Natural Disasters Topic 5a (Volcanic Eruptions)

Volcano Chemistry

Photo by Chip Clark

Minerals

From Hamblin & Christiansen (2001)

What is a Mineral?

A mineral is a crystalline solid with a specific internal structure and a consistent elemental composition.

One of the requirements to be a mineral is that it has a specific internal structure. This is referred to as its crystallinity and means that atoms, or groups of atoms, are arranged in three-dimensionally repeating patterns.

Crystallinity

Diamond and graphite are examples of minerals that have the same composition (C) but a different crystalline structure.


Composition

The other requirement is that a mineral have a consistent elemental composition. This means that a given mineral is always made up of the same elements in the same ratio, which is shown by a chemical formula.

The mineral galena is composed of the elements lead (Pb) and sulfur (S) in a one to one (1:1) ratio, giving it the formula PbS.

Galena

The mineral fluorite is composed of the elements calcium (Ca) and fluorine (F) in a one to two (1:2) ratio, giving it the formula CaF2.

Fluorite

Some minerals are composed of several elements and have highly complex formulas, such as muscovite, which includes the elements potassium (K), aluminum (Al), silicon (Si), oxygen (O), and hydrogen (H) and has the formula KAl2(AlSi3O10)(OH)2 .

Muscovite

Photos from Hamblin & Christiansen (2001)

Atomic Structure

Atoms are composed of smaller particles called protons, neutrons, and electrons. Protons and neutrons are responsible for almost the entire mass of an atom and are tightly packed within the nucleus. Electrons carry almost no mass and form a “cloud” around the nucleus where they are in constant motion. Protons have a positive charge; electrons are negatively charged, and neutrons have no charge.

What is an Element?An element is an atom that has a specific number of protons.

The Periodic Table includes all of the known elements, including several which are man-made. Each element has an atomic number, written in the upper left corner, which shows how many protons are present in the nucleus of that atom. If the atom were to gain or lose a proton, it would become a different element, which is the basis of the ancient practice of alchemy. Each row indicates the filling of an electron shell for electrically-neutral atoms. Columns represent elements with similar chemical properties.

Isotopes

Some elements can exist with a variable number of neutrons. These are referred to as isotopes of that mineral. A change in the number of neutrons does not change the atom to another element but does affect its mass. Some isotopes are unstable under conditions other than those in which they formed and “decay” to form different, more stable isotopes.

IonsIons are elements that have an electrical charge.

For instance, sodium (Na), which is found along the left margin of the periodic table tends to lose one electron. This loss results in an ion of Na. Since there is now one less electron than proton, the overall electrical charge of the ion is +1. Chlorine (Cl) is located near the right margin of the table and has a tendency to gain one electron. This gain produces a Cl ion with one more electron than proton and a net charge of -1.

An electrically neutral atom has the same number of protons and electrons. However, many atoms, particularly those in columns at either end of the periodic table, tend to lose or gain electrons, giving them an overall charge. This charge affects the manner in which these elements bond to one another and to other elements.

Chemical Bonding

Bonding refers to the manner in which elements combine with each other to form more complex substances, called compounds. The type and strength of the bond is controlled by the manner in which electrons interact between these elements. In an atom, electrons are arranged in layers (shells) around the nucleus. Each shell has a maximum number of electrons that it can hold. For instance, the first shell can hold a maximum of two, the second 8, and so forth. If a shell is not full, the element is electrically unstable. To become stable, the element will either lose, gain, or share electrons to produce an outer shell that is complete.

From Tarbuck & Lutgens

Ionic Bonds

An ionic bond is produced when electrons are transferred from one atom to another, forming electrically charged ions. Atoms that lose electrons develop a positive charge and are referred to as cations. Those that gain electrons have a negative charge and are called anions. The opposing charges attract one another, forming ionic bonds. Ionic bonding is particularly common between atoms on opposite ends of the periodic table.

