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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
slide2

Characteristic structure

    • Pair of single tetrahedrons
    • Share single O
    • Z/O ratio = 2/7
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)
slide5

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

slide7

Chains are parallel to b axis

  • Cleavage commonly occurs between b axis
  • Commonly elongate parallel to b axis
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

occurrence
Occurrence
  • Zoisite
    • Medium grade metamorphic rocks derived from Ca-rich sedimentary rocks
    • Amphibolite from mafic igneous rocks
slide11

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
slide12

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
slide18

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
slide21

All rings point in same direction

    • No center of symmetry
    • piezoelectric
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
slide25

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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