GEOS254Lec4

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Transcript GEOS254Lec4

AMPHIBOLES (double chains)
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General formula: •X2Y5Z8O22(OH) 2 the box is a
site that may be vacant and takes Na and K
X= Ca, Mg, Fe, Na; Y= Mg, Fe, Al; Z= Si, Al
Taking the first possibility for each site gives
Ca2Mg5Si8O22(OH)2
tremolite
Ca2(Mg,Fe)5Si8O22(OH)2
actinolite
(Mg,Fe)7Si8O22(OH)2
cummingtonite
Na2(Mg,Fe)3Al2Si8O22(OH)2
glaucophane
Na2Fe2+3Fe3+2Si8O22(OH) 2
riebeckite
Amphibole double chains
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Tremolite
Ca2Mg5Si8O22(OH)2
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Chains of Si4O11(each SiO4
tetrahedron shares two
oxygens to form the chain)
OPTICS
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Most amphiboles are
monoclinic and most
sections cut parallel to C
have inclined extinction
 Coloured amphiboles are
trichroic (glaucophane &
riebeckite strongly so)
 Well formed crystals
common in igneous rocks
 In low grade metamorphic
rocks amphiboles tend to be
fibrous but are more
polygonal at high grades
Igneous
hornblende
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Hornblende with
plagioclase
inclusions in gabbro
from Ben Bullen
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Similar hornblende
with plagioclase
inclusions in mafic
igneous rock
Hornblende
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Brown hornblende
from Zone 3 Broken
Hill
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Green hornblende
partially replacing
pyroxene in metaandesite
Where do they occur?
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Tremolite (colourless and fibrous) in very Mg-rich
metamorphic rocks (meta-dolomites, metaperidotites)
Actinolite (green & fibrous) in low grade meta-basalts
Glaucophane (lavender) in high pressure metabasalts
Riebeckite (pleochroic dark blue – yellow - green) in
trachytes, quartz syenites and alkali granites and in
meta-BIFs in part as the fibrous variety crocidolite
Cummingtonite (c’less or pale green, multiple twins)
occurs in some igneous rocks (rhyolites) and in
some amphibolites
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ACTINOLITE/HORNBLENDE
Hornblende is like actinolite with added Al, Na & K
 Ca2(Mg,Fe)5Si8O22(OH)2 actinolite
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Hornblende has Na+K in the vacant site, balanced by
Al replacing Si and Al+Al substituting for Mg+Si. Some
Ti and Fe3+ also in the Y site
 NaCa2(Mg,Fe)4Al
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Si6Al2O22(OH)2hornblende
Most diorites,some gabbros and many I-type
granodiorites have hornblende. It also occurs as a
phenocryst in some andesites and dacites. Some
pyroxene is replaced by actinolite as intrusions cool
 Meta-basalts have actinolite at low grade &
hornblende at intermediate to high grade. Granulites
have enstatite that forms as hornblende begins to
break down
Amphibolites and
granulites at Broken
Hill
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Zone 1 Blue-green
hornblende
 Zone 2 Green/brown
hornblende
 Zone 3 Brown
hornblende and cpx +
opx
 Changes involve
increases in Na, Al and
Ti and a decrease in
ferric iron.
X2Y4-6Z8O20(OH,F)4
MICAS
(If Y=4:dioctohedral; 6:trioctohedral)
X=K, Na, Ca; Y=Al, Fe, Mg ,Li; Z=Si, Al
Common micas are:
Muscovite (Na - paragonite) K2Al4Si6Al2O20(OH,F)4
Phlogopite (end-member)
K2Mg6Si6Al2O20(OH,F)4
Biotite
K2(Mg,Fe,Al)6Si5Al3O20(OH,F)4
Rare micas include:
Margarite Ca2Al4Si4Al4O20(OH,F)4
Lepidolite K2(Li,Al)5-6Si6-5Al2-3O20(OH,F)4
Optical properties
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Perfect cleavage
Parallel extinction
Birefingence .04-.08
Mottled extinction (soft)
Mica structure:
Sheets of (Si,Al)O4 tetrahedra
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2 sheets with opposite facing
are connected by octohedrally
coordinated atoms to form a
sandwich. The sandwiches are
weakly bonded with K atoms.
Monoclinic but pseudo-hexagonal
Biotite compositions
MgO
Micas in igneous rocks
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is common in K-rich plutonic rocks
from gabbro to granite. Biotite coexists with
hornblende in many I-type granites and with
cordierite in many S-type granites.
Phlogopite is a minor mineral in parts of the
mantle. Biotite also occurs in many potassic
andesites, dacites and rhyolites
 Muscovite occurs in a small number of
granites as a primary mineral but occurs in
many as a minor alteration minerals formed
as the pluton cools. A couple of very rare
rhyolites have muscovite phenocrysts
Micas in metamorphic rocks.
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Meta-mudstones and meta-granites contain biotite
at all except the lowest and highest grades. At low
grade muscovite and chlorite are present and at the
highest enstatite and K-feldspar
 Muscovite is present in many meta-mudstones
except at the highest grades when it is replaced by
andalusite/sillimanite plus K-feldspar
 K-rich meta-basalts/andesites will also contain
biotite
 Metalimestones with K and Al can have phlogopite
Micas in rocks continued…
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Micas are very common in pegmatites which were
mined for large books of muscovite that were used
as an electrical and thermal insulator.
(manufactured products have replaced mica in this
role)
 The clay mineral illite is very similar to muscovite
in composition (a little more Si and less K). On
deep burial this changes to muscovite
 Detrital muscovite is not uncommon in sandstones
but biotite is common only in pyroclastic
sedimentary rocks and their redeposited
equivalents
Biotite is easily
deformed and
perhaps
surprisingly, so is
quartz . S-type
granites (lots of
quartz & mica) are
significantly more
likely to show
deformation than Itype granites