This document provides an overview of minerals and igneous rocks. It defines minerals as crystalline solids with a specific internal structure and consistent elemental composition. Minerals have requirements of crystallinity and composition. Igneous rocks form from the cooling of magma and lava and are classified based on their mineral composition (felsic, mafic, intermediate, ultramafic) and texture (phaneritic, aphanitic, glassy). Common igneous rock types include granite, rhyolite, gabbro, and pegmatite.

1. Volcano Chemistry

2. Minerals
Photo by Chip Clark

3. What is a Mineral?
A mineral is a crystalline solid with a specific internal
structure and a consistent elemental composition.
From Hamblin & Christiansen (2001)

4. Crystallinity
Diamond and graphite
are examples of minerals
that have the same
composition (C) but a
different crystalline
structure.
From Hamblin & Christiansen (2001)
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.

5. Composition
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
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.
Photos from Hamblin & Christiansen (2001)

6. 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.

7. 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.

8. 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.

9. Ions
Ions are elements that have an electrical charge.
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.
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.

10. Chemical Bonding
From Tarbuck & Lutgens
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.

11. 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.

12. 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.

13. Metallic Bonds
From Jim Clark
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.

14. Molecule
Halite Halite Structure
NaCl Molecule
Na+ Cl-
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.

15. Complex Ions/Silica Tetrahedra
Complex ions are covalently-bonded compounds that have an electrical charge.
The most important comlex ion is SiO4,
which has a negative charge of -4.
Si4+ + O2- --> SiO4
4-
From Hamblin & Christiansen (2001)
Complex ions combine like other ions to form electrically-neutral compounds.

16. Ionic Substitution
From Hamblin & Christiansen (2001)
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.

17. 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)

18. Mineral Groups
Mineral Groups
and Their Associated Anions
Native elements
Silicates
Oxides
Sulfides
Carbonates
Sulfates
Halides
Not applicable
4-
SiO4
O2-
S2-
2-
CO3
4-
SO4
Mostly Cl- and F-Major
Common
Cations
Si4
Al3
Ca2
Na
K
Fe2 & Fe3
Ti
Mg2
Mn2
P5
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.

19. From Hamblin & Christiansen (2001)
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.

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

21. What is an
Igneous Rock?
An igneous rock is a
rock that formed
through the cooling
and crystallization of
magma.

22. Magma vs. Lava
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.
Lava
Magma
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.

23. Classification of Igneous Rocks
(orthoclase)
Felsic
Modified from Hamblin & Christiansen (2001)
(hornblend)
(augite)
Igneous rocks are classified on the basis of two factors, mineral composition
(felsic vs. mafic) and texture (aphanitic vs. phaneritic).

24. Composition
(orthoclase)
Felsic
(hornblend)
(augite)
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.

25. Felsic Igneous Rocks
The term felsic comes from fel
for feldspar and sic for silica.
These rocks are also commonly
referred to as siliceous.
Amphibole
(hornblend)
(orthoclase)
Na-plagioclase
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.

26. Quartz

27. Orthoclase

28. Plagioclase Group

29. Hornblende

30. Biotite

31. Mafic Igneous Rocks
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
(augite)
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.

32. Olivine

33. Augite

34. Intermediate Igneous Rocks
Intermediate composition igneous
rocks are transitional between
felsic and mafic and contain
minerals common to both.
K-feldspar
(orthoclase)
Biotite
Ca
Quartz
Plagioclase
Na Pyroxene
(hornblend)
(augite)
However, notice that quartz and pyroxene do not overlap and are rarely found
together. Also, olivine is typically not found in intermediate composition
rocks.

35. Ultramafic Igneous Rocks
Ultramafic refers to rocks that are
composed almost exclusively of
Fe- and Mg-rich minerals.
Ca-plagioclase
Pyroxene
(augite)

36. 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.

37. Texture
Texture refers to the size, shape, and relationship of mineral grains and records
the cooling history (speed, depth) and gas content of the magma.
Phaneritic (course-grained) Aphanitic (fine-grained)
From: British Columbia Ministry of Mines and Energy
Glassy (smooth) Vesicular (full of holes) Brecciated (fragmented)

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

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

40. Grain Shape
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.
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.

41. Grain Relationships
The primary texture of igneous rocks is that of interlocking crystals.

42. 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.

43. Glassy Texture
From Grace Davies Photography
In some cases the lava cools so rapidly that no crystals have time to form,
producing a glassy texture.

44. 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.

45. Pyroclastic Texture
From: British Columbia Ministry of Mines and Energy
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.

46. Textural Summary

47. Classification of Igneous Rocks
Felsic
Modified from Hamblin & Christiansen (2001)
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.

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

49. Pegmatite
Photo by: Deanna Greenwood
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.

50. 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.

51. Gabbro
From: British Columbia Ministry of Mines and Energy
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.

52. 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.

53. Diorite
From: British Columbia Ministry of Mines and Energy
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.

54. 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.

55. Peridotite
Dunite
Pyroxenite
From: British Columbia Ministry of Mines and Energy
From: Pamela Gore
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).

56. Komatiite
From: Univ. of Hawaii From: Univ. of Hawaii
Komatiite is ultramafic and aphanitic. It has the same composition as
peridotite, but the groundmass minerals cannot be seen without
magnification.

57. Classification of Igneous Rocks

58. Obsidian
From Grace Davies Photography
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.

59. Pummice and Scoria
From: Pamela Gore
Pummice 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.

60. Volcanic Breccia
From: British Columbia Ministry of Mines and Energy
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.

61. Welded Tuff
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.