Download mineralogy - West Virginia University

Dr. Helen Lang
Dept. of Geology & Geography
West Virginia University
FALL 2015
GEOLOGY 284:
MINERALOGY
Remember Bowen’s Reaction Series
hi T
olivine
Ca plagioclase
orthopyroxene
temperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
Mafic Minerals in Igneous Rocks
• Olivine
(isolated tetrahedral sil.)
• Pyroxenes
(single chain silicates)
– Enstatite
(orthopyroxene)
– Augite
(clinopyroxene)
• Amphiboles
(double-chain silicates)
– Hornblende
• Micas
(sheet silicates)
– Biotite
– Muscovite (OK,
not mafic)
We’ll start with Olivine
• Olivine occurs in high temperature,
mafic (hi Mg, low Si) igneous rocks
• Commonly the first mineral to
crystallize from a basaltic magma
• Olivine also makes up most of the
Earth’s Mantle (metamorphic rock)
We’ll start with Olivine
• Formula: (Mg, Fe)2SiO4
• An Isolated Tetrahedral Silicate
• complete solid solution between two
end-members:
– Mg2SiO4 Forsterite (Fo) and
– Fe2SiO4
Fayalite (Fa)
Olivine Structure
Relatively simple
Isolated SiO44tetrahedra
Divalent cations
(Mg2+, Fe2+) share
tetrahedral oxygens,
balance charge and
link tetrahedra
Olivine Structure Movie
http://socrates.berkeley.edu/~eps2/wisc/geo360/olivine.mov
Olivine Properties
•
•
•
•
•
Vitreous luster, glassy
Olive-green color
No good cleavages
H=6.5 G=3.2
Optical: high relief, moderately
high birefringence
Olivine Xenoliths from the Mantle
Clear, Gem-quality Olivine (Peridot)
Olivine in thin section
Bowen’s Reaction Series
hi T
olivine
Ca plagioclase
orthopyroxene
temperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
Pyroxenes
• Common in mafic and intermediate igneous
rocks
• Commonly crystallize from basaltic magma
after olivine (Bowen’s reaction series)
• Pyroxenes are Single Chain Silicates
Pyroxenes
• General Formula: (Ca2+,Mg2+,Fe2+)2Si2O6
• Two kinds:
– Orthopyroxenes (Mg,Fe)2Si2O6 have
perpendicular axes
• Enstatite
– Clinopyroxenes Ca(Mg,Fe)Si2O6 have
inclined axes
• Augite and Diopside
Pyroxenes are Single
Chain Silicates
Bigger
cations (Ca2+
if present) –
in octahedral
coordination
Smaller cations (Mg2+ and
Fe2+) – in octahedral coordination
Strips or “I-beams”
in the pyroxene
structure are tightly
held together
Cleavages (at ~90o) go
between “I-beams” where
bonds are weaker
Movie: Pyroxene I-beam
http://socrates.berkeley.edu/~eps2/wisc/geo360/diopibeam.mov
Orthopyroxene movie
http://socrates.berkeley.edu/~eps2/wisc/geo360/opx.mov
(note no net tilt)
Diopside movie
http://socrates.berkeley.edu/~eps2/wisc/geo360/diopside.mov
(note tilt)
HRTEM Image of Pyroxene
Pyroxene Compositions and the
Pyroxene Quadrilateral
Diopside
CaMgSi2O6
Enstatite
Mg2Si2O6
clinopyroxenes
Augite
Orthopyroxene
orthopyroxenes
Hedenbergite
CaFeSi2O6
Ferrosilite
Fe2Si2O6
Pyroxene Properties, General
• Stubby Prisms
• Vitreous luster, translucent
• Two perfect prismatic cleavages at
approximately 90o to eachother
Enstatite (Mg orthopyroxene)
Pyroxene
Relief, Color
and Cleavage in
PPL
Orthopyroxene Properties
• Enstatite (orthopyroxene)
– Color gray, bronze, brown
– Colorless to tan in thin section
– High relief
– Low birefringence
– Parallel extinction
Orthopyroxene has low birefringence
Clinopyroxene has higher (second order) birefringence
photo – Smith College, John Brady (?)
