AMPHIBOLES (double chains)AMPHIBOLES (double chains)

General formula: General formula: X X22YY55ZZ88OO2222(OH) (OH) 22 the box is a the box is a

site that may be vacant and takes Na and Ksite that may be vacant and takes Na and K XX= Ca, Mg, Fe, Na; = Ca, Mg, Fe, Na; YY= Mg, Fe, Al; = Mg, Fe, Al; ZZ= Si, Al= Si, Al Taking the first possibility for each site givesTaking the first possibility for each site gives CaCa22MgMg55SiSi88OO2222(OH)(OH)22 tremolite tremolite

CaCa22(Mg,Fe)(Mg,Fe)55SiSi88OO2222(OH)(OH)22 actinoliteactinolite

(Mg,Fe)(Mg,Fe)77SiSi88OO2222(OH)(OH)22 cummingtonitecummingtonite

NaNa22(Mg,Fe)(Mg,Fe)33AlAl22SiSi88OO2222(OH)(OH)2 2 glaucophaneglaucophane

NaNa22FeFe2+2+33FeFe3+3+

22SiSi88OO2222(OH) (OH) 2 2 riebeckiteriebeckite

Amphibole double chainsAmphibole double chains Tremolite Tremolite

CaCa22MgMg55SiSi88OO2222(OH)(OH)22

Chains of SiChains of Si44OO1111(each SiO(each SiO44

tetrahedron shares two tetrahedron shares two oxygens to form the chain)oxygens to form the chain)

OPTICSOPTICS Most amphiboles are Most amphiboles are

monoclinic and most monoclinic and most sections cut parallel to C sections cut parallel to C have inclined extinctionhave inclined extinction

Coloured amphiboles are Coloured amphiboles are trichroic (glaucophane & trichroic (glaucophane & riebeckite strongly so)riebeckite strongly so)

Well formed crystals Well formed crystals common in igneous rocks common in igneous rocks

In low grade metamorphic In low grade metamorphic rocks amphiboles tend to be rocks amphiboles tend to be fibrous but are more fibrous but are more polygonal at high gradespolygonal at high grades

Igneous Igneous hornblendehornblende

Hornblende with Hornblende with plagioclase plagioclase inclusions in gabbro inclusions in gabbro from Ben Bullenfrom Ben Bullen

Similar hornblende Similar hornblende with plagioclase with plagioclase inclusions in mafic inclusions in mafic igneous rockigneous rock

HornblendeHornblende

Brown hornblende Brown hornblende from Zone 3 Broken from Zone 3 Broken HillHill

Green hornblende Green hornblende partially replacing partially replacing pyroxene in meta-pyroxene in meta-andesiteandesite

Where do they occur?Where do they occur? Tremolite (colourless and fibrous) in very Mg-rich Tremolite (colourless and fibrous) in very Mg-rich

metamorphic rocks (meta-dolomites, meta-metamorphic rocks (meta-dolomites, meta-peridotites)peridotites)

Actinolite (green & fibrous) in low grade meta-basaltsActinolite (green & fibrous) in low grade meta-basalts Glaucophane (lavender) in high pressure meta-Glaucophane (lavender) in high pressure meta-

basaltsbasalts Riebeckite (pleochroic dark blue – yellow - green) in Riebeckite (pleochroic dark blue – yellow - green) in

trachytes, quartz syenites and alkali granites and in trachytes, quartz syenites and alkali granites and in meta-BIFs in part as the fibrous variety crocidolitemeta-BIFs in part as the fibrous variety crocidolite

Cummingtonite (c’less or pale green, multiple twins) Cummingtonite (c’less or pale green, multiple twins) occurs in some igneous rocks (rhyolites) and in occurs in some igneous rocks (rhyolites) and in some amphibolitessome amphibolites

ACTINOLITE/HORNBLENDEACTINOLITE/HORNBLENDE Hornblende is like actinolite with added Al, Na & KHornblende is like actinolite with added Al, Na & K CaCa2(2(Mg,Fe)Mg,Fe)55SiSi88OO2222(OH)(OH)2 2 actinoliteactinolite Hornblende has Na+K in the vacant site, balanced by Hornblende has Na+K in the vacant site, balanced by

Al replacing Si and Al+Al substituting for Mg+Si. Some Al replacing Si and Al+Al substituting for Mg+Si. Some Ti and FeTi and Fe3+3+ also in the Y also in the Y site site

NaCaNaCa22(Mg,Fe)(Mg,Fe)44Al SiAl Si66AlAl22OO2222(OH)(OH)22hornblendehornblende Most diorites,some gabbros and many I-type Most diorites,some gabbros and many I-type

granodiorites have hornblende. It also occurs as a granodiorites have hornblende. It also occurs as a phenocryst in some andesites and dacites. Some phenocryst in some andesites and dacites. Some pyroxene is replaced by actinolite as intrusions coolpyroxene is replaced by actinolite as intrusions cool

Meta-basalts have actinolite at low grade & hornblende Meta-basalts have actinolite at low grade & hornblende at intermediate to high grade. Granulites have enstatite at intermediate to high grade. Granulites have enstatite that forms as hornblende begins to break downthat forms as hornblende begins to break down

Amphibolites and Amphibolites and granulites at Broken granulites at Broken

HillHill Zone 1 Blue-green Zone 1 Blue-green

hornblendehornblende Zone 2 Green/brown Zone 2 Green/brown

hornblendehornblende Zone 3 Brown Zone 3 Brown

hornblende and cpx + hornblende and cpx + opxopx

Changes involve Changes involve increases in Na, Al and increases in Na, Al and Ti and a decrease in Ti and a decrease in ferric iron.ferric iron.

