UNIVERSITY OF SOUTH AFRICA (UNISA)
College of Science, Engineering and Technology
⋄
ENGINEERING GEOLOGY
Exam Question Answers
⋄
Module: Engineering Geology
Total Marks: 100
Submitted in partial fulfilment of the requirements for Engineering Geology
at the University of South Africa.
,UNISA | Engineering Geology Exam Answers
Question 1: Mineralogy and Rock Formation
1.1 Bowen’s Reaction Series – Discontinuous Series Diagram
The discontinuous series in Bowen’s reaction series describes a sequence in which each
mineral forms at a specific temperature range and does not continuously transform into the
next one; instead, it reacts with the remaining melt to form a completely different mineral
structure (Twiss and Moores, 2007).
K-Feldspar / Mus-
covite / Quartz Felsic / Granitic
KAlSi3 O8 / SiO2 T < 700–900°C
Comp. No. 3
Temperature Increasing
Biotite Mica
K(Mg,Fe)3 AlSi3 O10 (OH)2
Intermediate / Andesitic
Amphibole (Hornblende)
T ≈ 900–1100°C
Ca2 (Mg,Fe)5 Si8 O22 (OH)2 Comp. No. 2
Pyroxene (Augite)
(Ca,Mg,Fe)SiO3
Mafic / Basaltic
T > 1200°C
Comp. No. 1
Olivine
(Mg,Fe)2 SiO4
Rock Types: Olivine stage: Dunite (intrusive) / Komatiite (extrusive) – dark colour;
Pyroxene: Gabbro/Basalt; Amphibole: Diorite/Andesite – medium grey; Biotite-Quartz:
Granite/Rhyolite – light/pink
Figure 1: Bowen’s Discontinuous Reaction Series showing temperature range, magma compo-
sitions, and associated rock types. Olivine crystallises first (highest melting point).
Page 1 of 20
, UNISA | Engineering Geology Exam Answers
Key Distinction
Crystallisation order: Olivine has the highest melting point and crystallises first from a
mafic melt at temperatures above 1200°C. Quartz and potassium feldspar crystallise
last, at the lowest temperatures, from felsic melts. General rock colour progression
runs from dark (mafic, iron-magnesium rich) to light (felsic, silica-rich) as temperature
decreases (Nesse, 2012).
1.2 Seven Major Mineral Groups
The seven major mineral groups, each with a representative example and chemical formula,
are presented below (Klein and Dutrow, 2008):
Table 1: Seven Major Mineral Groups
Mineral Group Example Mineral Chemical Formula
Silicates Quartz SiO2
Carbonates Calcite CaCO3
Oxides Hematite Fe2 O3
Sulphides Pyrite FeS2
Sulphates Gypsum CaSO4 ·2H2 O
Halides Halite NaCl
Native Elements Gold Au
1.3 Geotechnical Site Investigation – Vaal River Bridge
a) Primary Rock-Forming Mineral in Peridotite and Dunite
Both peridotite and dunite are ultramafic rocks formed at the top of Bowen’s discontinuous
series. The primary rock-forming mineral abundant in these rocks is olivine (Mg,Fe)2 SiO4 .
Dunite is in fact composed almost entirely of olivine (greater than 90%), while peridotite con-
tains olivine as its dominant mineral alongside minor pyroxene (Wyllie, 1971).
b) Secondary (Altered) Mineral Expected After Alteration
When olivine undergoes hydrothermal alteration or regional metamorphism, as described in
the geological report for this site, the expected secondary mineral is serpentine (specifically
antigorite or lizardite, general formula Mg3 Si2 O5 (OH)4 ). This process, called serpentinisation,
occurs when olivine reacts with water at moderate temperatures (O’Hanley, 1996).
Page 2 of 20
College of Science, Engineering and Technology
⋄
ENGINEERING GEOLOGY
Exam Question Answers
⋄
Module: Engineering Geology
Total Marks: 100
Submitted in partial fulfilment of the requirements for Engineering Geology
at the University of South Africa.
,UNISA | Engineering Geology Exam Answers
Question 1: Mineralogy and Rock Formation
1.1 Bowen’s Reaction Series – Discontinuous Series Diagram
The discontinuous series in Bowen’s reaction series describes a sequence in which each
mineral forms at a specific temperature range and does not continuously transform into the
next one; instead, it reacts with the remaining melt to form a completely different mineral
structure (Twiss and Moores, 2007).
K-Feldspar / Mus-
covite / Quartz Felsic / Granitic
KAlSi3 O8 / SiO2 T < 700–900°C
Comp. No. 3
Temperature Increasing
Biotite Mica
K(Mg,Fe)3 AlSi3 O10 (OH)2
Intermediate / Andesitic
Amphibole (Hornblende)
T ≈ 900–1100°C
Ca2 (Mg,Fe)5 Si8 O22 (OH)2 Comp. No. 2
Pyroxene (Augite)
(Ca,Mg,Fe)SiO3
Mafic / Basaltic
T > 1200°C
Comp. No. 1
Olivine
(Mg,Fe)2 SiO4
Rock Types: Olivine stage: Dunite (intrusive) / Komatiite (extrusive) – dark colour;
Pyroxene: Gabbro/Basalt; Amphibole: Diorite/Andesite – medium grey; Biotite-Quartz:
Granite/Rhyolite – light/pink
Figure 1: Bowen’s Discontinuous Reaction Series showing temperature range, magma compo-
sitions, and associated rock types. Olivine crystallises first (highest melting point).
Page 1 of 20
, UNISA | Engineering Geology Exam Answers
Key Distinction
Crystallisation order: Olivine has the highest melting point and crystallises first from a
mafic melt at temperatures above 1200°C. Quartz and potassium feldspar crystallise
last, at the lowest temperatures, from felsic melts. General rock colour progression
runs from dark (mafic, iron-magnesium rich) to light (felsic, silica-rich) as temperature
decreases (Nesse, 2012).
1.2 Seven Major Mineral Groups
The seven major mineral groups, each with a representative example and chemical formula,
are presented below (Klein and Dutrow, 2008):
Table 1: Seven Major Mineral Groups
Mineral Group Example Mineral Chemical Formula
Silicates Quartz SiO2
Carbonates Calcite CaCO3
Oxides Hematite Fe2 O3
Sulphides Pyrite FeS2
Sulphates Gypsum CaSO4 ·2H2 O
Halides Halite NaCl
Native Elements Gold Au
1.3 Geotechnical Site Investigation – Vaal River Bridge
a) Primary Rock-Forming Mineral in Peridotite and Dunite
Both peridotite and dunite are ultramafic rocks formed at the top of Bowen’s discontinuous
series. The primary rock-forming mineral abundant in these rocks is olivine (Mg,Fe)2 SiO4 .
Dunite is in fact composed almost entirely of olivine (greater than 90%), while peridotite con-
tains olivine as its dominant mineral alongside minor pyroxene (Wyllie, 1971).
b) Secondary (Altered) Mineral Expected After Alteration
When olivine undergoes hydrothermal alteration or regional metamorphism, as described in
the geological report for this site, the expected secondary mineral is serpentine (specifically
antigorite or lizardite, general formula Mg3 Si2 O5 (OH)4 ). This process, called serpentinisation,
occurs when olivine reacts with water at moderate temperatures (O’Hanley, 1996).
Page 2 of 20
