EAS 302
Lab 3
From Atoms to Crystals: Introduction to Mineralogy
Name _____________
Introduction
Due 16 Feb 2005
Most of the Earth is made of crystalline
solids, things we call minerals. There
are thousands or tens of thousands of
different minerals, but only a few are
common. The most common minerals
are silicates, carbonates, and sulfides.
Of these, silicates are by far the most
abundant, because of the great
abundance of silicon and oxygen in the
Earth. At pressures prevailing in the
outer 700 km of the Earth, the silica
tetrahedron, a silicon surrounded by 4
oxygens, forms the basis of all silicates.
This is illustrated in Figure 1. The
Figure 1. The silica tetrahedron.
tetrahedra may or may not share
oxygens and hence be bound together.
Except in quartz, there are always other ions present between the tetrahedra. The
degree of sharing of oxygens between tetrahedra and the number and nature of other
ions present leads to several basic classes of silicates. These are orthosilicates,
chain silicates (single and double chains), sheet silicates, and framework silicates.
2. Mineral Classes
Minerals are subdivided or classified
based either on their symmetry or their
composition. Here we will briefly consider only compositional classification.
A. Native Metals: Perhaps the most fundamental group of minerals are native metals.
Copper, iron, gold, platinum, and a few other metals can occur naturally as pure (or
nearly so) metals. These naturally occurring metals are called “native” (e.g., “native
copper”). They are, however, extremely rare.
B. Sulfides are minerals in which a positively charged metal ion (called a “cation”)
such as iron (Fe), zinc (Zn), or lead (Pb) is bound to one or more negatively charged
sulfur ion (called an anion).
C. Sulfates are minerals in which a metal cation is bound to a sulfate (SO 24 – ) anion.
D. Carbonates are minerals composed of a metal cation bound to a carbonate (CO 23 – )
anion.
E. Oxides are minerals in which oxygen is the cation. Strictly speaking, minerals such
as quartz (SiO2) and feldspar (NaAlSi3O8) should be classified as oxides. However,
minerals such as these are placed in a class by themselves, called silicates.
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EAS 302
Lab 3
From Atoms to Crystals: Introduction to Mineralogy
F. Silicates are minerals containing silicon and oxygen and
are by far the most
common minerals on
Earth. They are so
important that we will
consider them in
much more detail
below.
G. Others: There are
many other groups,
including
halides,
phosphates, borates,
hydroxides, etc., but
Figure
The are
structures
of olivine
and pyroxene.
while2.they
sometimes
important,
they are less
common that the minerals of the groups listed
above.
3. Structure of Silicate Minerals
All silicates minerals are based on a single
structural unit, the silica tetrahedron, in which a silicon
(Si) atom is bonded to and surrounded by 4 oxygen
(O) atoms arranged as a tetrahedron or 3-sided
pyramid (Figure 1).
Among other things, this
arrangement results because this is the most efficient
way to pack 4 large oxygen ions around a silicon ion.
By joining tetrahedra in various ways, nature has Figure 3. Arrangement of
found a way to construct a vast array of minerals with tetrahedra in sheet silicates
a remarkable range of properties.
Orthosilicates (inosilicates): From a structural
viewpoint, the simplest silicates are the orthosilicates, in which the silica tetrahedra
stand alone, surrounded by metal cations to balance charge. A good example is the
mineral olivine ((Mg,Fe)2SiO4), one of the most abundant minerals in the Earth (Figure
2). Charge balance is needed since the each oxygen has a charge of –2, while the
silicon has a charge of +4. With four oxygens and 1 silicon, the silica tetrahedron has a
charge of –4. In olivine, the –4 charge of each tetrahedra is balance by the charge of 2
magnesium (Mg) or iron (Fe) atoms, each of which has a charge of +2.
Chain Silicates: In this group of minerals, one or two oxygen in each tetrahedra are
shared by two silicon atoms; as a result, the tetrahedra are joined together to form
chains, as illustrated in Figure 2. When one oxygen is shared, a single chain results,
when two oxygens are shared, a double chain results. Minerals formed by single chains
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EAS 302
Lab 3
From Atoms to Crystals: Introduction to Mineralogy
of tetrahedra are known as pyroxenes, for example enstatite (MgSiO3) and diopside
(CaMgSi2O6). Notice that since one oxygen is shared, the net charge on each
tetrahedra is now only +2. Hence in enstatite, only one magnesium (Mg) atom is needed
to balance the charge of each tetrahedra. Double-chain silicates are given the general
name “amphiboles”.
One of the most common amphiboles is hornblende
(NaCa2(Mg,Fe)5Si6AlO22(OH)2). Note that amphiboles always have OH ions substituting
for O, and usually have some Al substituting for Si.
Sheet silicates (phylosilicates): In sheet silicates, three of the oxygens in each
tetrahedra are shared between adjacent Si atoms. This results in the tetrahedra being
bound together to form sheets. The chemical bonds between sheets are usually much
weaker than the bonds within sheets. As a result, sheet silicates have a well-developed
cleavage.
