Transcript 1A_RocksEngPropertiesx

Introduction to Rocks and
Their Engineering Properties
Lecture by Dr. Ken Galli, Boston College
EESC116301
Environmental Issues and Resources
June 25, 2015
Please do not distribute beyond the EESC116301Class.
GEOLOGIC MATERIALS AND THEIR PROPERTIES
The abundant elements in the Earth’s crust are:
Si (Silicon)
Al (Aluminum)
O (Oxygen)
Ca (Calcium)
Mg (Magnesium)
Fe (Iron)
Na (Sodium)
K (Potassium)
(the big eight!)
They do not occur in elemental form, but as naturally occurring combinations (compounds) with specific and unique
properties known as Minerals.
Mineral: naturally occurring, inorganic, crystalline solid with a defined chemical composition and unique properties.
These elements make up virtually all of the mud,
sand and various types of rocks at the Earth’s
surface.
Element: a substance that cannot be split into
simpler substances by a chemical reaction. All
elements have the same number of protons in
their nuclei.
Atom: the smallest part of a substance that can
still exist and still retain the properties of the
substance.
Compound: a substance whose molecules contain
the atoms of two or more elements, chemically
bonded together.
Mixture: is a combination of two or more
substances that are not chemically united and do not
exist in fixed proportions to each other. It has no
chemical bonding and is therefore not a compound.
Molecule: the smallest naturally occurring particle
of a substance. Molecules can consist of any
number of atoms, from one (e.g., neon) to many
thousands (e.g. proteins). All the molecules of a
pure sample of a substance contain the same atoms
in the same arrangement.
Atoms: Building Blocks of Minerals
• Atoms
– Smallest particles of matter
Atomic Structure
• Nucleus – central part of an atom that contains
– Protons – mass of 1, positive electrical charges
– Neutrons – mass of 1, neutral electrical charges
• Electron shells
– Surround nucleus
– Contain electrons – negligible mass (1/1836th of a
proton’s mass), negative electrical charges
An electron generates an electric field surrounding it. An electron
moving relative to an observer generates a magnetic field.
External magnetic fields deflect an electron. Electrons radiate or
absorb energy in the form of photons when accelerated.
Atomic Structure
Atoms
• Atomic number
– Number of protons in an atom’s nucleus.
– Defines which element it is.
• Ions
– Positively or negatively charged atoms due to
electron gain or loss
Composition and Structure of
Minerals
• Elements
– Basic building blocks of minerals
– Over 100 are known
– Defined by their number of protons
– Groups of the same type of atoms
Inert
Share
electrons
Vertical columns
have elements with
similar properties
Lose
outer electrons
Galli, After - http://sciencenotes.org/wp-content/uploads/2015/01/PeriodicTableMuted.png
Gain
electrons
How to make a rock!
Start by bonding one or
more elements together
into crystals, the
building blocks of
minerals
Next, put together a
mixture of one or more
minerals to make an
aggregate—a rock!
Rocks can be observed
where they naturally
‘crop out,’ in Outcrops.
FYI—Mixture: an unbounded aggregate of
of one or more elements or compounds in
which the components retain their
properties, and in which there is no fixed
proportion of one compound to the other.
Some Important
Minerals
MINERALS
The Mineral QUARTZ
Properties:
Hardness (H) of 7 (very hard) [Diamonds Hmax = 10]
No Cleavage because there are no planes of weakness in its
molecular structure
Conchoidal (shell-like) Fracture
Very resistant to chemical and physical weathering
Hexagonal Crystals (i.e. six-sided)
Dissolved by strong bases
Many colors, but often clear
and transparent or translucent SiO2
To recap:
clear, hard, fractures, chemically inert
MINERALS
The Mineral PINK FELDSPAR (Orthoclase)
One of the Most
Properties:
common minerals
Hardness of 6.0 (hard)
Cleavage is good in 2 directions forming nearly right angled prisms
Color is off-white, yellow, or shades of red, orange to brown
Luster is vitreous to dull if weathered
Transparency crystals are usually opaque
Monoclinic Crystals
KAlSi3O8, Potassium Aluminum Silicate
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Clay Minerals
• Al, Si, K in sheets or layers that cannot be seen with
the naked eye nor a hand lens. The extremely fine
equivalents of micas.
• Many form by alteration of Feldspars at the Earth’s
surface. This happens quickly and can be seen as
light gray to white soft areas on the surface of
feldspar crystals or grains in many rocks.
• Probably the most common material at the Earth’s
immediate surface.
MINERALS
The Mineral MUSCOVITE, a type of Mica
Properties:
Hardness of 2.0-2.25 (very soft) [Talc Hmin =1]
One Plane of Perfect Cleavage because there are
horizontal planes of weakness in its molecular structure
Fairly resistant to chemical and physical weathering
Plate- or Sheet-Like Crystals; Tabular Crystals with
Pseudo-hexagonal or triangular outline
Color: Clear, transparent to translucent sheets,
often in thick stacks called “books!”
One of the most common minerals in rocks
Excellent electrical and heat insulator
KAl2(AlSi3O10 ) (OH)2
To recap: Clear, tough sheets that you can peel!
The Mica Sheet -->
MINERALS
The Mineral CALCITE
Properties:
Hardness (H) of 3
Light, with perfect Rhombohedral Cleavage
Very resistant to chemical and physical weathering
Hexagonal Crystals (i.e. six-sided; Dog-tooth spar)
Easily dissolved by weak acids, incl. dilute Hydrochloric acid (fizzes)
Many colors, but often clear and transparent or translucent
CaCO3; Main constituent of Limestone
To recap: Clear, Soft, Flat Cleavage Planes, Fizzes in weak acid/vinegar
Cleavage Unit Cell
Calcite Rhomb
Dogtooth Spar crystal
Double Refraction
Hot igneous melt
(magma) can intrude
and cross-cut preexisting rocks; if we
date the intrusive
igneous rock then we
know the rock it cuts
is older than that
absolute date. This is
one type of relative
dating.
James
Hutton’s
Rock Cycle
If regionally
metamorphosed by
the collision of terrane
with the edge of a
continent they display
FOLIATION- parallel
alignment of platy or
elongated minerals at
a right angle to the
maximum collisional
stress.
Sedimentary Rocks
Form in layers
(STRATA) Original
bedding, due to
vertical gravitational
settling
(sedimentation), with
oldest strata at base
and youngest at top of
an undisturbed
sequence.
Compaction
+/Cementation
= Lithification
(i.e., turned to
rock)
Sedimentary Rocks:
Sediment: Unconsolidated particles created by weathering and erosion of rock and made of
fragments or clasts, checmial precipitates, and organic debris.
(Source: R. H. Bailey)