Lecture_04_MinsII_120124

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GEOS 251


Physical Geology

24 January 2012


Those still wanting to add the class


Come to the front of the room


Labs started this week


Minerals, I


Gould
-
Simpson 201, ground floor, north side of building


Today

Minerals and Fluids


Continue systematic mineralogy


Mineral properties


Lead into:

Rocks and the Rock Cycle


Progress on web site/D2L connection

Colloquium talk this week


Dr. Katie Snell, Postdoctoral Fellow,
California Institute of Technology


“Paleoclimate and Paleoelevation in

the Western US Cordillera, 80 Ma to Present”


Thursday, 26 January, 4pm ,129 Haury


Coffee and cookies beginning at 3:30 pm in
lobby of Gould Simpson

Last Time


Minerals


Naturally occurring, solid
crystalline

materials, typically
inorganic, with a fixed chemical composition


Crystal structures


Repeated 3
-
D arrangement of atoms that are linked to
crystal form


Bonding


Electron sharing in various modes (ionic, covalent,
metallic…)


Lower energy configuration than atoms (stabilizes
compounds, solutions, etc.)


Charge and size


Charge balance, coordination


Reflects composition, governs properties

Some Key Physical Properties

Key to identification and to behavior in the Earth


Density (mass / volume)


Response to force


hardness


elastic
-
brittle
-
ductile behavior


cleavage and fracture


Interaction with light


color (many origins)


refraction and dispersion


Electrical and magnetic properties

more next lecture; now systematic mineralogy

Today


Minerals & Properties


Continue systematic mineralogy


Mainly silicates


Mineral properties


Response to stress


Response to light


Response to fields


Mineral stability


What minerals can tell us


Lead into:

Rocks and the Rock Cycle

Systematic Mineralogy


Non
-
silicate minerals


Classified by anion
: elements, oxides, sulfides,
carbonates, sulfates, phosphates . . .


Silicate minerals


Classified by SiO
4
4
-

polymerization
:
framework, sheet, chain (single and double),
orthosilicates

Hematite:
Fe
2
O
3


6
-
fold symmetry,
an “iron rose”


This is the most oxidized form of iron (other valences?)


Where would you expect to find hematite (and most
Fe
+3
) in the Earth
--

in the core, the mantle, or the crust?

Apatite: Ca
5
(PO
4
)
3
F



a phosphate


Phosphates, are constructed
with the PO
4
-
3

anion


What kind of polyhedron is PO
4
-
3

(think coordination #)?


Phosphates are examples of the
many non
-
silicates with complex
oxyanions (
cf.

CO
3
=
, SO
4
=
)


Location of most phosphorous
in rocks (an essential nutrient)


Where is most of the
phosphorous in this room?

(& why are calcium and
fluoride important?)

Silicate Minerals


Constructed of SiO
4
4
-

tetrahedra that share 0 to 4 of their
corners with other SiO
4
4
-

tetrahedra


With all corners shared

one gets
SiO
2

(e.g., quartz)


a
“framework” silicate (3
-
D connections)


With fewer shared or with some Al
+3

substituted for
Si
+4,

we need to charge balance with other ions


typically Na, K, Mg, Ca, Fe, Al, H


What charge needed with isolated SiO
4
4
-
?


What if we substitute 50% Al for Si in 4[SiO
2
] = Si
4
O
8
?


With fewer than 4 corners shared we get sheet, double
chain, single chain, and orthosilicates


Similar relationships apply in silicate melts



The
main groups of silicate units

that make up
most silicate minerals

progressively greater
polymerization of
SiO
4
4
-

:


Isolated (0 shared) to single and double chain to
sheet to framework (all shared) [text images wrong]


We now look at these mineral groups along with
their related physical properties

Major Silicate Mineral Groups

(examined in lab this week, used
repeatedly
)


Maf
ic minerals (
Mg
-
Fe

silicates)


Olivine {isolated SiO
4
}


Pyroxenes {single chain}


Amphiboles {double chain}


Micas {sheet}


Clay minerals {sheet}


Felsi
c minerals (
fel
dspars and
si
lica) {framework}


Plagioclase feldspar (Ca
-
Na)


Potassium feldspar (K)


Quartz

Mineral Structures and Properties



Similar to Table 3.4 in 6th Edition (but easier to read)

