Geol 2312 – Petrology Quiz 2 Name_______Key________ Spring

Geol 2312 – Petrology
Spring 2016
Quiz 2
Chapters 5-7
Name_______Key________
Score _____/15
1) Identify the word that fits the definitions below (1.5 pts)
Metastable
Energy
System
Surroundings
Unstable
a. A portion of the universe (PoU) that we can physically or mentally isolate = system
b. PoUs outside that isolated entity = surroundings
c. Changes to any PoU involves the exchange of energy
d. PoUs that change spontaneously are said to be unstable
e. PoUs that require the input of additional energy to change are said to be metastable
f. PoUs are send to be stable when they are at their highest or lowest energy state (circle one)
2) The Gibbs Free Energy of Formation of a mineral phase at its reference state is measured at
(circle one for each question): (1 pt)
What temperature? 0°C (273°K) 25°C (298°K) 100°C (373°K)
What pressure?
1 MPa (10 atm) 0.1 MPa (1 atm) 10MPa (100 atm)
What abundance?
1 cm3 100 grams 1 mole
From what reactants for Quartz? Si + O
Al + Si + O2
Si + O2
3) The change in Gibbs Free Energy of a phase with fixed composition under different
temperatures and pressures is defined by: dG = VdP - SdT. With this in mind and with the
figures showing G-P-T conditions, answer the following questions. (2 pts)
a) Under constant pressure, a change in G created by a change
in temperature is given by a negative value of S – Entropy. In
your own words, what does this parameter represents? Degree
of randomness
b) In the diagrams to the left, does point A, where the solid
phase is stable, or point B, where the liquid phase is stable ,
have a higher entropy? Liquid
c) What do the negative slopes of dG/dT for both solid and
liquid curves indicate about the affect of increasing temperate
on the G of all phases. Incr. T lowers Gibbs Free Energy
d) What is the significance of temperature Teq? Temperature of
melting, crystallization, or fusion
e) Under constant temperature, a change in G created by a
change in pressure is defined by what characteristic of the
mineral phase Volume Mass Enthalpy or Heat Capacity
4) Circle the true statement about the crystallization of a magma, as observed with the
Makaopuhi lava lake in Hawaii. (1 pt)
a. Temperatures must drop over a couple hundred degrees C for a magma to go from
100% liquid to 100% solid
b. During crystallization, the proportion of minerals crystallizing stays constant
c. Solid solution minerals change composition during crystallization
d. The composition of the magma stays constant.
5) Match the definition with the term (2 pt)
a. Closed system 5
b. Phase 4
c. Component 1
d. Intensive variables 2
e. Open system 3
1) a chemical constituent of a phase or system
2) a parameter affecting a system that is mass independent
(like density, molar volume, temperature, pressure,….
3) a portion of the universe that exchanges energy and
material with its surroundings
4) a mechanically separable part of a system
5) a portion of the universe that exchanges only energy
(heat) with its surroundings
6) In the phase diagram for the one-component system H2O in the figure, determine what phase (or
phases) are stable at the T and P indicated (2 pts):
a. T- 200°C, P-0.1 MPa (1 atm) - Vapor
b. T- 50°C, P-0.001 Mpa - Vapor
c. T- 0°C, P-0.1 MPa (1 atm) – Ice and Liquid
d. T- 0.01°C, P-0.0006 Mpa – Ice, Liquid and Vapor
7) Assuming equilibrium crystallization of liquid/magma a in the 2-component solidsolution phase diagram for plagioclase below (2 pts).
a. What is the An content of the original magma/liquid
a? An60
b. What is the An content of the first plagioclase to
crystallize? ~An85 represented by point? c
c. What is the An content of the last liquid to crystallize?
~An20 , represented by what point? g
d. What is the An content of plagioclase in the final rock
(anorthosite)? An60, and a what temperature?_ ~1345
8) Assuming equilibrium melting conditions in the Anorthite-Diopside two component
eutectic system, answer the following questions (1.5pt)
a. At what temperature will a gabbroic rock with
70% An (plagioclase) and 30% Di (pyroxene) start
to melt? ~1274
b. What will be the %An component of the melt
generated at that temperature? ~42%An
c. At what temperature will the rock be completely
melted? ~1450
9) In the ternary system An-Di-Fo (plagioclase-pyroxene-olivine), do the following (2 pt)
Y
Z
X
a. Plot the composition of a magma that has only
olivine on the liquidus, label it with an X
Anywhere in the Fo+Liq field
b. Plot the composition of a magma that would
have olivine and plagioclase on the liquidus, label
it with a Y.
Anywhere on the An-Fo cotectic line
c. Plot the composition of magma crystallizing Ol,
Pl and Cpx, label it with a Z. At eutectic point
d. What temperature would the magma have to be
to crystallize Ol+Pl+Cpx? 1270