MICASMICAS XX22YY4-64-6ZZ88OO2020(OH,F)(OH,F)44

(If Y=4:dioctohedral; 6:trioctohedral)(If Y=4:dioctohedral; 6:trioctohedral)

X=K, Na, Ca; Y=Al, Fe, Mg ,Li; Z=Si, AlX=K, Na, Ca; Y=Al, Fe, Mg ,Li; Z=Si, Al

Common micas are: Common micas are:

Muscovite (Na - paragonite) Muscovite (Na - paragonite) KK22AlAl44SiSi66AlAl22OO2020(OH,F)(OH,F)44

Phlogopite (end-member) Phlogopite (end-member) KK22MgMg66SiSi66AlAl22OO2020(OH,F)(OH,F)44

Biotite Biotite KK22(Mg,Fe,Al)(Mg,Fe,Al)66SiSi55AlAl33OO2020(OH,F)(OH,F)44

Rare micas include:Rare micas include:

Margarite CaMargarite Ca22AlAl44SiSi44AlAl44OO2020(OH,F)(OH,F)44

Lepidolite KLepidolite K22(Li,Al)(Li,Al)5-65-6SiSi6-56-5AlAl2-32-3OO2020(OH,F)(OH,F)44

Optical propertiesOptical properties Perfect cleavagePerfect cleavage Parallel extinctionParallel extinction Birefingence .04-.08Birefingence .04-.08 Mottled extinction (soft)Mottled extinction (soft)

Mica structure:Mica structure:Sheets of (Si,Al)OSheets of (Si,Al)O44 tetrahedra tetrahedra

2 sheets with opposite facing 2 sheets with opposite facing are connected by octohedrally are connected by octohedrally coordinated atoms to form a coordinated atoms to form a sandwich. The sandwiches are sandwich. The sandwiches are weakly bonded with K atoms.weakly bonded with K atoms.

Monoclinic but pseudo-hexagonal

Biotite compositionsBiotite compositions

MgO

Micas in igneous rocksMicas in igneous rocks Biotite is common in K-rich plutonic rocks Biotite is common in K-rich plutonic rocks

from gabbro to granite. Biotite coexists with from gabbro to granite. Biotite coexists with hornblende in many I-type granites and with hornblende in many I-type granites and with cordierite in many S-type granites. cordierite in many S-type granites. Phlogopite is a minor mineral in parts of the Phlogopite is a minor mineral in parts of the mantle. Biotite also occurs in many potassic mantle. Biotite also occurs in many potassic andesites, dacites and rhyolitesandesites, dacites and rhyolites

Muscovite occurs in a small number of Muscovite occurs in a small number of granites as a primary mineral but occurs in granites as a primary mineral but occurs in many as a minor alteration minerals formed many as a minor alteration minerals formed as the pluton cools. A couple of very rare as the pluton cools. A couple of very rare rhyolites have muscovite phenocrystsrhyolites have muscovite phenocrysts

Micas in metamorphic rocks.Micas in metamorphic rocks.

Meta-mudstones and meta-granites contain biotite Meta-mudstones and meta-granites contain biotite at all except the lowest and highest grades. At low at all except the lowest and highest grades. At low grade muscovite and chlorite are present and at the grade muscovite and chlorite are present and at the highest enstatite and K-feldsparhighest enstatite and K-feldspar

Muscovite is present in many meta-mudstones Muscovite is present in many meta-mudstones except at the highest grades when it is replaced by except at the highest grades when it is replaced by andalusite/sillimanite plus K-feldsparandalusite/sillimanite plus K-feldspar

K-rich meta-basalts/andesites will also contain K-rich meta-basalts/andesites will also contain biotitebiotite

Metalimestones with K and Al can have phlogopiteMetalimestones with K and Al can have phlogopite

Micas in rocks continued…Micas in rocks continued… Micas are very common in pegmatites which were Micas are very common in pegmatites which were

mined for large books of muscovite that were used mined for large books of muscovite that were used as an electrical and thermal insulator. as an electrical and thermal insulator. (manufactured products have replaced mica in this (manufactured products have replaced mica in this role)role)

The clay mineral illite is very similar to muscovite The clay mineral illite is very similar to muscovite in composition (a little more Si and less K). On in composition (a little more Si and less K). On deep burial this changes to muscovitedeep burial this changes to muscovite

Detrital muscovite is not uncommon in sandstones Detrital muscovite is not uncommon in sandstones but biotite is common only in pyroclastic but biotite is common only in pyroclastic sedimentary rocks and their redeposited sedimentary rocks and their redeposited equivalentsequivalents

Biotite is easily Biotite is easily

deformeddeformed and and perhaps perhaps

surprisingly, so is surprisingly, so is quartz . S-type quartz . S-type

granites (lots of granites (lots of

quartz & mica)quartz & mica) are are significantly more significantly more

likely to show likely to show deformation than I-deformation than I-

type granitestype granites