Response to Applied Stress (F/a)


When forces are applied to them, minerals and
fluids respond by changing shape by:


Elastic
behavior


can be restored [solids only]


Brittle

behavior


cannot be restored (materials
break) [solids only]


Plastic

(or
ductile
) to
fluid

behavior


cannot be
restored [solids and fluids]


In minerals: cleavage, fracture, hardness are all
aspects of these phenomena


Cleavage in


Calcite
-

right


Halite (salt)
-

below


Similar appearance and
behavior to impact


Break along weak
directions in structure


Different angles reflect
different crystal structures


Mg[SiO
3
]

Mg
+2
2
[SiO
4
]
-
4

Mg
+2

[SiO
3
]
-
2

Mg
+2
7
[Si
8
O
22
]
-
12
(OH)
-
1
2


Differences in cleavage (and form)

single chain silicates vs. double chain silicates


A key tool for identification...


cleavage & crystal form in thin section


which silicate group?


Perfect single cleavage in mica
is due to the sheet
-
like silicate
structure

There are many
other links between
crystal structures of
minerals and their
physical properties


Chrysotile asbestos
and talc (both Mg
minerals) are among
the many sheet
silicates

TEM image

Minerals in

the News II


Asbestos (mineral fibers) was widely used in industry and
building for many decades in the 20th Century


For several decades it has been recognized that exposure to
“asbestos” can be major health hazard, leading to cancer


Enormous litigation, illness, and economic impact


However, there are
two kinds of asbestos



chrysotile
(serpentine)

asbestos and
crocidolite (amphibole)

asbestos


It has been demonstrated (e.g., by the National Academy of
Science) that
only crocidolite constitutes a health hazard
, yet an
enormous high cost effort is undertaken to remove both types







MOREOVER >>>


Serpentine (and
chrysotile) are very
common on active
continental margins


e.g., California


Soils are mineral based


Wine grapes like crummy
soils


like those derived
from serpentinite

Fracture

Another mode of brittle failure


Tendency to break along
irregular

surfaces (other
than cleavage planes)


Related to how bond strengths are distributed in
directions that cut
across

crystal planes


Conchoidal


smooth, curved (like glass)


Fibrous or splintery (like wood)


Related to this is how hard it is to scratch or powder a
mineral: this is termed a mineral’s hardness >>>


Analogous to Table 3.2 in 6th Edition

Similarities between Melts and Crystals


Silicate melts (magmas) have no long
-
range
order (they are not crystalline) but they still
have local bonding that is much like minerals


SiO
2
-
rich (“felsic” = feldspar
-

and silica
-
rich)
melts are highly polymerized and thus more
viscous (other things being equal)

Light and Minerals


Light is
absorbed
,

reflected, and refracted,

leading to differences

in
color, streak, luster
, and
fluorescence


Causes of color (absorption)


Intrinsic: Major or minor constituents
(e.g., Fe in
mafic

minerals; Cu in copper
minerals; Cr in ruby [corundum] or
emerald [beryl])


Other: Inclusions, diffraction behavior


What causes light and dark areas on moon?

Like other materials, minerals can
transmit, reflect, and refract light


Response to Light


Color

portion of spectrum reflected (not
absorbed) by the mineral


Fully absorbed


black


Scattered


white


Passes through


clear


Luster


the way in which the
surface

of the
mineral reflects light


Streak


color of fine, abraded grains


Many ways to powder, e.g., scratch on unglazed
porcelain


Fluorescence

emission of light by a
mineral stimulated by radiant energy


Ruby is a rare form of
the mineral corundum
[Al
2
O
3
]


In rubies, a little
chromium replaces
some aluminum,
leading to the red
color (Al
+3
,Cr
+3
)
2
O
3


Ruby deposits form
by metamorphism of
Al
-
rich rocks such as
clay
-
rich limestones


Analogous to Table 3.3 in 6th Edition

Streak


Color can be
misleading, as
the same
mineral can
have many
colors


Streak (color of
powder) is
more diagnostic


What is this mineral?


What color would it make rocks?

Answer: Dispersed hematite (Fe
2
O
3
) and/or limonite (FeO(OH))
color many rocks red, brown, and gold


we see the “streak” color

Why are these and many other rocks red or brown?

