GEOS 254 Lecture 2: GEOS 254 Lecture 2: FELDSPARSFELDSPARS

Feldspars are the most abundant minerals in the Feldspars are the most abundant minerals in the crust (very minor in the mantle).crust (very minor in the mantle).

Three main end members:Three main end members: Albite Albite AbAb NaAlSiNaAlSi33OO88

AnorthiteAnorthite AnAn CaAlCaAl22SiSi22OO88

OrthoclaseOrthoclase OrOr KAlSiKAlSi33OO88

The plagioclase feldspars are solid solutions of The plagioclase feldspars are solid solutions of albitealbite and and anorthiteanorthite. Na. Na++ Si Si4+4+ is replaced by Ca is replaced by Ca2+2+ Al Al3+3+

The alkali feldspars are solid solutions of The alkali feldspars are solid solutions of albitealbite and and orthoclaseorthoclase. Na is replaced by K. Na is replaced by K

There is virtually no solid solution between An and Or.There is virtually no solid solution between An and Or.

Silicate minerals have silicon tetrahedra that Silicate minerals have silicon tetrahedra that are variably interconnected. In olivine they are are variably interconnected. In olivine they are isolated (none of the four oxygens are shared isolated (none of the four oxygens are shared

with the adjacent tetrahedra). In pyroxenes with the adjacent tetrahedra). In pyroxenes one oxygen is shared (chains). In amphiboles one oxygen is shared (chains). In amphiboles two are shared (double chain). In micas three two are shared (double chain). In micas three are shared (sheets) and in quartz & feldspar are shared (sheets) and in quartz & feldspar

all are shared (3D framework)all are shared (3D framework)

Plagioclase FeldsparsPlagioclase Feldspars Are “triclinic” (no axes at 90Are “triclinic” (no axes at 90oo)) Have multiple twinning (albite) & can also have Have multiple twinning (albite) & can also have

pericline twinning (multiple) forming a coarse grid pericline twinning (multiple) forming a coarse grid twinning.twinning.

In igneous rocks form rectangular crystals (more In igneous rocks form rectangular crystals (more elongate in many volcanic rocks) & commonly elongate in many volcanic rocks) & commonly have intricate zoning (oscillatory) and many twins. have intricate zoning (oscillatory) and many twins.

In metamorphic rocks form polygonal grains with In metamorphic rocks form polygonal grains with little zoning and sparse growth twinning.little zoning and sparse growth twinning.

Deformation twins can occur in both rock types. Deformation twins can occur in both rock types.

K-feldsparK-feldspar K-feldspar has three different structural K-feldspar has three different structural

states: K-feldspar formed at high states: K-feldspar formed at high temperature and cooled quickly is temperature and cooled quickly is sanidinesanidine. . If formed at high temperature and cooled If formed at high temperature and cooled slowly (plutonic) or formed at moderate slowly (plutonic) or formed at moderate temperatures are temperatures are orthoclase.orthoclase. If formed at If formed at low temperature are low temperature are microclinemicrocline. Sanidine . Sanidine and orthoclase are monoclinic and have no and orthoclase are monoclinic and have no multiple twinning. Microcline is triclinic and multiple twinning. Microcline is triclinic and has fine grid or tartan twinning.has fine grid or tartan twinning.

Feldspars are rectangular in Feldspars are rectangular in igneous rocks and polygonal in igneous rocks and polygonal in

mmmm

Complex zoning common in igneous not in mm.Complex zoning common in igneous not in mm. Growth twins common in igneous, sparse in Growth twins common in igneous, sparse in

mm.mm.

Feldspar in sedimentary Feldspar in sedimentary rocksrocks

A: Rectangular plagioclase (volcanic ash) forming A: Rectangular plagioclase (volcanic ash) forming nucleus for oolite in limestone at Keepit.nucleus for oolite in limestone at Keepit.

B: Microcline and perthite grains in sandstone. Pink B: Microcline and perthite grains in sandstone. Pink colour on right is a stain that shows the carbonate colour on right is a stain that shows the carbonate cement is calcite (not dolomite).cement is calcite (not dolomite).

3 main feldspar end-3 main feldspar end-members (Ab, An, Or)members (Ab, An, Or)

2 series (red 2 series (red arrows), arrows), plagioclase and plagioclase and alkali feldspar.alkali feldspar.

