1. Disilicates and Ring Silicates
Not a common group of minerals – not rock forming
Widely distributed
Most common are the epidote group
Epidote
Zoisite
Clinozoisite
Allanite

2. Characteristic structure
Pair of single tetrahedrons
Share single O
Z/O ratio = 2/7

3. General Structure Complex structure Links of silica tetrahedrons
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

4. Structure – Epidote Gp 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)

5. 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

6. Cross-linked double tetrahedronM2 M1
Cross-linked double tetrahedron M3
A = 7 to 11 fold coordination
Fig. 15-2

7. Chains are parallel to b axis
Cleavage commonly occurs between b axis
Commonly elongate parallel to b axis

8. Variation of cations in A and M3 sites control names
Chemistry – Epidote Gp
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

9. Epidote Gp Minerals A Site M3 site Epidote Ca Al, Fe3+ Clinozoisite Al
Piemontite
Mn, Fe3+, Al
Allanite
Ca, Ce3+, La3+, Na
Fe3+, Fe2+, Mg, Al

10. Occurrence
Zoisite
Medium grade metamorphic rocks derived from Ca-rich sedimentary rocks
Amphibolite from mafic igneous rocks

11. 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

12. Allanite Common accessory mineral in felsic igneous rocks
Granite, granodiorite, monzonite, syenite

13. Ring Silicates Only three common species All are 6 member rings
Tourmaline
Beryl
Cordierite
All are 6 member rings
Rings stacked on top of each other
Isostructural

14. Ring – Z/O = 1/3
Beryl and Cordierite
Tourmaline
Fig. 15-1

15. 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

16. Be in 4 fold coordination
Si tetrahedral rings
Be in 4 fold coordination
Al in 6 fold coordination
Fig. 15-6

17. 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

18. Occurrence Use Common in granitic pegmatites
Associations are quartz, K-spar, albite, muscovite, biotite, tourmaline
Use
Source of Be
Gemstone: aquamarine (blue-green), emerald (green)

19. 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

20. 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

21. All rings point in same direction
No center of symmetry
piezoelectric

22. 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
Fig. 15-6

23. 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

24. End members Schorl Dravite Elbaite NaFe3Al6(Si6O8)(BO3)3(O,OH,F)4
NaMg3Al6(Si6O8)(BO3)3(O,OH,F)4
Elbaite
Na(Li,Al)3Al6(Si6O8)(BO3)3(O,OH,F)4

25. Occurrence
Common in pegmatites, other igneous and metamorphic rocks
Resistant to weathering – heavy mineral fractions
Use
Gemstone
Pressure gauges and other electrical components