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Feldspar Group. Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase (K) Comprised of 2 series: Albite-anorthite (Na-Ca) Albite-orthoclase (Na-K). Tectosilicates.

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Feldspar Group

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Feldspar group

Feldspar Group

  • Most abundant mineral in the crust  6 of 7 most common elements

  • Defined through 3 end-members 

    • Albite (Na), Anorthite (Ca), Orthoclase (K)

  • Comprised of 2 series:

    • Albite-anorthite (Na-Ca)

    • Albite-orthoclase (Na-K)


Tectosilicates

Tectosilicates

Substitute Al3+ for Si4+ allows Na+ or K+ to be added

Albite-Orthoclase

Feldspars

Substitute two Al3+ for Si4+ allows Ca2+ to be added

Albite-Anorthite

Albite: NaAlSi3O8


Feldspar group albite anorthite series

Feldspar Group – Albite-Anorthite series

  • Complete solid solution Plagioclase Feldspars

  • 6 minerals

    • Albite (Na)

    • Oligoclase

    • Andesine

    • Labradorite

    • Bytownite

    • Anorthite (Ca)

  • Albite-Anorthite double duty

    • End-members (Pure Na or Ca)

    • Minerals 90-99.99% Na or Ca

  • Notation:

    • AnxAby An20Ab80=Oligoclase


Feldspar group albite anorthite series1

Feldspar Group – Albite-Anorthite series

  • Optical techniques to distinguish between plagioclase feldspars:

    • Michel-Levy Method – uses extinction angles of twinned forms to determine An-Ab content

    • Combined Carlsbad-Albite Method  uses Michel-Levy technique for both sides of a twin form


Feldspar group albite orthoclase series

monalbite

anorthoclase

1100

high albite

sanidine

900

intermediate albite

Temperature (ºC)

700

orthoclase

low albite

microcline

500

Miscibility Gap

300

10

70

30

50

90

Orthoclase

KAlSi3O8

Albite

NaAlSi3O8

% NaAlSi3O8

Feldspar Group – Albite-Orthoclase series

  • Several minerals – Alkali Feldspars

  • High – T minerals

    • Sanidine

    • Anorthoclase

    • Monalbite

    • High Albite

  • Low Temperature exsolution at solvus

    • Chicken soup separation

  • Forms 2 minerals, in igneous rocks these are typically intergrowths, or exsolution lamellae – perthitic texture


Alkali feldspar exsolution

Liquid

1100

monalbite

anorthoclase

900

high albite

sanidine

intermediate albite

Temperature (ºC)

700

orthoclase

low albite

microcline

500

Miscibility Gap

300

10

70

30

50

90

Orthoclase

KAlSi3O8

Albite

NaAlSi3O8

% NaAlSi3O8

Alkali Feldspar Exsolution

  • Melt cools past solvus (line defining miscibility gap)

  • Anorthoclase, that had formed (through liquidus/solidus) separates (if cooling is slow enough) to form orthoclase and low albite

  • In hand sample – schiller effect  play of colors caused by lamellae


Alkali feldspar lamellae

Alkali Feldspar lamellae


Feldspathoid group

Feldspathoid Group

  • Very similar to feldspars and zeolites

  • Include Nepheline, Analcime, and Leucite

  • Also framework silicates, but with another Al substitution for Si

  • Only occur in undersaturated rocks (no free Quartz, Si-poor) because they react with SiO2 to form feldspars


Nesosilicates independent sio 4 tetrahedra

b

M1 in rows and share edges

M2 form layers in a-c that share corners

Some M2 and M1 share edges

a

Nesosilicates: independent SiO4 tetrahedra

Olivine (001) view blue = M1 yellow = M2


Feldspar group

Olivine – complete solid solution

Forsterite-Fayalite  FoxFay

Fayalite – Fe end-member

Forsterite – Mg end-member

Olivine Occurrences:

Principally in mafic and ultramafic igneous and meta-igneous rocks

Fayalite in meta-ironstones and in some alkalic granitoids

Forsterite in some siliceous dolomitic marbles

Monticellite CaMgSiO4

Ca  M2 (larger ion, larger site)

High grade metamorphic siliceous carbonates


Distinguishing forsterite fayalite

Distinguishing Forsterite-Fayalite

  • Petrographic Microscope

    • Index of refraction  careful of zoning!!

    • 2V different in different composition ranges

    • Pleochroism/ color slightly different

  • Spectroscopic techniques – many ways to determine Fe vs. Mg

  • Same space group (Pbnm), Orthorhombic, slight differences in unit cell dimensions only


Inosilicates single chains pyroxenes

b

Diopside: CaMg [Si2O6]

a sin

Where are the Si-O-Si-O chains??