White mica, or muscovite (KAl3Si3O10(OH)2), black mica, or biotite
(K(Mg,Fe)3AlSi3O10(OH)2), and talc (Mg3Si4O10(OH)2) are familiar examples. As with the
amphiboles, Al and (OH) are essential constituents.
Framework Silicates (tectosilicates): When all oxygens in the tetrahedra are
shared, a three-dimensional framework results. This is the crystal lattice structure found
in quartz and feldspars, which are among the most abundant minerals in the Earth’s
crust. In the feldspars, the trivalent aluminum ion, Al3+, substitutes for the tetravalent
silicon ion, Si4+. This reduction in positive charge is balanced by the addition of another
metal ion, either sodium (Na), potassium (K), or calcium (Ca).
A mineral’s properties are determined by its crystal lattice structure. For example,
olivine “weathers” quite rapidly, breaking down in contact with water at the surface of
Figure 4. Structure of pyrophyllite and talc, simple sheet silicates.
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EAS 302
Lab 3
From Atoms to Crystals: Introduction to Mineralogy
the earth to form other minerals. One reason it weathers so easily is that the silica
tetrahedra are not joined together. Once the surrounding metal ions are leached away,
the tetrahedra is no longer bound to its neighbors. Usually, pyroxenes and amphiboles
have two good cleavages (planes along which the mineral breaks more easily). This is
because the bonds within chains of tetrahedra are stronger than the bonds between
chains, so the cleavage planes are parallel to the chains of tetrahedra. Micas generally
have one extremely good cleavage: as you might expect, this plane is parallel to the
sheets of tetrahedra. Quartz does not have a good cleavage. Since all the tetrahedra
are bound together, there is no plane along which quartz easily breaks.
4. Mineral Identification
On the next page, you will find a list of minerals and a rough description of them. They
are divided into groups based on composition and structure (as you can see by the
chemical formulas). Your task is to match the minerals in the lab to the one that you
find on your list. You don’t have to do them in order, but make sure that you look at all
of the minerals.
Use the following criteria to identify the minerals
• Color
• Luster
• Cleavage/Fracture Surfaces
• Magnetism
• Density (specific gravity)
• Hardness
• HCl solubility
• Crystal Habit
• Streak
Use the following scale to determine relative hardness.
Fingernail - 2.5
Glass - 5 to 5.5
Knife - 6.5
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Quartz – 7
EAS 302
Mineral name
Lab 3
From Atoms to Crystals: Introduction to Mineralogy
Box #
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EAS 302
Lab 3
From Atoms to Crystals: Introduction to Mineralogy
Galena – PbS – most important ore of lead.
Characteristics:
perfect cleavage in three directions
metallic luster; grey color and streak
high density
hardness between 2.5 and 5.5
Pyrite – FeS2 – “fool’s gold”
Characteristics:
lacks cleavage, but usually occurs in cubes
metallic luster and gold color
greenish or brownish black streak.
parallel striations
hardness of 6-6.5
Gypsum – CaSO4 + H2O
Characteristics:
perfect cleavage in one direction
curved and splintery fracture
commonly found in arrow-head twins
colors include colorless, white, gray, yellow, red, and brown
hardness of 2
Calcite – CaCO3 – common mineral for invertebrate shells
Characteristics:
perfect cleavage in three directions forming rhombohedrons
transparent to translucent
effervesces when HCl is added to its surface
colors include white, gray, yellow and red
hardness of 3
Magnetite – Fe3O4 – common ore of iron
Characteristics:
magnetic!!!!
no cleavage
iron black color; black streak
hardness of 6
metallic luster
slowly soluble in HCl
Hematite– Fe2O3– ferric iron oxide; also an iron ore
Characteristics:
Redish brown to black; red streak
Luster is metallic in well-formed crystals
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EAS 302
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From Atoms to Crystals: Introduction to Mineralogy
Usually not magnetic
Habit is commonly tabular
Hardness 5.5 to 6.5
Soluble in HCl
Quartz – SiO2 – popular gemstone, including amethyst
Characteristics:
lacks cleavage and has a curved fracture
may be prismatic
colors include clear, purple, pink, gray, and yellow
hardness of 7
Potassium-Feldspar – KalSi3O8 – this is a major component of many granites!
Characteristics:
two good perpendicular cleavages
striations can be found on crystal faces
color can be pink, blue, green, white and pale yellow
hardness of 6
Mica – (chemical composition can vary depending on type)
Biotite – K(Mg,Fe)3(AlSi3O10)(OH)2
Muscovite – KAl2(AlSi3O10)(OH)2
Characteristics:
one perfect direction of cleavage
typically looks like sheets of paper in a book
colors are dark for biotite, and lighter for muscovite...and we have both!
Hardness of 2-2.5
Amphibole – (and what a complicated chemical composition!) – frequently a component
of granite
perfect cleavage in two directions in the shape of a diamond
fracture is typically uneven to splintery
crystals generally prismatic
colors are dk green, dk brown, and black
hardness of 5-6
Olivine – (Mg,Fe)2SiO4 – this mineral is a huge component of the green beaches in
Hawaii
cleavage indistinct
curved and uneven fracture
transparent to translucent
colors are olive-green, yellowish-brown, and reddish
hardness of 6.5-7
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