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: & for O like this:

The electron dot models are therefore: :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 SiO4configuration. 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.

.

.

-1

-1

-1-1

“Tetrahedron” translates “four faces”. Imagine placing a sheet of paper cut into appropriately sized equilateral 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 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.

Let’s look at some characteristics of the tetrahedron before we go on.

-1

-1-1

-1

There are two ways to deal with the dangling -1 charge at a tetrahedral corner.

Si-O ratio is 1:4

(or 2:8)

Si-O ratio is 2:7

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

3

2 sh

ared

ON

o sh

ared

O

2 sh

ared

O2

shar

ed O

3sh

ared

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.

3sh

ared

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°.

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°.

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?

5 - No preferred direction of weaker bonds. NO CLEAVAGE.

1- OLIVINE

2- PYROXENE

3- AMPHIBOLE

4- MICA

5- QUARTZ and FELDSPAR

1- OLIVINE

2- PYROXENE

3- AMPHIBOLE

4- MICA

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.

First to crystallize

Last to Crystallize

BOWEN’S REACTION SERIES“SERIES” IS PLURAL HERE

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!