Plagioclase, Plagioclase, anorthoclase & anorthoclase & microcline are microcline are triclinic (blue triclinic (blue arrows) and have arrows) and have multiple twinning.multiple twinning.

Orthoclase & Orthoclase & sanidine are sanidine are monoclinic (green monoclinic (green arrows) and only arrows) and only have simple have simple twins.twins.

Feldspars in Feldspars in igneous igneous rocks.rocks.

Basalt An> 50; Basalt An> 50; andesite An 30 andesite An 30 –50; dacite & –50; dacite & rhyolite An< 30.rhyolite An< 30.

High High temperature temperature volcanic alkali-volcanic alkali-feldspars from feldspars from Ab to Or Ab to Or (sanidine or (sanidine or anorthoclase).anorthoclase).

Plutonic: K-rich Plutonic: K-rich or Na-rich.or Na-rich.

Plagioclase in meta basalt.Plagioclase in meta basalt. Basalts & high-Basalts & high-

grade meta-basalts grade meta-basalts have An>50have An>50

Low grade Low grade metabasalts have metabasalts have An<10 (albite). The An<10 (albite). The Ca & Al is in epidote Ca & Al is in epidote and hornblende.and hornblende.

Meta-limestones Meta-limestones (marl with clay (marl with clay minerals for Al) minerals for Al) have pure anorthitehave pure anorthite

K-feldspar in K-feldspar in metamorphic metamorphic

rocksrocks

Sanidine only in Sanidine only in rare ultra high rare ultra high temp contact temp contact mm.mm.

Orthoclase in Orthoclase in moderate and moderate and high temp mmhigh temp mm

Microcline in low Microcline in low temp. mm rocks.temp. mm rocks.

Deformation Deformation promotes change promotes change from meta-stable from meta-stable orthoclase to orthoclase to microcline. microcline.

Order/disorder in Order/disorder in K-feldspars.K-feldspars.

Sanidine and orthoclase Sanidine and orthoclase (monoclinic) have 2 different (monoclinic) have 2 different sites for Si and Al. sites for Si and Al.

Sanidine has no preference with Sanidine has no preference with 25% Al & 75% Si in both. 25% Al & 75% Si in both.

In orthoclase the red site takes In orthoclase the red site takes 30% Al and the blue only 20%30% Al and the blue only 20%

Microcline (triclinic) has four sites. Microcline (triclinic) has four sites. Red dots have 56% Al, blue Red dots have 56% Al, blue dots 7%, red + 25%, blue + 8%.dots 7%, red + 25%, blue + 8%.

How the How the change from change from monoclinic to monoclinic to

triclinic is triclinic is detected by X-detected by X-ray diffractionray diffraction

X-rays are X-rays are “reflected” by “reflected” by each lattice each lattice plane when the plane when the angle of angle of incidence is incidence is correct.correct.

nnλλ = 2D Sin = 2D SinΘΘ

MONOCLINIC

TRICLINIC

Two lattice planes have the same spacing

Two lattice planes have different spacing

Orthoclase/microclineOrthoclase/microcline Deformation of a Deformation of a

granite near the granite near the fault has allowed fault has allowed the meta-stable the meta-stable orthoclase to orthoclase to transform to transform to microcline.microcline.

What is the cause What is the cause of the microcline of the microcline in slide 14 at in slide 14 at Bathurst? Is there Bathurst? Is there a hidden fault?a hidden fault?

3 metamorphic 3 metamorphic zones at Broken zones at Broken

HillHill Grade increases to the Grade increases to the

SE.SE. Granitic gneisses in Granitic gneisses in

Zone 1 (lowest grade) Zone 1 (lowest grade) contain microcline as contain microcline as the K-feldspar.the K-feldspar.

Granitic gneisses in Granitic gneisses in Zone 2 (intermediate) Zone 2 (intermediate) and Zone 3 (high grade) and Zone 3 (high grade) contain orthoclase.contain orthoclase.

Orthoclase/microclineOrthoclase/microcline Contact Contact

metamorphism of one metamorphism of one granite by another granite by another later granite.later granite.

In the outer aureole In the outer aureole microcline forms.microcline forms.

In the inner it is In the inner it is disordered to disordered to orthoclase.orthoclase.

Allows the relative Allows the relative age of granites that age of granites that are close together to are close together to be determined. be determined.