Key Physical Properties

(and help in identifying minerals)


Density (mass / volume)


can be clue to composition


Response to force


hardness (Mohs scale)
-

relates to bonding, structure


elastic
-
brittle
-
ductile behavior


cleavage

(regular, commonly shiny, many parallel planes)

and fracture
(irregular
and/or curved, not shiny, not parallel planes)

-

like crystal forms (habit),
these relate to symmetry and structure


Interaction with light


color (many origins)
-

relates to composition, impurities


refraction and dispersion
-

key in microscopic study


Electrical and magnetic properties
-

specific clues

An approach to mineral ID


Hard (≥5) vs. soft (≤5)



(~ steel hardness)




If a silicate:


cleavage and habit


intense color (black/green)


If a non
-
silicate:


luster, distinguishing
cleavage, hardness


Ionic / metallic vs. covalent


most silicates, except sheets ,are >5


except for a few oxides and diamond,
most non
-
silicates are soft


Distinguish main silicate groups


distinctive cleavage / fracture


Fe(Mg)
-
rich vs. Fe(Mg)
-
poor


Distinguish main groups


metallic luster / opaque (metals, most
common sulfides, oxides)


reacts with acid (carbonates)


soft or dense (sulfates)

Mineral Stability


A major clue to
Earth’s behavior


Depends on:
pressure (P),
temperature (T),
composition (X)


Examples


SiO
2
(P
-
T[
-
X])


C (P
-
T)


CaCO
3
(P
-
T
-
X)

What are these
crystals?


Phase equilibria (stability relationships) of SiO
2

C



Region of
diamond
formation


Thermal
decomposition
of CaCO
3

Pressure
-
temperature phase diagram


Geotherms



what are
they? Why are some
hot and some cold?


Diamond
-
graphite /
where can diamond
form?


Aragonite
--
orthorhombic


Calcite

rhombohedral

(hex)


Decomposition of
CaCO
3

with
temperature (practical
application?)


Thermal
decomposition
of CaCO
3

Region of
diamond
formation

Other kinds of mineral reactions


In addition to reactions that take place with
changes with pressure and temperature,
many mineral reactions take place as
minerals react with or form from fluids


Among the many important examples are
those related to weathering, ore formation,
and magmatism (later lectures)


One example, with calcite >>>

Chemical
reactions:

i.d. &

process

Reaction of calcite with acid:

CaCO
3
+ 2H
+

=

Ca
+2
+ H
2
O + CO
2

Summary of Minerals


Naturally occurring crystalline materials (other solids
can be geologically important)


Major groups are non
-
silicates and silicates


Properties follow from composition, chemical bonding,
and structure


Properties include: density, responses to stress
(hardness) and fields (magnetism, electricity), and
interaction with light (color, refraction, etc.)


Stability:


Depends on pressure, temperature, and composition


Can reveal much about the Earth

Relationships to Fluids and Rocks


Minerals make up rocks and, in addition:


Melts

are fluids of molten silicates that share structural
features with minerals (
magmas

are mixtures of melts
and crystals)



Cooling of melts and magmas makes
igneous rocks


Liquid water

is a fluid that reacts with minerals
(especially those with Na, K, Ca, Mg)



Fundamental to
weathering and sedimentary rocks
,
but also to
metamorphism and mineral (ore) deposits


Air

is a gaseous fluid important at the surface



The oxygen, water, and carbon dioxide in the
atmosphere are involved in many geologic processes

Remember: Major Rock
-
Forming Minerals

Their groups, properties, general compositions, & occurrence

** minerals are important
throughout

geology (i.e., 251)


Mafic minerals (Mg
-
Fe silicates)


Olivine


Pyroxenes


Amphiboles


Micas


Felsic minerals (
fel
dspars and
si
lica)


Plagioclase feldspar (Ca
-
Na)


Potassium feldspar (K)


Quartz


Clay minerals


Non
-
silicates


Carbonates (calcite [Ca], dolomite [Ca
-
Mg])


Sulfates ([Ca] gypsum & anhydrite)


Halite


Oxides (magnetite, hematite)


Minerals

Building Blocks of Rocks


Rock

Cycle

Next Time


Rock groups


Igneous rocks
: solidified from melt


Sedimentary rocks
: generated by surface fluids


Metamorphic rocks
: form by recrystallization


Rock cycle: Intra
-
conversion of rock types


Controlled by
plate tectonics and climate