viscous). They are probably more like
something between cold honey and silly putty when they erupt!