Clinopyroxene Properties
• Diopside
– Color light green
– Colorless in thin section
– Moderate birefringence, inclined extinction
• Augite
– Dark green to black
– Tan, light brown or green in thin section
– Moderate birefringence, inclined extinction
Augite
Augite (moderate birefringence, twinning
and cleavage)
Bowen’s Reaction Series
hi T
olivine
Ca plagioclase
orthopyroxene
temperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
Hornblende (Hb)
• is the main amphibole in igneous rocks
• crystallizes after olivine and pyroxenes
from mafic magmas, and at lower
temperature
• is most common in intermediate
composition igneous rocks
Amphiboles
• General Formula:
– (Na,K)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
– large
medium
small
tetrahedral cations
• Simple Amphibole (Tremolite):
– _Ca2Mg5Si8O22(OH)2
• Double Chain Silicates (Inosilicates)
• note (Si8O22)12- in formula
• they’re hydrous (OH-bearing) Minerals
Amphiboles are DoubleChain Silicates
Medium
cations (Ca)
Small cations
(Fe,Mg)
Large cations
(Na+,K+) or empty
Amphibole Structure
View Tremolite Movie
http://socrates.berkeley.edu/~eps2/wisc/geo360/tremolite.mov
Igneous Amphiboles
Igneous Amphiboles
Big Hornblende Crystals called Phenocrysts
Igneous Amphiboles
Tightly bonded
“I-beams”- neg.
charged chains
bonded by strip
of many positive
cations
Cleavages (at ~60o and 120o) go
between “I-beams” where bonds
are weaker
Hornblende Properties
•
•
•
•
Black or dark green
Vitreous luster, translucent
H=5-6
G=3.0-3.5
two perfect prismatic cleavages at about 60o and
120o to eachother, sometimes “splintery” cleavage
• commonly in prisms with flattened hexagonal
cross-sections
Hornblende’s Optical Properties
• Various shades of brown, red-brown, green,
blue-green or tan in thin section
• Moderate to strong pleochroism is typical
• Moderate birefringence, commonly
obscured by its color
• Two good cleavages at ~60o and 120o to
eachother
• You have to be looking down the prism to
see 2 crossing sets of cleavages, in many
views you will see only one set of cleavages
Hornblende shape, cleavage & birefringence
IG7-Crater Lake cumulate
Photo Billie Jo Fortney
Hornblende color and cleavage
Kaersutite (Na,Ti-rich igneous amphibole)
with strong red-brown pleochroism
Digression to talk about Asbestos
The term asbestiform describes
a mineral habit characterized by
long, thin, strong, flexible fibers
equivalent to hairs or whiskers
Uses of asbestos?
Asbestos Minerals
amphiboles
sheet silicate
Amphiboles are chain silicates
and are inherently and predictably
elongate or fibrous
It’s not surprising that most
asbestos minerals are amphiboles
Crocidolite (blue) Asbestos
Other Fibrous Amphiboles
Actinolite
Chrysotile “white” asbestos
• Is a form of serpentine
Mg3Si2O5(OH)4, a sheet silicate
• Sheet silicates are usually flaky,
formed of thin flexible sheets, or leaflike
• How can a sheet silicate be fibrous?
Serpentine is a tetrahedraloctahedral (t-o) Sheet Silicate
Mg3Si2O5 (OH)4
Si2O5 tetrahedral (t) sheet
Mg surr. by O and OH
forms octahedral (o) sheet
The trouble with serpentine: the o-sheet is bigger
than the t-sheet
In normal serpentine, Antigorite,
the sheets are wavy or corrugated
In asbestos serpentine, Chrysotile, the
sheets roll to accommodate the misfit
HRTEM pictures down fibers
Chrysotile (white)
Asbestos
Most of the asbestos used worldwide is Chrysotile (>95% in U.S.)
Chrysotile is much less dangerous than
amphibole asbestos, in fact, there is little
evidence that chrysotile poses any health
hazard to those who are exposed casually
(e.g., in buildings)!!!!
Back to Igneous Amphiboles
Tightly bonded
“I-beams”- neg.
charged chains
bonded by strip
of many positive
cations
Cleavages (at ~60o and 120o) go
between “I-beams” where bonds
are weaker
Pyroxene Compositions and the
Pyroxene Quadrilateral
Diopside
CaMgSi2O6
Enstatite
Mg2Si2O6
clinopyroxene (Cpx)
Augite
Orthopyroxene
orthopyroxene (Opx)
Hedenbergite
CaFeSi2O6
Ferrosilite
Fe2Si2O6
Asbestos Minerals
amphiboles
sheet silicate