Disilicates and ring silicates
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Disilicates and Ring Silicates. Disilicates Not a common group of minerals – not rock forming Widely distributed Most common are the epidote group Epidote Zoisite Clinozoisite Allanite. Characteristic structure Pair of single tetrahedrons Share single O Z/O ratio = 2/7.

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Disilicates and ring silicates
Disilicates and Ring Silicates

  • Disilicates

    • Not a common group of minerals – not rock forming

    • Widely distributed

    • Most common are the epidote group

      • Epidote

      • Zoisite

      • Clinozoisite

      • Allanite



General structure
General Structure

  • Complex structure

    • Leads to much substitution and complicated compositions

  • Links of silica tetrahedrons

    • Edge sharing Al octahedrons

    • Octahedrons linked by double silica tetrahedrons

    • Other cations link tetrahedrons


Structure epidote gp
Structure – EpidoteGp

  • One edge sharing octahedral chain

    • M2 sites

  • Second edge sharing octahedral chain

    • M1 site with Al

    • M3 site with other cations (usually Fe and Mn)


  • Chains and cross linked tetrahedron

    • produce 7 to 11 fold coordination sites – A sites

    • Commonly contain Ca

  • Overall – 5 sites:

    • Tetrahedron, 3 octahedral sites (M1, M2, & M3), one high coordination number site - A


Fig 15 2

M2

Fig. 15-2

M1

Cross-linked double tetrahedron

M3

A = 7 to 11 fold coordination



Chemistry epidote gp
Chemistry – EpidoteGp

M3 site

  • Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH)

A site

M1, M2, & M3 site

Tetrahedral Site

Variation of cations in A and M3 sites control names


Epidote gp minerals
EpidoteGp Minerals


Occurrence
Occurrence

  • Zoisite

    • Medium grade metamorphic rocks derived from Ca-rich sedimentary rocks

    • Amphibolite from mafic igneous rocks


  • Clinozoisite, epidote

    • Regional and contact metamorphic rocks

    • Pelites, metacarbonates, felsic to mafic meta-igneous rocks

    • Clinozoisite in Al-rich rocks

    • Epidote in Fe-rich rocks

    • Resistant to weathering, so they are common detrital grains, heavy minerals


  • Allanite

    • Common accessory mineral in felsic igneous rocks

    • Granite, granodiorite, monzonite, syenite


Ring silicates
Ring Silicates

  • Only three common species

    • Tourmaline

    • Beryl

    • Cordierite

  • All are 6 member rings

  • Rings stacked on top of each other

Isostructural


Fig 15 1

Ring – Z/O = 1/3

Fig. 15-1

Beryl and Cordierite

Tourmaline


Beryl al 2 be 3 si 6 o 18
Beryl – Al2Be3Si6O18

  • Structure

    • Rings rotated relative to the ones above and below

    • Each column are concentric cylinders of oxygen atoms

    • Columns arranged hexagonally

    • Al in distorted 6-fold coordination sites

    • Be in 4-fold sites between Si tetrahedrons

    • Could be considered tectosilicates


Fig 15 6
Fig. 15-6

Si tetrahedral rings

Be in 4 fold coordination

Al in 6 fold coordination


Compositional variations
Compositional variations

  • Inclusion of Li, Na, K, and Cs

  • Li substitutes for Be and Al

  • Other located in channels

  • Charge balance from substitution of Fe, Mg, Li for Al or vacancies in Be sites

  • Cr substitution makes emerald


  • Occurrence

    • Common in granitic pegmatites

    • Associations are quartz, K-spar, albite, muscovite, biotite, tourmaline

  • Use

    • Source of Be

    • Gemstone: aquamarine (blue-green), emerald (green)


Cordierite
Cordierite

  • Identical to Beryl except

    • Substitute Mg2+ for Al3+ in octahedral sites

    • Substitute Al for Be in cross linked tetrahedral sites

    • Replace one Si4+ with one Al3+ in rings for charge balance


Tourmaline
Tourmaline

  • Hydrated Na, Mg, Fe, Li, Al, B silicate

  • Structure

    • Single 6-member ring

    • 3 octahedral sites at base with X cations coordinating

    • One large W cation site in axis of rings

    • B trigonally coordinated as base

    • Columns held together with octahedrally coordinated Y cation



Fig 15 61
Fig. 15-6

W = Na, less commonly Ca, K

X = Mg & Fe, less commonly Mn, Li, Al

Y = Al, less commonly Fe, Mg

Trigonally coordinated B


Composition
Composition

  • General formula

    • WX3Y6(Si6O8)(BO3)3(O,OH,F)4

    • W = Na, less commonly Ca, K

    • X = Mg and Fe, less commonly Mn, Li, Al

    • Y = Al, less commonly Fe, Mg


End members
End members

  • Schorl

    • NaFe3Al6(Si6O8)(BO3)3(O,OH,F)4

  • Dravite

    • NaMg3Al6(Si6O8)(BO3)3(O,OH,F)4

  • Elbaite

    • Na(Li,Al)3Al6(Si6O8)(BO3)3(O,OH,F)4


  • Occurrence

    • Common in pegmatites, other igneous and metamorphic rocks

    • Resistant to weathering – heavy mineral fractions

  • Use

    • Gemstone

    • Pressure gauges and other electrical components


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