Covalent Bonds

A covalent bond is produced when electrons are shared between atoms. The electrons are neither lost nor gained but belong to both atoms equally. Covalent bonding is particularly common between atoms in the middle of the periodic table and is much stronger than ionic bonds.

Metallic Bonds

A metallic bond is produced when electron orbitals overlap and all electrons are shared between atoms. This is often referred to as a “sea of electrons,” and is responsible for the high conductivity, reflectivity, malleability, and ductility of metals. Metallic bonding is restricted to atoms of a single element.

From Jim Clark

Molecule

A molecule is a compound made of bonded atoms. A compound typically consists of two or more different elements but can be composed of one element only. A mineral is composed of a single type of molecule that combines into an identifiable crystal structure.

Halite StructureHalite

NaCl Molecule

Na+ Cl-

Complex Ions/Silica TetrahedraComplex ions are covalently-bonded compounds that have an electrical charge.

Complex ions combine like other ions to form electrically-neutral compounds.

The most important comlex ion is SiO4, which has a negative charge of -4.

Si4+ + O2- --> SiO44-


Ionic Substitution

A small amount of variation in a mineral’s chemical composition is permitted due to ionic substitution, in which ions of similar charge and size (ionic radius) can substitute for one another. Some possible substitutions are shown by the rows in the above illustration.


Physical and Chemical Properties

Because a particular mineral has a specific composition and internal arrangement of constituent atoms, all specimens will exhibit the same physical and chemical properties.

Photos from Hamblin & Christiansen (2001)

Mineral Groups

There are 92 known elements that combine to form over 4,000 different minerals. However, more than 99% of the earth’s crust (by weight) is composed of only 14 of these elements, and 90% of the crust is made up of the 10 most common minerals. Several important mineral groups have been identified and are named after their dominant anion. These anions combine with various cations to form the specific minerals within the group.

Native elements

Oxides

Sulfides

Sulfates

Halides

Silicates

Carbonates

SiO44-

O2-

S2-

CO32-

SO44-

Mostly Cl- and F-

Major Mineral Groupsand Their Associated Anions

Not applicable

CommonCations

Si4

Al3

Fe2 & Fe3

Ca2

NaK

Mg2

Ti

P5

Mn2


What is a Rock?

A rock is most commonly defined as an aggregate of different minerals that combine to make a single, heterogeneous mass. However, if a mass is sufficiently large, it may be composed of a single mineral and still be considered to be a rock.

Rock vs. Mineral

Minerals are always homogeneous substances with a single chemical formula; whereas, rocks are an aggregate of one or more minerals. Minerals, therefore, can be considered to be the building blocks of rocks.

This sample of granite is composed of the minerals quartz, plagioclase, potassium feldspar, and biotite.

What is an Igneous Rock?

An igneous rock is a rock that formed through the cooling and crystallization of magma.

Magma and lava are essentially the same material. We use the term magma for molten rock that is present within chambers beneath the earth’s surface. Magma becomes lava once it exits a volcano and flows over the earth’s surface.

Magma vs. Lava

Rocks that form while still in the subsurface from cooling magma are referred to as intrusive igneous rocks. Those that form on the surface as a result of solidification from lava are called extrusive igneous rocks.

Magma

Lava

Classification of Igneous Rocks

Igneous rocks are classified on the basis of two factors, mineral composition (felsic vs. mafic) and texture (aphanitic vs. phaneritic).

Modified from Hamblin & Christiansen (2001)

Felsic

(hornblend)(augite)

(orthoclase)

Composition

Most igneous rocks can be classified compositionally as felsic, mafic, or intermediate, depending on the mineral composition. Though less common, some rocks can be ultramafic in composition.

Felsic

(hornblend)

(augite)

(orthoclase)

Felsic Igneous Rocks

A felsic igneous rock always contains the minerals quartz and K-feldspar (orthoclase) and commonly includes Na-plagioclase, biotite, and amphibole (hornblende). Because of this mineral assemblage, felsic igneous rocks tend to be lighter in color than mafic ignous rocks, but this is not always the case.