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes1

b

a sin

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes2

b

a sin

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes3

b

a sin

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes4

b

a sin

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes5

b

a sin

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes6

Perspective view

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes7

SiO4 as polygons

(and larger area)

IV slab

VI slab

IV slab

a sin

VI slab

Inosilicates: single chains- pyroxenes

IV slab

VI slab

IV slab

b

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes8

M1 octahedron

Inosilicates: single chains- pyroxenes


Inosilicates single chains pyroxenes9

M1 octahedron

Inosilicates: single chains- pyroxenes


Inosilicates single chains pyroxenes10

(+)

M1 octahedron

(+) type by convention

Inosilicates: single chains- pyroxenes


Inosilicates single chains pyroxenes11

M1 octahedron

This is a (-) type

(-)

Inosilicates: single chains- pyroxenes


Inosilicates single chains pyroxenes12

T

M1

T

Creates an “I-beam” like unit in the structure.

Inosilicates: single chains- pyroxenes


Inosilicates single chains pyroxenes13

(+)

T

M1

T

Creates an “I-beam” like unit in the structure

Inosilicates: single chains- pyroxenes


Feldspar group

(+)

(+)

(+)

(+)

(+)

Inosilicates: single chains- pyroxenes

The pyroxene structure is then composed of alternating I-beams

Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation

Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains


Feldspar group

(+)

(+)

(+)

(+)

(+)

Inosilicates: single chains- pyroxenes

The pyroxene structure is then composed of alternation I-beams

Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation

Orthopyroxenes have alternating (+) and (-) orientations


Feldspar group

Inosilicates: single chains- pyroxenes

Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms


Feldspar group

Inosilicates: single chains- pyroxenes

The tetrahedral chain above the M1s is thus offset from that below

The M2 slabs have a similar effect

The result is a monoclinic unit cell, hence clinopyroxenes

(+) M2

c

a

(+) M1

(+) M2


Feldspar group

Inosilicates: single chains- pyroxenes

Orthopyroxenes have alternating (+) and (-) I-beams

the offsets thus compensate and result in an orthorhombic unit cell

c

(-) M1

(+) M2

a

(+) M1

(-) M2


Pyroxene chemistry

Pyroxene Chemistry

The general pyroxene formula:

W1-P (X,Y)1+P Z2O6

Where

  • W = Ca Na

  • X = Mg Fe2+ Mn Ni Li

  • Y = Al Fe3+ Cr Ti

  • Z = Si Al

    Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles


Pyroxene chemistry1

Pyroxene Chemistry

The pyroxene quadrilateral and opx-cpx solvus

Coexisting opx + cpx in many rocks (pigeonite only in volcanics)

Wollastonite Ca2Si2O6

  • Orthopyroxenes – solid soln between Enstatite-Ferrosilite

  • Clinopyroxenes – solid soln between Diopside-Hedenbergite

Hedenbergite

CaFeSi2O6

Diopside

CaMgSi2O6

clinopyroxenes

Joins – lines between end members – limited mixing away from join

pigeonite

orthopyroxenes

Ferrosilite

Fe2Si2O6

Enstatite

Mg2Si2O6


Orthopyroxene clinopyroxene

Wollastonite Ca2Si2O6

Hedenbergite

CaFeSi2O6

Diopside

CaMgSi2O6

clinopyroxenes

pigeonite

orthopyroxenes

Ferrosilite

Fe2Si2O6

Enstatite

Mg2Si2O6

Orthopyroxene - Clinopyroxene

OPX and CPX have different crystal structures – results in a complex solvus between them

Coexisting opx + cpx in many rocks (pigeonite only in volcanics)

pigeonite

1200oC

orthopyroxenes

clinopyroxenes

1000oC

CPX

Solvus

800oC

(Mg,Fe)2Si2O6

Ca(Mg,Fe)Si2O6

OPX

OPX

CPX


Orthopyroxene clinopyroxene solvus t dependence

Orthopyroxene – Clinopyroxenesolvus T dependence

  • Complex solvus – the ‘stability’ of a particular mineral changes with T. A different mineral’s ‘stability’ may change with T differently…

  • OPX-CPX exsolution lamellae  Geothermometer…

CPX

CPX

Hd

Di

Di

Hd

augite

augite

Subcalcic augite

Miscibility

Gap

Miscibility

Gap

pigeonite

pigeonite

orthopyroxene

orthopyroxene

Fs

En

Fs

En

OPX

OPX

800ºC

1200ºC

Pigeonite + orthopyroxene


Feldspar group

17.4 A

12.5 A

7.1 A

5.2 A

Pyroxenoids

“Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites

Pyroxene

2-tet repeat

Wollastonite

(Ca  M1)

 3-tet repeat

Rhodonite

MnSiO3

 5-tet repeat

Pyroxmangite

(Mn, Fe)SiO3

 7-tet repeat


Inosilicates double chains amphiboles

b

Tremolite:

Ca2Mg5 [Si8O22] (OH)2

a sin

Inosilicates: double chains- amphiboles

Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)

yellow = M4 (Ca)


Inosilicates double chains amphiboles1

b

Hornblende:

(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

a sin

Inosilicates: double chains- amphiboles

Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2

light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)

little turquoise ball = H


Inosilicates double chains amphiboles2

Hornblende:

(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

Same I-beam architecture, but the I-beams are fatter (double chains)

