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

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

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

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)

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

Cross-linked double tetrahedron M2 M1 Cross-linked double tetrahedron M3 A = 7 to 11 fold coordination Fig. 15-2

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

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

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

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 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

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

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

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

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

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 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

All rings point in same direction No center of symmetry piezoelectric

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

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 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

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