The term felsic comes from fel for feldspar and sic for silica. These rocks are also commonly referred to as siliceous.

Amphibole (hornblend)

Na-plagioclase

(orthoclase)

Quartz

Orthoclase

Plagioclase Group

Hornblende

Biotite

Mafic Igneous Rocks

A mafic igneous rock is made up mostly of a combination of the minerals olivine, pyroxene, and Ca-plagioclase, though amphibole can be present in small amounts. Because of the iron content, these minerals tend to have a higher specific gravity than do felsic minerals.

The term mafic comes from ma for magnesium and fic from ferric (iron). These rocks are also referred to as ferro-magnesian.

Amphibole (hornblend)

Olivine

Ca-Plagioclase

(augite)

Olivine

Augite

Intermediate Igneous Rocks

However, notice that quartz and pyroxene do not overlap and are rarely found together. Also, olivine is typically not found in intermediate composition rocks.

Intermediate composition igneous rocks are transitional between felsic and mafic and contain minerals common to both.

Biotite

PyroxeneNa

Ca

Plagioclase

Quartz

K-feldspar

(hornblend)

(orthoclase)

(augite)

Ultramafic Igneous Rocks

Ultramafic refers to rocks that are composed almost exclusively of Fe- and Mg-rich minerals.

Pyroxene

Ca-plagioclase

(augite)

Bowen Reaction Series

A given mineral will crystallize and melt within a specific range of temperatures, thus the order of formation or destruction can be predicted. Mafic minerals tend to form and melt at higher temperatures than felsic minerals.

Texture

Phaneritic (course-grained) Aphanitic (fine-grained)

Texture refers to the size, shape, and relationship of mineral grains and records the cooling history (speed, depth) and gas content of the magma.

Brecciated (fragmented)Vesicular (full of holes)Glassy (smooth)

From: British Columbia Ministry of Mines and Energy

Grain size in igneous rocks is a function of cooling rate and magma viscosity.

Grain Size

High viscosity inhibits the free migration of atoms through the magma. Therefore, a magma with high viscosity tends to produce smaller crystals and a higher proportion of glass than less viscous magmas.

Rapid cooling does not allow for extensive migration of individual atoms within the magma, resulting in a large number of small crystals. Slower cooling permits for more extensive migration, producing fewer, but larger crystals. "Instantaneous" cooling results in a glassy texture in which no crystals are formed.

Rapid cooling or viscous magma

Slow cooling or fluid magma

Phaneritic vs. Aphanitic

Most igneous rocks are classified as aphanitic or phaneritic depending on crystal size. Aphanitic means invisible and refers to igneous rocks where the crystals are so small that they cannot been seen without magnification. In phaneritic rocks, however, the mineral grains are easily seen without magnification. The larger the crystals, the slower the cooling rate of the rock.

Phaneritic Aphanitic

Texture refers to the size, shape, and relationship of mineral grains and records the cooling history of the rock.

Grain shape in igneous rocks is a function of the space that is available in which the mineral can grow. As minerals become larger and more numerous, they come into contact with each other and lose their crystal forms.

Grain Shape

Euhedral Anhedral

A crystal in which all faces are visible is called euhedral. If no crystal faces are evident, the crystal is anhedral. When some, but not all, faces are present, the crystal is subhedral.

Grain Relationships

The primary texture of igneous rocks is that of interlocking crystals.

Porphyritic Texture

If an igneous rock contains more than one distinctive crystal size, it is considered to be porphyritic. The larger grains are referred to as phenocrysts and the more abundant smaller-grained material surrounding them as the groundmass. Phenocrysts are often euhedral in shape due to their early formation; whereas, crystals in the groundmass are almost always anhedral. Both aphanitic and phaneritic rocks can be porphyritic. A porphyritic texture indicates a dual cooling history that began relatively slowly and then became more rapid.

From Grace Davies Photography

Glassy Texture

In some cases the lava cools so rapidly that no crystals have time to form, producing a glassy texture.