Inosilicates: double chains- amphiboles

Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2

light blue = M3 (all Mg, Fe)


Inosilicates double chains amphiboles3

(+)

(+)

(+)

(+)

(+)

b

Hornblende:

(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

Same I-beam architecture, but the I-beams are fatter (double chains)

a sin

Inosilicates: double chains- amphiboles

All are (+) on clinoamphiboles and alternate in orthoamphiboles

Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2

light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)

little turquoise ball = H


Inosilicates double chains amphiboles4

Hornblende:

(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

M1-M3 are small sites

M4 is larger (Ca)

A-site is really big

Variety of sites 

great chemical range

Inosilicates: double chains- amphiboles

Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2

light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)

little turquoise ball = H


Inosilicates double chains amphiboles5

Hornblende:

(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

(OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra

Inosilicates: double chains- amphiboles

(OH)

Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2

light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)

little turquoise ball = H


Feldspar group

Amphibole Chemistry

See handout for more information

General formula:

W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2

W = Na K

X = Ca Na Mg Fe2+ (Mn Li)

Y = Mg Fe2+ Mn Al Fe3+ Ti

Z = Si Al

Again, the great variety of sites and sizes  a great chemical range, and hence a broad stability range

The hydrous nature implies an upper temperature stability limit


Feldspar group

Amphibole Chemistry

Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)

Tremolite

Ferroactinolite

Actinolite

Ca2Mg5Si8O22(OH)2

Ca2Fe5Si8O22(OH)2

Clinoamphiboles

Cummingtonite-grunerite

Anthophyllite

Fe7Si8O22(OH)2

Mg7Si8O22(OH)2

Orthoamphiboles

Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite  gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions


Feldspar group

Amphibole Chemistry

Hornblende has Al in the tetrahedral site

Geologists traditionally use the term “hornblende” as a catch-all term for practically any dark amphibole. Now the common use of the microprobe has petrologists casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member.

Sodic amphiboles

Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2

Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2

Sodic amphiboles are commonly blue, and often called “blue amphiboles”


Feldspar group

Inosilicates

+

+

+

+

+

+

+

a

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

Clinopyroxene

Clinoamphibole

+

+

+

+

+

a

+

-

-

-

-

-

-

Orthopyroxene

Orthoamphibole

Pyroxenes and amphiboles are very similar:

  • Both have chains of SiO4 tetrahedra

  • The chains are connected into stylized I-beams by M octahedra

  • High-Ca monoclinic forms have all the T-O-T offsets in the same direction

  • Low-Ca orthorhombic forms have alternating (+) and (-) offsets


Inosilicates

pyroxene

amphibole

Inosilicates

b

a

Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them)

Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  60-120o cleavages in amphiboles


Tectosilicates1

Tectosilicates

After Swamy and Saxena (1994)J. Geophys. Res., 99, 11,787-11,794.


Tectosilicates2

Tectosilicates

Low Quartz

001 Projection Crystal Class 32


Tectosilicates3

Tectosilicates

High Quartz at 581oC

001 Projection Crystal Class 622


Tectosilicates4

Tectosilicates

Cristobalite

001 Projection Cubic Structure


Tectosilicates5

Tectosilicates

Stishovite

High pressure  SiVI


Tectosilicates6

Tectosilicates

Low Quartz Stishovite

SiIVSiVI


Feldspar group

Phyllosilicates

SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]

Apical O’s are unpolymerized and are bonded to other constituents


Feldspar group

Phyllosilicates

Tetrahedral layers are bonded to octahedral layers

(OH) pairs are located in center of T rings where no apical O


Feldspar group

Phyllosilicates

Octahedral layers can be understood by analogy with hydroxides

Brucite: Mg(OH)2

Layers of octahedral Mg in coordination with (OH)

Large spacing along c due to weak van der waals bonds

c


Feldspar group

Phyllosilicates

a2

a1

Gibbsite: Al(OH)3

Layers of octahedral Al in coordination with (OH)

Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasons

Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral


Feldspar group

Phyllosilicates

T O T K T O T K T O T

Muscovite:K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV)

T-layer - diocathedral (Al3+) layer - T-layer - K

K between T - O - T groups is stronger than vdw


Feldspar group

Phyllosilicates

T O T K T O T K T O T

Phlogopite: K Mg3 [Si3AlO10] (OH)2

T-layer - triocathedral (Mg2+) layer - T-layer - K

K between T - O - T groups is stronger than vdw


Igneous minerals

Igneous Minerals

  • Quartz, Feldspars (plagioclase and alkaline), Olivines, Pyroxenes, Amphiboles

  • Accessory Minerals – mostly in small quantities or in ‘special’ rocks

    • Magnetite (Fe3O4)

    • Ilmenite (FeTiO3)

    • Apatite (Ca5(PO4)3(OH,F,Cl)

    • Zircon (ZrSiO4)

    • Sphene (a.k.a. Titanite) (CaTiSiO5)

    • Pyrite (FeS2)

    • Fluorite (CaF2)


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