Vesicular Texture

As magma rises the confining pressure decreases and gases (mostly H2O) come out of solution, producing bubbles. This is similar to what happens when the pressure is released upon opening a bottle of carbonated drink. Solidification of the magma leaves cavities called vesicles.

Pyroclastic Texture

During a volcanic eruption fragments from earlier deposits can be plucked from vent walls. These exit the volcano together with ash and other materials, falling to the ground and forming a texture similar to that of a sedimentary breccia.


Phaneritic

Aphanitic

Ext

rusi

ve

Porphyritic Phaneritic

PyroclasticGlassy

Intr

usi

ve

VesicularPorphyriticAphanitic

Textural Summary


Names for specific igneous rock types are determined by composition (felsic, intermediate, mafic, ultramafic) and texture (aphanitic, phaneritic, glassy). Modifying textural terms, such as vesicular and breccia can be added to the rock name.

Modified from Hamblin & Christiansen (2001)

Felsic

Granite

Granite is felsic and phaneritic. It will always contain the minerals quartz and K-feldspar and may include plagioclase, biotite, and amphibole.

Pegmatite

Pegmatite is felsic and phaneritic with very large crystals. It’s composition is similar to that of granite, and many consider it to be granite that is very coarse-grained.

Photo by: Deanna Greenwood

Rhyolite

Rhyolite is felsic and aphanitic. It has the same composition as granite, but the groundmass minerals cannot be seen without magnification. The most common phenocrysts in porphyritic samples are quartz and K-feldspar.

Gabbro

Gabbro is mafic and phaneritic. Olivine and pyroxene are the most dominant minerals. Ca-plagioclase is also common; amphibole and biotite are less common; K-feldspar is rare; and quartz is never present.


Basalt

Basalt is mafic and aphanitic. It has the same composition as gabbro, but the groundmass minerals cannot be seen without magnification. The most common phenocrysts in porphyritic samples are pyroxene and plagioclase.

Diorite

Diorite is intermediate in composition and phaneritic. Plagioclase (Na and Ca) is the most common mineral. Amphibole and pyroxene are also common, with lesser amounts of K-feldspar and biotite. Quartz can be present as a minor constituent.


Andesite

Andesite is intermediate in composition and aphanitic. It has the same composition as diorite, but the groundmass minerals cannot be seen without magnification. The most common phenocrysts in porphyritic samples are amphibole and plagioclase.

Peridotite

Peridotite is ultramafic and phaneritic. These rocks are composed almost entirely of the minerals olivine and pyroxene. If the sample is composed completely of one or the other, it takes on the name of that mineral and is referred to as dunite (all olivine) or pyroxenite (all pyroxene).

Dunite

Pyroxenite


From: Pamela Gore

Komatiite

Komatiite is ultramafic and aphanitic. It has the same composition as peridotite, but the groundmass minerals cannot be seen without magnification.

From: Univ. of HawaiiFrom: Univ. of Hawaii


Obsidian

Obsidian is felsic and glassy. It has the same chemical composition as granite, but elements have not formed into minerals. Glass is not stable, and over time will convert to minerals. It does this by initially forming structures called spherulites.

From Grace Davies Photography

Pummice and Scoria

Pummice is felsic with the same composition as granite. Scoria is mafic and has the composition of gabbro. Both are highly vesicular in texture and actually contain more open space than rock material.

From: Pamela Gore

Pummice Scoria

Volcanic Breccia

Volcanic breccia resembles sedimentary breccia in texture and is made up of angular fragments torn from the walls of a volcanic vent during en eruption. The matrix is typically volcanic ash. Composition is usually similar to that of diorite.


Tuff is a general term for ash, breccia, pummice, and other materials ejected by felsic and intermediate composition volcanic eruptions. As these deposits are buried they become compressed, eliminating the open space and taking on a banded appearance. As with obsidian, over time the unstable glass fragments devitrify and form into minerals reflecting the chemistry of the original eruption.

Welded Tuff

Science

Natural Disasters Topic 5a (Volcanic Eruptions)